U.S. patent number RE30,738 [Application Number 06/118,957] was granted by the patent office on 1981-09-08 for apparatus and method for in situ heat processing of hydrocarbonaceous formations.
This patent grant is currently assigned to IIT Research Institute. Invention is credited to Jack Bridges, Allen Taflove.
United States Patent |
RE30,738 |
Bridges , et al. |
September 8, 1981 |
**Please see images for:
( Certificate of Correction ) ** |
Apparatus and method for in situ heat processing of
hydrocarbonaceous formations
Abstract
The disclosure describes a technique for uniform heating of
relatively large blocks of hydrocarbonaceous formations in situ
using radio frequency (RF) electrical energy that is substantially
confined to the volume to be heated and effects of dielectric
heating of the formations. An important aspect of the disclosure
relates to the fact that certain hydrocarbonaceous earth
formations, for example raw unheated oil shale, exhibit dielectric
absorption characteristics in the radio frequency range. In
accordance with the system of the invention, a plurality of
conductors are inserted in the formations and bound a particular
volume of the formations. The phrase "bounding a particular volume"
is intended to mean that the volume is enclosed on at least two
sides thereof. Electrical excitation is provided for establishing
alternating electric fields in the volume. The frequency of the
excitation is selected as a function of the dimensions of the
volume so as to establish a substantially non-radiating electric
field which is substantially confined in the volume. In this
manner, volumetric dielectric heating of the formations will occur
to effect approximately uniform controlled heating of the
volume.
Inventors: |
Bridges; Jack (Park Ridge,
IL), Taflove; Allen (Wilmette, IL) |
Assignee: |
IIT Research Institute
(Chicago, IL)
|
Family
ID: |
22381777 |
Appl.
No.: |
06/118,957 |
Filed: |
February 6, 1980 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
828621 |
Aug 29, 1977 |
04144935 |
Mar 20, 1979 |
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Current U.S.
Class: |
166/248; 166/245;
166/272.1; 166/60; 166/66.5; 219/770 |
Current CPC
Class: |
E21B
36/04 (20130101); E21B 43/305 (20130101); E21B
43/2401 (20130101) |
Current International
Class: |
E21B
43/00 (20060101); E21B 36/04 (20060101); E21B
43/30 (20060101); E21B 36/00 (20060101); E21B
43/24 (20060101); E21B 43/16 (20060101); E21B
043/24 (); E21B 043/30 () |
Field of
Search: |
;166/248,60,57,302,245,50,52,65R ;219/1.55R,10.65,10.81 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
DuBow, J., "Electrical and Thermal Properties of Oil Shale of
Interest to In-Situ Shale Oil Extraction," National Science
Foundation Report--NSF/RA-770043, PB 267136, Jan. 1977, pp. 40-44.
.
Fu, "Gasification of Fossil Fuels in a Microwave Discharge in
Argon", Chemistry 2d Industry, Jul. 31, 1971, pp. 876, 877. .
Fu et al., "Pyrolysis of Coals in a Microwave Discharge",
Industrial & Engineering Chemistry, Process Design and
Development, vol. 8, No. 2, Apr. 1969, pp. 257-262..
|
Primary Examiner: Novosad; Stephen J.
Claims
We claim:
1. A system for in situ heat processing of hydrocarbonaceous earth
formations, comprising:
a plurality of conductive means inserted in said formations and
bounding a particular volume of said formations;
electrical excitation means for establishing alternating electric
fields in said volume;
the frequency of said excitation means being selected as a function
of the volume dimensions so as to establish substantially
non-radiating electric fields which are substantially confined in
said volume;
whereby volumetric dielectric heating of the formations will occur
to effect approximately uniform heating of said volume.
2. A system as defined by claim 1 wherein the frequency of said
excitation is in the radio frequency range.
3. A system as defined by claim 2 wherein said conductive means
comprise opposing spaced rows of conductors disposed in opposing
spaced rows of boreholes in said formations.
4. A system as defined by claim 3 wherein the conductors of each
row comprise spaced elongated conductors.
5. A system as defined by claim .[.4.]. .Iadd.20 .Iaddend.wherein
said excitation is applied as a voltage as between the conductors
of the outer rows and the conductors of the central row.
6. A system as defined by claim 4 wherein said electrical
excitation is a source of current applied to at least one current
loop in said volume.
7. A system as defined by claim 4 wherein said electrical
excitation is applied across at least one electrical dipole in said
volume.
8. A system as defined by claim 4 wherein the conductors of the
central row are of substantially shorter length than the conductors
of the outer rows so as to reduce radiation at the ends of said
conductors.
9. A system as defined by claim 8 wherein the frequency of said
excitation is selected such that a half wavelength of
electromagnetic energy in the region beyond the center conductor is
substantially greater than the spacing between the outer rows to
give rise to a cutoff condition in said region.
10. A system as defined by claim 9 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
l/e attenuation distance of the electric field in any direction in
said volume is more than twice the physical dimension of said
volume in that direction.
11. A system as defined by claim 9 further comprising means for
modifying the electric field pattern so as to average the electric
field intensity in said volume to enhance the uniformity of heating
of said volume.
12. A system as defined by claim 8 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
l/e attenuation distance of the electric field in any direction in
said volume is more than twice the physical dimension of said
volume in that direction.
13. A system as defined by claim 8 further comprising means for
modifying the electric field pattern so as to average the electric
field intensity in said volume to enhance the uniformity of heating
of said volume.
14. A system as defined by claim 2 wherein said excitation in
applied as to a voltage as between different groups of said
conductive means.
15. A system as defined by claim 2 wherein said electrical
excitation is a source of current applied to at least one current
loop in said volume.
16. A system as defined by claim 2 wherein said electrical
excitation is applied across at least one electrical dipole in said
volume.
17. A system as defined by claim 2 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
l/e attenuation distance of the electric field in any direction in
said volume is more than twice the physical dimension of said
volume in that direction.
18. A system as defined by claim 2 further comprising means for
modifying the electric field pattern so as to average the electric
field intensity in said volume to enhance the uniformity of heating
of said volume.
19. A system as defined by claim 1 wherein said conductive means
comprise opposing spaced rows of conductors disposed in opposing
spaced rows of boreholes in said formations.
20. A system as defined by claim 19 wherein said rows of conductors
comprise three spaced rows of conductors.
21. A system as defined by claim 20 wherein the conductors of each
row comprise spaced elongated conductors.
22. A system as defined by claim 21 wherein said excitation is
applied as a voltage as between the conductors of the outer rows
and the conductors of the central row.
23. A system as defined by claim 22 wherein the conductors of the
central row are of substantially shorter length than the conductors
of the outer rows so as to reduce radiation at the ends of said
conductors.
24. A system as defined by claim 23 wherein the frequency of said
excitation is selected such that a half wavelength of
electromagnetic energy in the region beyond the center conductor is
substantially greater than the spacing between the outer rows to
give rise to a cutoff condition in said region.
25. A system as defined by claim 21 wherein said electrical
excitation is a source of current applied to at least one current
loop in said volume.
26. A system as defined by claim 25 wherein the conductors of the
central row are of substantially shorter length than the conductors
of the outer rows so as to reduce radiation at the ends of said
conductors.
27. A system as defined by claim 26 wherein the frequency of said
excitation is selected such that a half wavelength of
electromagnetic energy in the region beyond the center conductor is
substantially greater than the spacing between the outer rows to
give rise to a cutoff condition in said region.
28. A system as defined by claim 21 wherein said electrical
excitation is applied across at least one electrical dipole in said
volume.
29. A system as defined by claim 20 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
l/e attenuation distance of the electric field in any direction in
said volume is more than twice the physical dimension of said
volume in that direction.
30. A system as defined by claim 29 wherein said rows of conductors
are inserted in said formations at angles such that said rows are
closer together at far ends thereof to compensate for attenuation
of the electrical field at said far end.
31. A system as defined by claim 20 further comprising means for
modifying the electric field pattern so as to average the electric
field intensity in said volume to enhance the uniformity of heating
of said volume.
32. A system as defined by claim 31 wherein said means for
modifying the electric field pattern comprises means for modifying
the effective length of the conductors of the central row.
33. A system as defined by claim 32 wherein said means for
modifying the effective length of the conductors of the central row
comprises means for physically shortening the length of said
conductors.
34. A system as defined by claim 32 wherein said means for
modifying the effective length of said conductors comprises means
for electrically modifying the effective length thereof.
35. A system as defined by claim 20 wherein said rows of conductors
are inserted in said formations at angles such that said rows are
closer together at far ends thereof to compensate for attenuation
of the electrical field at said far end.
36. A system as defined by claim 19 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
l/e attenuation distance of the electric field in any direction in
said volume is more than twice the physical dimension of said
volume in that direction.
37. A system as defined by claim 19 further comprising means for
modifying the electric field pattern so as to average the electric
field intensity in said volume to enhance the uniformity of heating
of said volume.
38. A system as defined by claim 19 wherein said rows of conductors
are inserted in said formations at angles such that said rows are
closer together at far ends thereof to compensate for attenuation
of the electrical field at said far end.
39. A system as defined by claim 1 wherein said excitation is
applied as a voltage as between different groups of said conductive
means.
40. A system as defined by claim 39 wherein the conductors of the
central row are of substantially shorter length than the conductors
of the outer rows so as to reduce radiation at the ends of said
conductors.
41. A system as defined by claim 1 wherein said electrical
excitation is a source of current applied to at least one current
loop in said volume.
42. A system as defined by claim 1 wherein said electrical
excitation is applied across at least one electrical dipole in said
volume.
43. A system as defined by claim 1 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
l/e attenuation distance of the electric field in any direction in
said volume is more than twice the physical dimension of said
volume in that direction.
44. A system as defined by claim 43 further comprising means for
modifying the electric field pattern so as to average the electric
field intensity in said volume to enhance the uniformity of heating
of said volume.
45. A system as defined by claim 1 further comprising means for
modifying the electric field pattern so as to average the electric
field intensity in said volume to enhance the uniformity of heating
of said volume.
46. A method for in situ heating of hydrocarbonaceous earth
formations, comprising the steps of:
forming a plurality of boreholes which bound a particular volume of
said formations;
inserting elongated electrical conductors in said boreholes;
and
introducing electrical excitation to said formations to establish
alternating electric fields in said volume;
the frequency of said excitation being selected as a function of
the volume dimensions so as to establish substantially
non-radiating electric fields which are substantially confined in
said volume;
whereby volumetric dielectric heating of the formations will occur
to effect approximately uniform heating of said volume.
47. A method as defined by claim 46 wherein the frequency of said
excitation is in the radio frequency range.
48. A method as defined by claim 47 wherein the step of introducing
electrical excitation comprises applying a voltage as between
different groups of said conductors.
49. A method as defined by claim 47 wherein the step of introducing
electrical excitation comprises applying electrical current to at
least one current loop in said volume.
50. A method as defined by claim 47 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
l/e attenuation distance of the electric field in any direction in
said volume is more than twice the physical dimension of said
volume in that direction.
51. A method as defined by claim 47 further comprising the step of
modifying the electric field pattern so as to average the electric
field intensity in said volume to enhance the uniformity of heating
of said volume.
52. A method as defined by claim 51 wherein the step of modifying
the electric field pattern comprises the step of modifying the
effective length of some of said conductors.
53. A method as defined by claim 47 further comprising the step of
withdrawing through said boreholes the valuable constituents
resulting from said heating.
54. A method as defined by claim 47 wherein said dielectric heating
is continued to heat said volume to a temperature below the
temperature required for extraction of valuable constituents from
said volume, and further comprising the steps of applying further
nonelectrical heating means to said volume and withdrawing through
said boreholes valuable constituents from said volume.
55. A method as defined by claim 46 wherein said boreholes are
formed in opposing spaced rows in said formations.
56. A method as defined by claim 55 wherein said rows comprise
three spaced rows.
57. A system for in situ heat processing of an oil shale bed,
comprising:
a plurality of conductive means bounding a particular volume of
said bed;
electrical excitation means for establishing alternating electric
fields in said volume;
the frequency of said excitation means being selected as a function
of the volume dimensions so as to establish substantially
non-radiating electric fields which are substantially confined in
said volume;
whereby volumetric dielectric heating of the bed will occur to
effect approximately uniform heating of said volume.
58. A system as defined by claim 57 wherein the frequency of said
excitation is in the radio frequency range.
59. A system as defined by claim 57 wherein the frequency of said
excitation is in the range between about 1 MHz and 40 MHz.
60. A system as defined by claim 59 wherein said conductive means
comprise opposing spaced rows of conductors disposed in opposing
spaced rows of boreholes in said bed.
61. A system as defined by claim 60 wherein said rows of conductors
comprise three spaced rows of conductors.
62. A system as defined by claim 61 wherein the conductors of the
central row are of substantially shorter length than the conductors
of the outer rows so as to reduce radiation at the ends of said
conductors.
63. A system as defined by claim 62 wherein the frequency of said
excitation is selected such that a half wavelength of
electromagnetic energy in the region beyond the center conductor is
substantially greater than the spacing between the outer rows to
give rise to a cutoff condition in said region.
64. A system as defined by claim 59 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
l/e attenuation distance of the electric field in any direction in
said volume is more than twice the physical dimension of said
volume in that direction.
65. A system as defined by claim 57 wherein said conductive means
comprise opposing spaced rows of conductors disposed in opposing
spaced rows of boreholes in said bed.
66. A system as defined by claim 57 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
l/e attenuation distance of the electric field in any direction in
said volume is more than twice the physical dimension of said
volume in that direction.
67. A system for in situ heat processing of a tar sand deposit,
comprising:
a plurality of conductive means inserted in said deposit and
bounding a particular volume of said deposit;
electrical excitation means for establishing alternating electric
fields in said volume;
the frequency of said excitation means being selected as a function
of the volume dimensions so as to establish substantially
non-radiating electric fields which are substantially confined in
said volume;
whereby volumetric dielectric heating of the deposit will occur to
effect approximately uniform heating of said volume.
68. A system as defined by claim 67 wherein the frequency of said
excitation is in the radio frequency range.
69. A system as defined by claim 68 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
skin depth of the electric field in any direction in said volume is
more than twice the physical dimension of said volume in that
direction.
70. A system as defined by claim 67 wherein the frequency of said
excitation is selected as a function of the electrical lossiness of
the formations in said volume to be sufficiently low such that the
skin depth of the electric field in any direction in said volume is
more than twice the physical dimension of said volume in that
direction. .Iadd. 71. A system for in situ heat processing of
hydrocarbonaceous earth formations, comprising:
a waveguide structure comprising a plurality of elongate electrodes
and configured such that the direction of propagation of aggregate
modes of wave propagation therein is approximately parallel to an
elongate axis of said electrodes and bounding a particular volume
of earth formations as a dielectric medium bounded therein; and
means for supplying electromagnetic energy to said waveguide
structure at a frequency selected to confine said electromagnetic
energy in said structure and to dissipate said electromagnetic
energy to the earth formations, thereby to substantially uniformly
heat the bounded volume. .Iaddend..Iadd. 72. A system for in situ
heat processing of hydrocarbonaceous earth materials,
comprising:
a waveguide structure having an elongate shape which penetrates and
bounds a particular volume of earth formations therein and wherein
the aggregate direction of propagation of electromagnetic wave
modes in said structure is in a direction approximately parallel to
an elongate axis of said structure; and
means for supplying electromagnetic energy to said waveguide
structure at a frequency selected to confine said energy and to
dissipate said electromagnetic energy to said bounded volume
thereby to substantially uniformly heat said bounded volume.
.Iaddend. .Iadd. 73. A system for in situ heat processing of
hydrocarbonaceous earth formations, comprising:
field confining means bounding a particular volume of earth
formations and forming an elongate waveguide structure having a
direction of aggregate electromagnetic wave propagation mode
direction in a direction approximately parallel to an elongate axis
of said structure; and
means for supplying electromagnetic energy to said waveguide
structure at a frequency to confine said electromagnetic energy in
said structure and to cause dielectric heating of said bounded
volume to a substantially uniform degree. .Iaddend..Iadd. 74. A
system for in situ heat processing of hydrocarbonaceous earth
formations comprising:
a plurality of electrodes placed in a pattern bounding a particular
volume of hydrocarbonaceous earth formation, said pattern defining
a waveguide structure having said volume bounded therein as a
dielectric medium; and,
means for applying an alternating current to said electrodes in the
frequency range of the order of 100 kilohertz to 100 megahertz, the
frequency of said current being selected as a function of a volume
dimension so as to establish substantially non-radiating and
uniform electromagnetic fields in said volume, thereby obtaining
volumetric dielectric heating of said volume to a temperature
sufficient to permit production of hydrocarbonaceous components
thereof. .Iaddend. .Iadd. 75. A system of in situ heat processing
of hydrocarbonaceous earth formations comprising:
a pattern of conductors bounding a particular volume of
hydrocarbonaceous earth formation, said pattern defining an
unbalanced transmission line structure having said bounded volume
integral therewith as a dielectric medium; and
means for supplying alternating current in the frequency range of
the order of 100 kilohertz to 100 megahertz, to said conductors,
the frequency of said current being selected as a function of at
least one volume dimension so as to establish substantially
non-radiating electromagnetic fields in said volume.
.Iaddend..Iadd. 76. A system for in situ heat processing of
hydrocarbonaceous earth formations comprising:
a substantially tri-plate pattern of electrodes placed in a
particular volume of hydrocarbonaceous earth formation and forming
a waveguide structure having said volume bounded therein as a
dielectric medium wherein adjacent portions of electrodes within a
plate are at approximately the same potential; and
means for supplying a time varying electric field in the frequency
range from 100 kilohertz to 100 megahertz to said electrodes so as
to establish substantially non-radiating electric fields in said
volume. .Iadd. 77. A system for in situ heat processing of
hydrocarbonaceous earth formations comprising:
waveguide means formed by a pattern of electrodes placed in a
particular volume of hydrocarbonaceous earth formation to bound
said volume therein as a dielectric medium; and
means for supplying alternating current to said waveguide structure
at a frequency of the order of from 100 kilohertz to 100 megahertz
to effectively confine electromagnetic fields in said structure and
to effect substantially uniform dielectric heating of said volume.
.Iaddend..Iadd. 78. A system for in situ heat processing of
hydrocarbonaceous earth formations comprising:
an unbalanced transmission line structure deployed in a particular
volume of hydrocarbonaceous earth formation, said structure
bounding said volume and employing said formation material as a
dielectric medium therein; and
means for supplying electrical energy having a frequency of the
order of from 100 kilohertz to 100 megahertz to said transmission
line structure, thereby confining said energy in said structure and
providing dielectric heating to a controllable degree in said
volume. .Iaddend. .Iadd. 79. A system for in situ heat processing
of hydrocarbonaceous earth formations comprising:
a waveguide structure formed by bounding a particular volume of
earth formations with a pattern of electrodes bounding said volume
and including said volume as a dielectric medium therein; and
means for establishing alternating electromagnetic fields in the
frequency range between 100 kilohertz and 100 megahertz in said
bounded volume, the frequency of said alternating fields being
selected as a function of a volume dimension, thereby causing
volumetric dielectric heating of said volume to an approximately
uniform degree. .Iaddend..Iadd. 80. A system for in situ heat
processing of hydrocarbonaceous earth formations comprising:
electrode means bounding a particular volume of earth formations in
such a manner as to comprise a waveguide structure having said
volume bounded therein as a dielectric medium; and
means for supplying electromagnetic energy to said waveguide
structure at a frequency selected to confine said energy
substantially in said volume and to cause heating of said volume by
displacement currents to a substantially uniform degree in said
volume. .Iaddend. .Iadd. 81. A system for in situ heat processing
of hydrocarbonaceous earth formations, comprising:
electrode means bounding a particular volume of earth formations in
such a manner as to comprise an unbalanced transmission line
structure having said volume bounded therein as a dielectric
medium; and
means for supplying electromagnetic energy to said unbalanced
transmission line at a frequency selected to cause heating of said
volume by displacement currents to a substantially uniform degree
in said volume. .Iaddend..Iadd. 82. A system for in situ heat
processing of hydrocarbonaceous earth formations comprising:
electrode means bounding a particular volume of earth formations in
such a manner as to comprise an approximately tri-plate
transmission line structure having said volume bounded therein as a
dielectric medium; and
means for supplying electromagnetic energy to said triplate
transmission line structure at a frequency and field intensity
selected to cause heating of said volume to a substantially uniform
degree in said volume without significant heat loss to the adjacent
unbounded regions and without electrical breakdown of said bounded
volume. .Iaddend. .Iadd. 83. A system for in situ heat processing
of hydrocarbonaceous earth formations, comprising
a waveguide structure comprising a plurality of electrodes bounding
a particular volume of earth formations as a dielectric medium
bounded therein; and
means for supplying electromagnetic energy to said waveguide
structure at a frequency selected to dissipate said electromagnetic
energy substantially only to said bounded medium thereby to
substantially uniformly heat said bounded volume. .Iaddend..Iadd.
84. A system for in situ heat processing of hydrocarbonaceous earth
formations, comprising:
an unbalanced transmission line structure comprising a plurality of
electrodes bounding a particular volume of earth formations as a
dielectric medium bounded therein; and
means for supplying electromagnetic energy to said unbalanced
transmission line structure at a frequency selected to
substantially confine said energy to said structure and to
dissipate said electromagnetic energy to said dielectric medium by
displacement current heating thereof, thereby to substantially
uniformly heat said bounded volume without significant heat loss to
the adjacent unbounded regions and without electrical breakdown of
said bounded volume. .Iaddend. .Iadd. 85. A system for in situ heat
processing of hydrocarbonaceous earth formations, comprising:
an approximately tri-plate transmission line structure comprising a
plurality of electrodes bounding a particular volume of earth
formations as a dielectric medium bounded therein; and
means for supplying electromagnetic energy to said approximately
tri-plate transmission line structure at a frequency and field
intensity selected to dissipate said electromagnetic energy to said
dielectric medium, thereby to substantially uniformly heat said
bounded volume without significant heat loss to the adjacent
unbounded regions and without electrical breakdown of said bounded
volume. .Iaddend..Iadd. 86. A system for in situ heating a volume
of hydrocarbonaceous earth formation to an elevated temperature
comprising:
electrical excitation means for providing an electrical waveform of
a frequency of the order of from 100 kilohertz to 100
megahertz;
a conductor array located approximately centrally in said volume to
which the electrical waveform is applied, said central conductor
array comprising a line of conductors inserted in boreholes in the
formation, wherein adjacent conductors in the line are separated by
a distance of about 1/8 of a wavelength of less of the electrical
waveform; and
a bounding conductor array comprising at least one line of
electrical conductors inserted in boreholes in the formation,
adjacent of said conductors in a line being separated by about 1/8
of a wavelength or less of the electrical waveform wherein bounding
conductors are at approximately the same potential as the adjacent
unbounded earth formations whereby radiation of electrical energy
outside the volume of the hydrocarbonaceous earth formation is
minimized. .Iaddend..Iadd. 87. A method of heating a volume of
hydrocarbonaceous earth formations to an elevated temperature
comprising:
applying an electrical waveform to a first row of elongated
conductors penetrating a volume of the formation, adjacent
conductors being separated by a distance less than 1/8 of the
wavelength of the electrical waveform;
confining the electromagnetic field in the volume by bounding said
volume with at least two rows of elongated conductors, adjacent
conductors in a row being separated by a distance less than 1/8 of
the wavelength of the electrical waveform; and
varying at least one of; (a) the frequency of the electrical
waveform; (b) the physical length of individual conductors in a row
of conductors, (c) the series capacitance of conductors in a row;
(d) the effective electrical length of conductors in a row; to
facilitate uniform heating of the formation in the direction of the
principal axis of the elongated conductors. .Iaddend..Iadd. 88. A
method for in situ heat processing of hydrocarbonaceous earth
formations comprising the steps of:
placing a plurality of electrodes into a particular volume of
hydrocarbonaceous material in a pattern which bounds said volume
and defines an unbalanced transmission line structure having said
bounded volume present as a dielectric medium bounded therein;
applying alternating current at a radio frequency on the order of
from 100 kilohertz to 100 megahertz to said electrodes, said radio
frequency being chosen as a function of a volume dimension so as to
establish substantially non-radiating electromagnetic fields which
are substantially confined in said volume, thereby effecting
approximately uniform heating of said volume to a temperature
sufficient to permit production of
hydrocarbonaceous components thereof. .Iaddend..Iadd. 89. A method
for in situ heating processing of hydrocarbonaceous earth
formations comprising the steps of:
placing a plurality of electrodes into a particular volume of
hydrocarbonaceous material in a pattern which bounds said volume
and defines a waveguide structure having said bounded volume
present as a dielectric medium bounded therein;
applying alternating current at a radio frequency on the order of
100 kilohertz to 100 megahertz to said electrodes, said radio
frequency being chosen as a function of at least one volume
dimension so as to establish substantially nonradiating
electromagnetic fields which are substantially confined in said
volume, thereby effecting approximately uniform heating;
modifying the electromagnetic field pattern so as to time average
the electromagnetic field in said volume to enhance the uniformity
of heating of said volume. .Iaddend. .Iadd. 90. A method for in
situ heat processing of hydrocarbonaceous earth formations,
comprising the steps of:
forming a plurality of holes which bound a particular volume of
hydrocarbonaceous material and spaced from each other so as to
define an approximately triplate structure having said bounded
volume of hydrocarbonaceous material present as a dielectric medium
bounded therein;
inserting electrical conductors into said holes; and,
applying alternating current at a radio frequency on the order of
from 100 kilohertz to 100 megahertz to said conductors, said radio
frequency being chosen as a function of at least one volume
dimension so as to establish substantially nonradiating
electromagnetic fields which are substantially confined in said
volume, thereby effecting approximately uniform heating of said
volume. .Iaddend..Iadd. 91. A method for in situ heat processing of
hydrocarbonaceous earth formations, comprising the steps of:
enclosing a particular volume of earth formations on at least two
sides thereof with a plurality of spaced electrodes to define a
waveguide structure having said enclosed volume present therein as
a dielectric medium; and
establishing alternating electromagnetic fields in the frequency
range between 100 kilohertz to 100 megahertz in said enclosed
volume, the frequency of said alternating fields being selected as
a function of a volume dimension, so as to establish substantially
non-radiating, confined, electromagnetic fields in said volume,
thereby causing volumetric dielectric heating of said volume to
effect approximately
uniform heating of said volume. .Iaddend..Iadd. 92. A method for in
situ heat processing of hydrocarbonaceous earth formations
comprising the steps of:
bounding a particular volume of earth formations with a waveguide
structure comprising elongate electrodes having outer electrodes
which are at approximately the same potential as the adjacent
unbounded earth formations; and
propagating electromagnetic energy through the waveguide structure
in an aggregate mode of propagation generally parallel to the
direction of an elongate axis of said electrodes, thereby
substantially confining the electromagnetic energy in the waveguide
structure and uniformly heating the bounded volume of earth
formations. .Iaddend. .Iadd. 93. A method for in situ heat
processing of hydrocarbonaceous earth formations, comprising the
steps of:
bounding a particular volume of earth formations with a
transmission line structure having an inner elongate shaped
propagating electrode structure and an outer elongate shaped
electrode structure which is at approximately the same potential as
the adjacent unbounded earth formations; and
propagating modes of electromagnetic energy in said structure in an
aggregate direction generally parallel to an elongate axis of said
propagating electrodes, thereby confining said electromagnetic
energy in said bounded volume and uniformly heating said bounded
volume. .Iaddend..Iadd. 94. A system for in situ heat processing of
hydrocarbonaceous earth formations, comprising:
a multi mode cavity structure comprising a plurality of elongate
electrodes and configured such that the direction of wave
propagation of a particular mode is parallel to an elongate axis of
at least one set of said electrodes, said multi mode cavity
structure bounding a particular volume of earth formations as a
dielectric medium bounded therein wherein the outermost electrodes
are at approximately the same potential as the adjacent unbounded
earth formations; and
means for supplying electromagnetic energy to said multi mode
cavity structure at a frequency selected to confine said
electromagnetic energy in said structure and to dissipate said
electromagnetic energy to the earth formations; thereby to
substantially uniformly heat the bounded volume. .Iaddend..Iadd.
95. The system of claim 94 and further including means for time
averaging said electromagnetic energy along the direction of
propagation, thereby to enhance the uniformity of heating of the
bounded volume of earth formations. .Iaddend..Iadd. 96. A system
for in situ heat processing of hydrocarbonaceous earth formations,
comprising:
a waveguide structure having a plurality of rows of conductors, the
spacing of conductors in a row being less than the spacing of said
rows of conductors and bounding a particular volume of earth
formations as a dielectric medium bounded therein; and,
means for supplying electromagnetic energy to said waveguide
structure at a frequency selected to dissipate said electromagnetic
energy substantially only to said bounded medium, thereby to
substantially uniformly heat said bounded volume. .Iaddend..Iadd.
97. The system of claim 96 and further including means for time
averaging said electromagnetic energy along a direction of its
propagation in said waveguide structure. .Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates to the exploitation of hydrocarbon-bearing
earth formations, and, more particularly, to a system and method
for the in situ heating processing of hydrocarbon-bearing earth
formations such as oil shale, tar sands, coal, heavy oil, and other
bituminous or viscous petroliferous deposits. The present subject
matter is related to subject matter set forth in the copending U.S.
application Ser. No. 828,904, of Jack Bridges, Allen Taflove and
Richard Snow, filed of even date herewith and assigned to the same
assignee as the present application.
Large scale commercial exploitation of certain hydrocarbon-bearing
resources, available in huge deposits on the North American
continent, has been impeded by a number of problems, especially
cost of extraction and environmental impact. The United States has
tremendous coat resources, but deep mining techniques are hazardous
and leave a large percentage of the deposits in the earth. Strip
mining of coal involves environmental damage or expensive
reclamation. Oil shale is also plentiful in the United States, but
the cost of useful fuel recovery has been generally noncompetitive.
The same is true for tar sands which occur in vast amounts in
Western Canada. Also, heavy or viscous oil is left untapped, due to
the extra cost of extraction, when a conventional oil well is
produced.
Materials such as oil shale, tar sands, and coal are amenable to
heat processing to produce gases and hydrocarboneous liquids.
Generally, the heat develops the porosity, permeability and/or
mobility necessary for recovery. Oil shale is a sedimentary rock
which, upon pyrolysis or distillation, yields a condensable liquid,
referred to as a shale oil, and non-condensable gaseous
hydrocarbons. The condensable liquid may be refined into products
which resemble petroleum products. Oil sand is an erratic mixture
of sand, water and bitumen with the bitumen typically present as a
film around water-enveloped sand particles. using various types of
heat processing the bitumen can, with difficulty, be separated.
Also, as is well known, coal gas and other useful products can be
obtained from coal using heat processing.
In the destructive distillation of oil shale or other solid or
semi-solid hydrocarbonaceous materials, the solid material is
heated to an appropriate temperature and the emitted products are
recovered. This appears a simple enough goal but, in practice, the
limited efficiency of the process has prevented achievement of
large scale commercial application. Regarding oil shale, for
example, there is no presently acceptable economical way to extract
the hydrocarbon constituents. The desired organic constituent,
known as kerogen, constitutes a relatively small percentage of the
bulk shale material, so very large volumes of shale need to be
heated to elevated temperatures in order to yield relatively small
amounts of useful end products. The handling of the large amounts
of material is, in itself, a problem, as is the disposal of wastes.
Also, substantial energy is needed to heat the shale, and the
efficiency of the heating process and the need for relatively
uniform and rapid heating have been limiting factors on success. In
the case of tar sands, the volume of material to be handled, as
compared to the amount of recovered product, is again relatively
large, since bitumen typically constitutes only about ten percent
of total, by weight. Material handling of tar sands is particularly
difficult even under the best of conditions, and the problems of
waste disposal are, of course, present here too.
There have been a number of prior proposals set forth for the
extraction of useful fuels from oil shales and tar sands in situ
but, for various reasons, none has gained commercial acceptance.
One category of such techniques utilizes partial combustion of the
hydrocarbonaceous deposits, but these techniques have generally
suffered one or more of the following disadvantages: lack of
precise control of the combustion, environmental pollution
resulting from disposing of combustion products, and general
inefficiency resulting from undesired combustion of the
resource.
Another category of proposed in situ extraction techniques would
utilize electrical energy for the heating of the formations. For
example, in the U.S. Pat. No. 2,634,961 there is described a
technique wherein electrical heating elements are imbedded in pipes
and the pipes are then inserted in an array of boreholes in oil
shale. The pipes are heated to a relatively high temperature and
eventually the heat conduits through the oil shale to achieve a
pyrolysis thereof. Since oil shale is not a good conductor of heat,
this technique is problematic in that the pipes must be heated to a
considerably higher temperature than the temperature required for
pyrolysis in order to avoid inordinately long processing times.
However, overheating of some of the oil shale is inefficient in
that it wastes input electrical energy, and may undesirably
carbonize organic matter and decompose the rock matrix, thereby
limiting the yield. Further electrical in situ techniques have been
termed as "ohmic ground heating" or "electrothermic" processes
wherein the electric conductivity of the formations is relied upon
to carry an electric current as between electrodes placed in
separated boreholes. An example of this type of technique, as
applied to tar sands, is described in U.S. Pat. No. 3,848,671. A
problem with this technique is that the formations under
consideration are generally not sufficiently conductive to
facilitate the establishment of efficient uniform heating ourrents.
Variations of the electrothermic techniques are known as
"electrolinking", "electrocarbonization", and "electrogasification"
(see, for example, U.S. Pat. No. 2,795,279). In electrolinking or
electrocarbonization, electric heating is again achieved via the
inherent conductivity of the fuel bed. The electric current is
applied such that a thin narrow fracture path is formed between the
electrodes. Along this fracture path, pyrolyzed carbon forms a more
highly conducting link between the boreholes in which the
electrodes are implanted. Current is then passed through this link
to cause electrical heating of the surrounding formations. In the
electrogasification process, electrical heating through the
formations is performed simultaneously with a blast of air or
steam. Generally, the just described techniques are limited in that
only relatively narrow filament-like heating paths are formed
between the electrodes. Since the formations are usually not
particularly good conductors of heat, only non-uniform heating is
generally achieved. The process tends to be slow and requires
temperatures near the heating link which are substantially higher
than the desired pyrolyzing temperatures, with the attendant
inefficiencies previously described.
Another approach to in situ processing has been termed
"electrofracturing". In one variation of this technique, described
in U.S. Pat. No. 3,103,975, conduction through electrodes implanted
in the formations is again utilized, the heating being intended,
for example, to increase the size of fratures in a mineral bed. In
another version, disclosed in U.S. Pat. No. 3,696,866, electricity
is used to fracture a shale formation and a thin viscous molten
fluid core is formed in the fracture. This core is then forced to
flow out to the shale by injecting high pressured gas in one of the
well bores in which an electrode is implanted, thereby establishing
an open retorting channel.
In general, the above described techniques are limited by the
relatively low thermal and electrical conductivity of the bulk
formations of interest. While individual conductive paths through
the formations can be established, heat does not radiate at useful
rates from these paths, and efficient heating of the overall bulk
is difficult to achieve.
A further proposed electrical in situ approach would employ a set
of arrays of dipole antennas located in a plastic or other
dielectric casing in a formation, such as a tar sand formation. A
VHF or UHF power source would energize the antennas and cause
radiating fields to be emitted therefrom. However, at these
frequencies, and considering the electrical properties of the
formations, the field intensity drops rapidly as a function of
distance away from the antennas. Therefore, once again, non-uniform
heating would result in the need for inefficient overheating of
portions of the formations in order to obtain at least minimum
average heating of the bulk of the formations.
A still further proposed scheme would utilize in situ electrical
induction heating of formations. Again, the inherent (although
limited) conduction ability of the formations is relied upon. In
particular, secondary induction heating currents are induced in the
formations by forming an underground toroidal induction coil and
passing electrical current through the turns of the coil. The
underground toroid is formed by drilling vertical and horizontal
boreholes and conductors are threaded through the boreholes to form
the turns of the toroid. It has been noted, however, that as the
formations are heated and water vapors are removed from it, the
formations become more resistive, and greater currents are required
to provide the desired heating.
The above described techniques are limited by either or both of the
relatively low thermal and electrical conductivity of the bulk
formations of interest. Electrical techniques utilized for
injecting heat energy into the formations have suffered from
limitations given rise to by the relatively low electrical
conductivity of the bulk formations. In situ electrical techniques
appear well capable of injecting heat energy into the formations
along individual conductive paths or around individual electrodes,
but this leads to non-uniform heating of the bulk formations. The
relatively low thermal conductivity of the formations then comes
into play as a limiting factor in attaining a relatively uniformly
heated bulk volume. The inefficiencies resulting from non-uniform
heating have tended to render existing techniques slow and
inefficient.
It is an object of the present invention to provide in situ heat
processing of hydrocarbonaceous earth formations utilizing
electrical excitation means, in such a manner that substantially
uniform heating of a particular bulk volume of the formations is
efficiently achieved.
Further objects of the present invention are to provide a system
and method for efficiently heat processing relatively large blocks
of hydrocarbonaceous earth formations with a minimum of adverse
environmental impact and for yielding a high net energy ratio of
energy recovered to energy expended.
SUMMARY OF THE INVENTION
Applicants have devised a technique for uniform heating of
relatively large blocks of hydrocarbonaceous formations using radio
frequency (RF) electrical energy that is substantially confined to
the volume to be heated and effects dielectric heating of the
formations. An important aspect of applicants' invention relates to
the fact that certain hydrocarbonaceous earth formations, for
example raw unheated oil shale, exhibit dielectric absorption
characteristics in the radio frequency range. As will be described,
various practical constraints limit the range of frequencies which
are suitable for the RF processing of commercially useful blocks of
material in situ. The use of dielectric heating eliminates the
reliance on electrical conductivity properties of the formations
which characterize most prior art electrical in situ approaches.
Also, unlike the proposed schemes which attempt to radiate
electrical energy from antennas in uncontrolled fashion, applicants
provide field confining structures which maintain most of the input
energy in the volume intended to be heated. Conduction currents,
which are difficult to establish on a useful uniform basis, are
kept to a minimum, and displacement currents dominate and provide
the desired substantially uniform heating. Since it is not
necessary for the resultant heat to propagate over substantial
distances in the formations (as in the above described prior art
ohmic heating schemes) the relatively poor thermal conductivity of
the formations is not a particular disadvantage in applicants'
technique. Indeed, in already-processed formations from which the
useful products have been removed, the retained heat which is
essentially "stored", can be advantageously utilized. In an
embodiment of the invention, initial heating of adjacent blocks of
hydrocarbonaceous formations is implemented using this retained
heat.
In particular, the present invention is directed to a system and
method for in situ heat processing of hydrocarbonaceous earth
formations. In accordance with the system of the invention, a
plurality of conductive means are inserted in the formations and
bound a particular volume of the formations. As used herein, the
phrase "bounding a particular volume" is intended to mean that the
volume is enclosed on at least two sides thereof. As will become
understood, in the most practical implementations of the invention
the enclosed sides are enclosed in an electrical sense and the
conductors forming a particular side can be an array of spaced
conductors. Electrical excitation means are provided for
establishing alternating electric fields in the volume. The
frequency of the excitation means is selected as a function of the
dimensions of the bound volume so as to establish a substantially
non-radiating electric field which is substantially confined in
said volume. In this manner, volumetric dielectric heating of the
formations will occur to effect approximately uniform heating of
the volume.
In the preferred embodiment of the invention, the frequency of the
excitation is in the radio frequency range and has a frequency
between about 1 HMz and 40 MHz. In this embodiment, the conductive
means comprise opposing spaced rows of conductors disposed in
opposite spaced rows of boreholes in the formations. One
particularly advantageous structure in accordance with the
invention employs three spaced rows of conductors which form a
triplate-type of waveguide structure. The stated excitation may be
applied as a voltage, for example across different groups of the
conductive means or as a dipole source, or may be applied as a
current which excites at least one current loop in the volume. When
a triplate-type of structure is employed, the conductors of the
central row are preferably substantially shorter than the length of
the conductors of the outer rows so as to reduce radiation, and
resultant heat loss, at the ends of the conductors.
In accordance with a further feature of the invention, the
frequency of the excitation is selected as a function of the
electrical lossiness of the formations in the confined volume to be
sufficiently low that the (l/e) attenuation distance of the
electric field in any direction in the volume is more than twice
the physical dimension of the volume in that direction. In this
manner, the diminution of the electric field in any direction due
to transfer of energy to the formations (as in, of course,
desirable to effect the needed heating) is not so severe as to
cause undue nonuniformity of heating in the volume and wasteful
overheating of portions thereof. As will be described, a further
technique is employed for obtaining relatively uniform heating by
modifying the electric field pattern during the heating process so
as to effectively average the electric field intensity in the
volume to enhance the uniformity of heating of the volume.
The electrical heating techniques disclosed herein the applicable
to various types of hydrocarbon-containing formations, including
oil shale, tar sands, coal, heavy oil, partially depleted petroleum
reservoirs, etc. The relatively uniform heating which results from
the present techniques, even in formations having relatively low
electrical conductivity and relatively low thermal conductivity,
provides great flexibility in applying recovery techniques.
Accordingly, as will be described, the in situ electrical heating
of the present invention can be utilized either alone or in
conjunction with other in situ recovery techniques to maximize
efficiency for given applications.
Further features and advantages of the invention will become more
readily apparent from the following detailed description when taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an in situ twin lead transmission line in earth
formations.
FIG. 2 illustrates an in situ biplate transmission line in earth
formations.
FIG. 3 illustrates an in situ triplate transmission line in earth
formations.
FIG. 4A is a plan view of an in situ structure in accordance with
an embodiment of the invention.
FIG. 4B is an end view of the structure of FIG. 4A as taken through
a section defined by arrows 4b--4b of FIG. 4A.
FIG. 4C is a side view of the structure of FIG. 4A as taken through
a section defined by arrows 4c--4c of FIG. 4A.
FIG. 5 illustrates an alternate configuration of the structure of
FIG. 4B wherein the outer rows of conductors taper toward each
other.
FIG. 6 illustrates implementation of the invention in a situation
of a moderately deep resource bed.
FIG. 7 illustrates implementation of the invention in a situation
where a relatively thick resource bed is located relatively deep in
the earth's surface.
FIG. 8 is a graph of the electric field and heating patterns
resulting from a standing wave pattern in a triplate-type live
configuration.
FIG. 9 illustrates a smoothly varying exponential heating pattern
which results from modifying of the electric field pattern during
operation.
FIG. 10 is a graph of operating frequency versus skin depth for an
in situ oil shale heating system.
FIG. 11 is a graph of operating frequency versus processing time
for an in situ oil shale heating system.
FIG. 12A illustrates an embodiment of the invention wherein current
loop excitation is employed.
FIG. 12B is an enlargement of a portion of FIG. 12A.
FIG. 13 is a simplified schematic diagram of a system and facility
for recovery of shale oil and related products from an oil shale
bed.
FIG. 14 is a simplified schematic diagram of a system and facility
for recovery of useful constituents from a tar sand formation.
FIG. 15 is a simplified schematic diagram which illustrates how
residual heat in "spent" formations can be utilized for pre-heating
resources to be subsequently processed.
FIG. 16 illustrates an embodiment of the invention wherein electric
dipole excitation is employed.
FIG. 17 shows a diagram of a non-resonant processing technique.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Before describing the preferred implementations of practical forms
of the invention, the principles of the invention can be initially
understood with the air of the simplified diagrams of FIGS. 1, 2
and 3. FIG. 1 illustrates a twin-lead transmission line defined by
a pair of elongated conductors 101 and 102 which are inserted into
hydrocarbonaceous earth formations 10, for example an oil shale or
coal deposit. A source 110 of radio frequency excitation is coupled
to the twin-lead transmission line. The resultant electric field
causes heating, the heating being indicated in the FIGURES by the
dots. The intensity of the heating is represented by the density of
the dots. In FIG. 1, the field lines, which are in a general
standing wave pattern, extend well outside the region between the
transmission line leads and substantial radiation occurs from
various points with resultant loss of heating control. (The actual
field pattern will depend, inter alia, upon frequency, as will be
discussed below, and the illustrations of FIGS. 1, 2 and 3 are for
an appropriately chosen exemplary frequency.) In FIG. 2, there is
illustrated a biplate transmission line consisting of spaced
parallel conductive plates 201 and 202 in the formations. When
excited by a source 210 of RF energy, a standing wave field pattern
is again established. Radiation is particularly prevalent at the
edges and corners of the transmission line plates. Radiation
outside the transmission line confined region is less than in FIG.
1, but still substantial, as is evident from the heating pattern.
FIG. 3 illustrates a triplate transmission line which includes
spaced outer parallel plate conductors 301 and 302 and a central
parallel plate conductor 303 therebetween. Excitation by an RF
source 310, as between the central plate and the outer plate,
establishes a fairly well confined field. The central plate 303 is
made shorter than the outer plates 301 and 302, and this
contributes to minimizing of fringing effects. Standing waves would
also normally be present (as in FIGS. 1 and 2) but, as will be
described further hereinbelow, the periodic heating effects caused
by standing wave patterns can be averaged out, such as by varying
the effective length of the center plate 303 during different
stages of processing. The resultant substantially uniform average
heating is illustrated by the dot density in FIG. 3.
It is seen from the FIGS. 2 and 3 that alternating electric fields
substantially confined within a particular volume of
hydrocarbonaceous formations can effect dielectric heating of the
bulk material in the volume. The degree of heating at each
elemental volume unit in the bulk will be a function of the
dielectric lossiness of the material at the particular frequency
utilized as well as a function of the field strength. Thus, an
approximately uniform field in the confined volume will give rise
to approximately uniform heating within the volume, the heating not
being particularly dependent upon conduction currents which are
minimal (as compared to displacement currents) in the present
techniques.
As previously indicated, the illustrations of FIGS. 1, 2 and 3 are
intended for the purpose of aiding in an initial understanding of
the invention. The structures of FIGS. 2 and 3, while being within
the purview of the invention, are not presently considered as
preferred practical embodiments since plate conductors of large
size could not be readily inserted in the formations. As will
become understood, the confining structures of FIGS. 2 and 3 can be
approximated by rows of conductors which are inserted in boreholes
drilled in the formations.
One preferred form of applicants' invented system and method is
illustrated in conjunction with FIGS. 4A, 4B and 4C. FIG. 4A shows
a plan view of a surface of a hydrocarbonaceous deposit having
three rows of boreholes with elongated conductors therein. This
structure is seen to be analagous to the one in FIG. 3, except that
the solid parallel plate conductors are replaced by individual
elongated tubular conductors placed in boreholes that are drilled
in relatively closely spaced relationship to form outer rows
designated as row 1 and row 3, and a central row designated as row
2. The rows are spaced relatively far apart as compared to the
spacing of adjacent conductors of a row. FIG. 4B shows one
conductor for each row; viz., conductor 415 from row 1, conductor
425 from row 2, and conductor 435 from row 3. FIG. 4C illustrates
the conductors of the central row, row 2. In the embodiment shown,
the boreholes of the center row are drilled to a depth of L.sub.1
meters into the formations where L.sub.1 is the approximate depth
of the bottom boundary of the hydrocarbonaceous deposit. The
boreholes of the outer rows are drilled to a depth of L.sub.2,
which is greater than L.sub.1 and extends down into the barren rock
below the useful deposit. After inserting the conductors into the
boreholes, the conductors of row 2 are electrically connected
together and coupled to one terminal of an RF voltage source 450
(see FIG. 4B). The conductors of the outer rows are also connected
together and coupled to the other terminal of the RF voltage source
450. The zone heated by applied RF energy is approximately
illustrated by the cross-hatching of FIG. 4A. The conductors
provide an effective confining structure for the alternating
electric fields established by the RF excitation. As will become
understood, heating below L.sub.1 is minimized by selecting the
frequency of operation such that a cutoff condition substantially
prevents propagation of wave energy below L.sub.1.
The use of an array of elongated cylindrical conductors to form a
field confining structure is advantageous in that installation of
these units in boreholes is more economical than, for example,
installation of continuous plane sheets on the boundaries of the
volume to be heated in situ. Also, enhanced electric fields in the
vicinities of the borehole conductors, through which recovery of
the hydrocarbonous fluids ultimately occurs, is actually a benefit
(even though it represents a degree of heating non-uniformity in a
system where even heating is striven for) since the formations near
the borehole conductors will be heated first. This tends to create
initial permeability, porosity and minor fracturing which
facilitates orderly recovery of fluids as the overall bound volume
later rises in temperature. To achieve field confinement, the
spacing between adjacent conductors of a row should be less than
about a quarter wavelength apart and, preferably, less than about
an eighth of a wavelength apart.
Very large volumes of hydrocarbonaceous deposits can be heat
processed using the described technique, for example volumes of the
order of 10.sup.5 cubic meters of oil shale. Large blocks can, if
desired, be processed in sequence by extending the lengths of the
rows of boreholes and conductors. Alternative field confining
structures and modes of excitation are possible and will be
described further hereinbelow. At present, however, two
alternatives will be mentioned. First, further field confinement
can be provided by adding conductors in boreholes at the ends of
the rows (as illustrated by the dashed boreholes 490 of FIG. 4A) to
form a shielding structure. Secondly, consider the configuration of
FIG. 5 (analagous to the cross-sectional view of FIG. 4B) wherein
the conductors of the outer rows are tapered toward the central
rows at their deep ends so as to improve field uniformity (and
consequently, heating uniformity) further from the source.
FIGS. 1-5 it was assumed, for ease of illustration, that the
hydrocarbonaceous earth formations had a seam at or near the
surface of the earth, or that any overburden had been removed.
However, it will be understood that the invention is equally
applicable to situations where the resource bed is less accessible
and, for example, underground mining is required. In FIG. 6 there
is shown a situation wherein a moderately deep hydrocarbonaceous
bed, such as an oil shale layer of substantial thickness, is
located beneath barren rock formations. In such instance, a drift
or adit 640 can be mined and boreholes can be drilled from the
surface, as represented by the boreholes 601, 602 and 603 of FIG.
6, or from the drift. Again, each of these boreholes represents one
of a row of boreholes for a triplate-type configuration as is shown
in FIG. 4. After the boreholes have been drilled, tubular
conductors 611, 612 and 613 are respectively lowered into the lower
borehole portions in the resource bed. The coaxial lines 660
carrying the RF energy from a source 650 to the tubular conductors
can now be strung down an upper portion of one or more of the
boreholes and then connected across the different rows of tubular
conductors at drift 640. In this manner, there is no substantial
heating of the upper barren rock as might be the case if the
conductors were coupled from the surface of each borehole.
FIG. 7 illustrates a situation wherein a relatively thin
hydrocarbonaceous deposit is located well below the earth's
surface. In such case, a drift or adit 640 is first provided, and
horizontal boreholes are then drilled for the conductors. The FIG.
7 again illustrates a tri-plate type configuration of three rows of
boreholes, with the conductors 701, 702 and 703 being visible in
the FIGURE.
The selection of suitable operating frequencies in the present
invention depends upon various factors which will now be described.
As radio frequency (RF) electromagnetic wave energy propagates
within the hydrocarbon-bearing media of interest, electrical energy
is continuously converted to heat energy. The two primary energy
conversion mechanisms are ohmic heating, which results from the
conductivity of the formations, and dielectric heating, which
results from rotation of molecular dipoles by the alternating
electric field of the wave energy. At any elemental volume point,
x, within the formations of interest, the dielectric permittivity
at a frequency f can be expressed as
where .epsilon..sub.r '(x,f) is the relative real part of the
complex dielectric permittivity, .epsilon..sub.r "(x,f) is the
relative imaginary part of the dielectric permittivity and
represents both conductivity and dielectric losses and
.epsilon..sub.o is the permittivity of free space. The heating
power density, U(x,f) at point x can be expressed as
where E(x) is the electric field intensity at the point x. At radio
frequencies (0.3 MHz. to 300 MHz.) dielectric heating predominates
for the types of formations of interest herein, and the shale, tar
sand, and coal desposits to be treated can be considered as "lossy
dielectrics".
As the electromagnetic wave energy is converted to heat, the
electric field wave progressively decays in exponential fashion as
a function of distance along the path of the wave propagation. For
each electrical skin depth, .DELTA., that the wave traverses, there
is a reduction in the wave electric field by about 63%. The skin
depth, .DELTA., is related to the propagation medium's permittivity
and the electromagnetic wave frequency by the relationship
##EQU1##
The heating resulting from electromagnetic waves in
hydrocarbon-bearing formations diminishes progressively as the wave
energy penetrates further into the formations and away from the
source thereof. Thus, the use of RF energy does not, per se, yield
uniform heating of the formations of interest unless particular
constraints are applied in the selection of frequency and field
confining structure.
An idealized in situ heating technique would elevate all points
within the defined heating zone to the desired processing
temperature and leave volumes outside the heating zone at their
original temperature. This cannot be achieved in practice, but a
useful goal is to obtain substantially uniform final heating of the
zone, e.g. temperatures which are within a .+-.10% range
throughout. Since the heating power density, U(x,f), is a function
of the square of the electric field intensity, E, it is desirable
to have E within the range of about .+-.5% of a given level in most
of the processing zones. Consider, for example, the triplate line
structure of FIG. 4 as being imbedded in an oil shale formation. An
electromagnetic wave is excited by the RF power source 450 at the
surface of the oil shale seam and propagates down the triplate line
into the shale. The wave decays exponentially with distance from
the surface because of conversion of electrical energy into heat
energy. Upon reaching the end of the center conductor, at a depth
of L.sub.1 meters, it is desired that the wave undergo
substantially total reflection. This is achieved by selecting the
excitation frequency such that the half wavelength .lambda..sub.l
/2 along the tri-plate line is substantially greater than the
spacing between the outer rows, thereby giving rise to a cutoff
condition.
The result of the wave attenuation and reflection is the generation
of a standing wave along the length of the triplate line. At a
point, x, on the line, the magnitude of the total standing wave
electric field, E.sub.T --x, from the end of the center conductor
is ##EQU2## where .DELTA..sub.l is the electrical skin depth for a
wave traveling along the triplate line, and .lambda..sub.l is the
wavelength along the triplate line, (.DELTA..sub.l and
.lambda..sub.l being assumed constant along the length of the
line.)
To illustrate the nature of the standing wave pattern and heating
potential resulting from the triplate-type line of structure of
FIG. (4), equation (4) can be used to compute the ratios E.sub.T
(x)/E.sub.T (O) and U(x)/U(O)=[E.sub.T (x)/E.sub.T (O)].sup.2 for
the triplate line. Typical results are shown in the graph of FIG.
8. It is seen that E.sub.T and U decay with depth and exhibit an
oscillatory behavior near L.sub.1, with interleaved peaks and nulls
separated by a constant distance, .lambda..sub.l /4, from each
other. The position of the deepest peak coincides with the end of
the center conductor at L.sub.1 ; the position of the deepest null
is at L.sub.1 --.lambda..sub.l /4.
An in situ triplate-type of structure having a heating potential
distribution as shown in FIG. 8 will more easily meet heating
uniformity goals over its length if the oscillatory pattern could
be smoothed out. This can be done by modifying the electric field
pattern so as to effectively average the electric field intensity
in the volume being heated. This may be achieved by physically
decreasing the insertion depth of the center conductor by
.lambda..sub.l /4 units midway through the heating time. Pulling
each tube of the center conductor .lambda..sub.l /4 units out of
its respective borehole, or employing small explosive charges to
sever the deepest .lambda..sub.l /4 units of each tube are two ways
this can be done. Shifting the end of the center conductor in this
manner would shift the entire standing wave pattern toward the
surface of the oil shale seam by a distance of .lambda..sub.l /4
units. Thus, heating peaks would be moved to the positions of
former heating nulls, and vice versa. Averaged over the entire
heating time, the spatially oscillatory behavior of U would largely
disappear. This can be demonstrated mathematically using equations
(2) and (3): ##EQU3## where K is a constant set by the power level
of the source. Equation (5) represents a smoothly varying
exponentially decreasing distribution of U, as shown in FIG. 9. It
will be understood that electrical means could alternatively be
utilized to modify the electric field pattern so as to average the
electric field intensity in the volume being heated. Modification
of the phase or frequency of the excitation could also be
employed.
The described technique of effectively averaging the electric field
substantially eliminates peaking-type heating non-uniformities, but
it is seen that the exponential decay of the electric field still
poses difficulties in attaining substantially uniform heating. In
order to minimize the latter type of heating non-uniformity, the
frequency of operation is selected such that the (l/e) attenuation
distance .DELTA..sub.l is greater than the length L.sub.1 and,
preferably, greater than twice the length L.sub.1.
The value of .DELTA..sub.l which is allowable for a particular
heating uniformity criterion can be determined from equation (5) by
setting the heating potential at x=L.sub.1 --.lambda..sub.l /4 (the
final position of the end of the center conductor) to be a desired
percentage of the heating potential at x=0. For example, a heating
goal of .+-.10% in the volume would indicate that the desired
percentage is 80%, so we have: ##EQU4## assuming the
.epsilon."(L.sub.1 --.lambda..sub.l /4)=.epsilon."(0). For the
present situation, the following inequalities hold true:
Using these inequalities, equation (6) can be rewritten as:
##EQU5## or equivalently as:
which has the solution
Thus, the length of the center conductor row of the triplate-type
line should not exceed 35% of the line (l/e) attenuation distance
in order to insure heating uniformity within .+-.10% over the
length of the line. Stated another way, to meet this heating
uniformity requirement the frequency of excitation should be
sufficiently low to insure a skin depth of about three times
L.sub.1.
For an in situ triplate line type of structure (e.g. FIG. 4) with
no artificial loading by either lumped capacitances or inductances,
the expression for .DELTA. is given by (3) above, and combining (3)
and (10) gives: ##EQU6##
To determine the variation of L.sub.1.sbsb.max with frequency for
oil shale, laboratory tests were conducted to obtain the electrical
permittivity of dry, Mahogany-type, Colorado oil shale over the
frequency range of 1 MHz to 40 MHz. Using the data in conjunction
with equations (3) and (11) curves for .DELTA. and L.sub.1.sbsb.max
were plotted versus frequency, as shown in FIG. 10. It is seen, for
example, that to allow the use of a single triplate-type structure
to process in situ a complete top to bottom section of an oil shale
bed with a thickness of 100 meters, the maximum operating frequency
which meets the stated heating uniformity criterion would be 18
MHz. In a similar manner, FIG. 9 can be used to determine the
maximum operating frequency for triplate-type structures used to
heat process shale beds ranging in thickness from 10 meters
(f.sub.max =95 MHz) to 2500 meters (f.sub.max =1 MHz). It will be
understood that trade-offs as between line length and frequency can
be effected when, for example, it is desirable to select a
particular frequency to comply with government radio frequency
interference requirements.
Capacitive loading could also be employed to minimize amplitude
reduction effects. For example, series capacitors can be inserted
at regular intervals along the tubes of the center conductor of the
triplate line. These capacitors would act to partially cancel the
effective series inductance of the center conductor. Using the
expression for .DELTA..sub.l of an arbitrary lossy transmission
line, it can be shown that ##EQU7## for an in situ triplate-type
line, where .DELTA. is the nominal (l/e) attenuation distance at
the operating frequency, and r is the percentage reduction of the
center conductor inductance caused by the inserted capacitors. For
example, if the effective center conductor inductance were reduced
by 75%, .DELTA..sub.l would increase by 100% to a value of
2.DELTA..
Having set forth considerations which are used in determining
maximum operating frequency, attention is now turned to the
selection of suitable minimum operating frequency.
The rate of resource heating is controlled by U(x,f), the heating
power density generated by the electromagnetic field. As seen from
relationship (2), there are two types of factors influencing the
rate of heating: a frequency-independent amplitude factor, E.sup.2
(x); and a frequency-dependent factor, f.epsilon." (x,f). To
achieve rapid heating of the resource body, it would be desirable
to generate a large value of E. However, if E is increased beyond
some maximum value, designated E.sub.max, the RF electric field
could cause arc-over or breakdown of the rock matrix and
carbonized, conducting paths might form between the inner and outer
conductors of the in situ confining structure. This could lead to
undesirable short circuiting of the system. To avoid this
possibility, the average RF electric field within the structure is
constrained to be no more than (S)E.sub.max, where S is a
dimensionless safety factor in the range 0.01-0.1. In this way,
reliable operation is insured despite electric field enhancement at
the surfaces of the conducting tubes of the FIG. 4 structure and
possible local variations of the breakdown level of the resource. A
pilot or demonstration scale RF in situ facility could operate with
a typical S factor close to 0.1 so that simulated production runs
could be completed rapidly. However, a large scale, commercial
facility would likely be designated more conservatively, i.e., with
an S factor close to 0.01, to assure normal operation of an
associated high power RF generator under "worst case" conditions.
Using E.sub.avg. =SE.sub.max in relationship (2) yields
The RF heating power density varies as the square of S, so
selection of S has an important impact on the processing time and,
as will be seen, selection of minimum operating frequency. It is
seen from relationships (2) and (13) that increasing the product
term, f.epsilon..sub.r "(x,f), increases the electromagnetic
heating power density regardless of the electric field amplitude.
This product term is found to increase monotonically in the
frequency range of 1 MHz to 40 MHz for oil shale. Thus, for a given
RF electric field, increasing the operating frequency causes the
shale heating rate to increase. Considerations of maximum operating
frequency, set forth above, must be borne in mind, however.
The minimum processing time at a particular operating frequency,
t.sub.min (f), can be derived as a function of the fraction, R, of
spent shale sensible heat that can be recycled (this aspect to be
treated below), the RF electric field breakdown level, E.sub.max,
of the shale rock, the safety factor, S, and the loss component,
.epsilon..sub.r " (f), of the shale. First, the total RF heating
energy required to process one cubic meter of raw oil shale can be
calculated, assuming an oil shale density of 1.6 g/cm.sup.3
(1.6.multidot.10.sup.3 kg/m.sup.3) and assuming ##EQU8## Now,
t.sub.min (f) can be found by dividing the RF heating requirement
of Equation (14a) by the maximum RF heating power density of
Equation (13): ##EQU9##
FIG. 11 uses Equation (14b) to plot versus frequency the minimum
processing time (with S=0.01 and S=0.1) for RF heating of dry,
Mahogany-type Colorado oil shale. It is assumed that E.sub.max
=10.sup.6 V/m and is independent of the operating frequency, and
the R=0.5. From FIG. 11, it is seen that, for S=0.1, t.sub.min
ranges from 0.6 hours at 40 MHz to 36 hours at 1 MHz, and to an
extrapolated time of about 300 hours at 0.1 MHz. For S=0.01
t.sub.min ranges from 60 hours at 40 MHz to 3600 hours (5 months)
at 1 MHz.
During the processing cycle of a block of shale using the present
technique, heat conduction to adjacent shale regions can tend to
degrade the desired heating uniformity by causing cooling of the
boundary planes of the shale block being processed. Further, such
thermal conduction results in heat energy flowing outside the block
of interest, complicating the problem of controlling the extent and
efficiency of the heating process. Such an outflow of heat further
increases the necessary heating time. Actual determination of heat
flow effects is a complex function of the size and shape of the
shale blocks being heated; however, an illustration of such effects
on the graphs of FIG. 11 is depicted by the dotted lines curves for
a hypothetical block of shale.
In order to limit these undesired consequences of resource heat
conduction, it is desirable to complete the processing cycle of the
block being treated before appreciable heat energy can flow out of
the block. Based on these considerations, applicants have selected
a maximum electrical processing time of about two weeks, with
preferred processing times being less than this time. From FIG. 11,
this condition would mean that the operating frequency could be no
lower than 0.1 MHz for the S=0.1 case, and could be no lower than
10 MHz for the S=0.01 case. An intermediate value of S would
accordingly yield an intermediate "order of magnitude" frequency of
1 MHz. The frequency lower bound (based on considerations of heat
conduction away from the electrically heated zone and conservative
design relative to shale breakdown) can be combined with the
frequency upper bound obtainable from FIG. 10 (based on
considerations of heating uniformity within the zone and shale skin
depth) to define the preferred frequency range. For blocks of
commercially practical size, a maximum frequency of about 40 MHz is
preferred, so that preferred frequency range is about 1 MHz to 40
MHz. It should be noted that other confining structures within the
purview of the invention, such as waveguides and cavities, will
have somewhat different optimum operating frequency ranges because
of differences in the electromagnetic field patterns and heat
conduction times peculiar to a given geometry.
It will be understood that there are other possible techniques for
exciting the alternating electric field patterns to obtain
dielectric heating of the formations bound by the confining
conductor structures of the invention: i.e., alternative to the
previously described technique of applying voltages across
different groups of the conductors. In FIG. 12 there is again shown
a triplate-type of configuration having rows of conductors
designated as row 1, row 2 and row 3, the conductors again being
inserted in boreholes drilled into hydrocarbonaceous formations
such as an oil shale bed. In the embodiment of FIG. 12, the desired
field pattern in the confined volume of formations is established
using a current loop excitation.
The conductors of the central row have loop exciters 121 and 122
formed integrally therewith, the loop exciters 121 providing
magnetic field excitation to the left of the central row conductors
and the loop exciters 122 providing magnetic field excitation to
the right of the central row conductors. The established
alternating electric field pattern, concomitant with the varying
magnetic field, provides substantially uniform dielectric heating
in the manner previously described. The conductors of the central
row have an outer tubular metal shell 123 and an inner conductor
124, shown in dashed line in FIG. 4A. Slots 125 and 126 are formed
in the outer tube and the loops 121 and 122 extend from the inner
conductor, through the slots, and then reconnect with the outer
conductor as shown by the dashed line. The lower portion 120 of the
central row conductor extends from the bottom of the loop.
In operation, an RF current source 127 is coupled between the outer
tubular conductor 123 and the inner conductor 124 and drives
current through the loop 121 and 122, thereby establishing
alternating magnetic fields and concomitant electric fields which
are confined in the volume bound by the rows of conductors in row 1
and row 3. A quarter wave stub 128 is provided at about the top of
the hydrocarbonaceous deposit and, in effect, creates an open
circuit which isolates the conductor passing through the overburden
from the lower portion thereof. This technique prevents energy from
propagating back toward the source and heating the overburden.
Considerations of frequency are similar to those discussed above.
An advantage of the approach of FIG. 12 is that the voltage
carrying capability of the cables can be reduced since the
possibility of a voltage breakdown is diminished when using a
current drive scheme.
It will be understood that various alternate techniques for
excitation of the electric fields can be implemented to obtain
dielectric heating as defined herein. For example, electric dipole
excitation could be employed to generate the electric fields in the
confined volume, so long as the previously described frequency
limitations are met for establishing relatively uniform dielectric
heating. FIG. 16 illustrates an arrangement wherein electric dipole
excitation is used. Center conductor 166 is coupled to electrodes
166A and 166B which protrude from slots in outer conductor 163, and
a voltage source 167 is coupled between the inner and outer
conductors.
In the configuration of FIG. 12, wherein a current loop drive is
utilized, it is advantageous to use a source position which results
in an odd number of quarter wavelengths from the position of the
current loop to each end of the central conductor, since the source
is at a voltage minimum and it is desirable to have voltage maxima
at the open circuited terminations to achieve a resonance
condition. Similarly, in FIG. 16 the dipole source is preferably
located an even number of quarter wavelengths from the ends of the
central conductor.
Referring to FIG. 13, there is shown a simplified schematic diagram
of a system and facility for recovery of shale oil and related
products from an oil shale bed. A tri-plate-type configuration of
the nature previously described is used in this system. Three rows
of boreholes, designated as row 1, row 2 and row 3, are drilled
through the overburden and into the oil shale bed, the central row
of boreholes preferably being of a lesser depth than the outer
rows. A drift 131 is mined in the overburden above the oil shale
formation so that electrical connections can be made in the manner
described in conjunction with FIG. 6. Tubular conductors are
inserted into the lower portions of the boreholes of each row. An
RF source 132 is provided and obtains its power from a suitable
power plant which may or may not be located at the site. For ease
of illustration, the electrical connections are not shown in FIG.
13, but they may be the same as those of FIG. 6. A network of pipes
for injection of suitable media are provided, the horizontal feed
pipes 133, 134 and 135 being coupled to the boreholes of row 1, row
2 and row 3, respectively, and suitable valves and cross-couplings
also being provided. The art of injecting suitable media and
recovering subsurface fluids is well developed and not, taken
alone, the subject of this invention, so the description thereof is
limited to that necessary for an understanding of the present
system and techniques. Recovered fluids are coupled to a main
discharge pipe 136 and then to suitable processing plant equipment
which is also well known in the art. Again, these well known
techniques will not be described in full detail herein, but a
conduit 137 represents the process of separation of shale oil vapor
and high and low BTU gas, whereas the conduit 138 represents the
processing of shale oil vapor, in well known manner, to obtain
synthethic crude. The overall processing system of FIG. 13 will
vary somewhat in its structure and use, depending upon which of the
to-be-described versions of the present technique are utilized to
recover valuable constituents from the oil shale bed.
It will be recognized that the heating can be advantageously
performed to different degrees in order to implement useful
extraction of the organic resources from the formations. These
techniques will also vary with the type of resource form which the
fuel is being recovered. In the case of oil shale, three versions
of extraction techniques utilizing the invention are set forth,
although it will become clear that variations or combinations of
these techniques could be readily employed by those skilled in the
art. The first version aims only for recovery of shale oil and
by-product gases that correspond to the recovery aims of previously
proposed in situ oil shale processing techniques. Electrical radio
frequency energy is applied, for example using the system of FIG.
13, to heat a relatively large block of oil shale in situ to above
500.degree. C. As the temperature passes the point where inherent
shale moisture flashes into steam, some fracturing, at least along
bedding planes, will typically be experienced. Additional
interconnecting voids will also form within unfractured pieces of
oil shale during pyrolysis in the 400.degree.-500.degree. C. range.
While substantially uniform heating is striven for, heating is not
exactly uniform and the oil shale nearer the electrodes will be
heating slightly more rapidly than the shale further away. As a
result, permeability is progressively established outward from the
electrodes, permitting passage of shale oil vapors up the hollow
electrode tubes for collection. In the same way, the considerable
quantity of hydrocarbon gases liberated at shale temperatures
between about 200.degree. C. to 500.degree. C. will pass to the
surface via the tubes. At the surface of the earth, the shale oil
vapors and bi-product gases are collected and processed using known
techniques, as depicted broadly in FIG. 13. In this first version
there is not necessarily any attempt to utilize the carbonaceous
residue left in the spent shale formations.
Another in situ processing version which utilizes the electrical
radio frequency heating techniques of the invention would aim to
increase the yield of useful products from the oil shale resource
and to reduce process energy consumption by making full use of the
unique attributes of the disclosed in situ heating technique. Since
heating to relatively precise temperatures is possible with the
invented technique, this second version would apply heating to
about 425.degree. C. to recover cracked kerogen in liquid form. In
this manner, the substantial electric energy needed to apply the
additional heat to volatilize the shale oil product would be
saved.
In either version of the process, a relatively high degree of
porosity and permeability will be present after removal of the
liquid kerogen. Thus, if desirable, subsequent recovery of the
carbonaceous residue on the spent shale could be achieved by
injection of steam and either air or oxygen to initiate a
"water-gas" reaction. Upon injection, the steam and oxygen react
with the carbonaceous residue to form a low BTU gas which is
recovered and can be used, for example, for the hydrogenation of
the raw shale oil, or for on-site generation of electric power. The
water-gas reaction would also result in a higher spent shale
temperature, for example 600.degree. C., than in the case of the
first processing version. This would be advantageous when
techniques, such as those described below in conjunction with FIGS.
15, 16, are employed for using residual heat for preheating the raw
shale in other blocks in the shale bed. An overall saving of
electrical energy would thereby be achieved. The creation of shale
permeability and wetability after removal of the liquid kerogen
would also permit extraction, in situ, of various coproducts such
as aluminum hydroxide, nahcolite, uranium or related minerals
present in the shale by leaching methods.
In a third processing version, the electrical heating techniques of
the invention are employed only to relatively lower temperatures,
below about 200.degree. C. to obtain fast fracturing of the shale
by vaporization of moisture content, whereupon combustion or
thermal in situ extraction techniques can be used to obtain the
useful products.
It will be understood that various "hybrid" extraction approaches,
which include the electrical heating techniques of this invention,
can be employed, depending upon the type of oil shale formations in
a particular region, availability of electrical energy, and other
factors relating to costs. For example, the disclosed electrical
radio frequency heating techniques could be employed in either the
middle range temperatures or to "top off" temperature distributions
obtained by other heating methods.
Applicants have observed that raw unheated tar sand, heavy oil
matrices, and partially depleted petroleum deposits exhibit
dielectric absorption characteristics at radio frequencies which
render possible the use of the present techniques for heating of
such deposits (tar sands being generally referred to hereafter, for
convenience) so that bitumen can be recovered therefrom. Again, the
relatively low electrical conductivity and relatively low thermal
conductivity of the tar sands is not an impediment (as in prior art
techniques) since dielectric heating is employed. The selection of
a suitable range of frequencies in the radio frequency band is
based on considerations that are similar to those set forth above.
If the selected frequencies of operation are too high, the
penetration of energy into the deposit is too shallow (i.e., a
small skin depth, as discussed above) and relatively large volumes
of in situ material cannot be advantageously processed due to large
non-uniformities of heating. On the other hand, if the frequency of
operation is selected below a certain range, the absorption of
energy per unit volume will be relatively low (since dielectric
absorption is roughly proportional to frequency over the range of
interest), so the amplitude of the electrical excitation must be
made relatively large in order to obtain the necessary heating to
prevent processing times from becoming inordinately long. However,
practical considerations limit the degree to which the applied
excitation can be intensified without the risk of electrical
breakdown. Thus, once a maximum excitation amplitude is selected,
the minimum frequency is a function of desired processing time.
Applicants have discovered that the dielectric absorption
characteristics of tar sands are generally in a range similar to
that described above in conjunction with oil shale, but somewhat
lower frequencies within the radio frequency range are anticipated.
However, it will be understood that variations in the optimum
frequencies will occur for different types of mineral deposits,
different confining structures, and different heating time
objectives.
In FIG. 14 there is shown a simplified schematic diagram of a
system and facility for recovery and processing of bitumen from a
subterranean tar sand formation. A triplate-type configuration is
again utilized with three rows of boreholes, designated at row 1,
row 2 and row 3, being drilled or driven through the overburden and
into the tar sand formation, as in FIG. 13. A drift 141 is mined in
the overburden above the tar sand formation so that electrical
connections can be made in the manner described in conjunction with
FIG. 6. Again, tubular conductors are inserted into the lower
portions of the boreholes of each row. An RF source 142 is provided
and, as before, for ease of illustration, the electrical
connections are not shown in FIG. 14, although they may be the same
as those of FIG. 6. As in FIG. 13, a network of pipes for injection
of suitable drive media is provided, the horizontal feedpipes 143
and 145 being coupled to the boreholes of row 1 and row 3,
respectively, in this instance. Pipe 146 is the main collection
pipe and suitable valves and cross-couplings are also provided. In
the present instance, after suitable heating of the resource, steam
or hot chemical solutions can typically be injected into at least
some of the boreholes and the hot mobile tars are forced to the
surface for collection via collection pipes 144 and 146 and
collection tank 147. Subsequent processing of the recovered tars is
a well developed art and will not be described herein. In the
illustration of FIG. 14, the boreholes of rows 1 and 3 are utilized
as "injection wells" and the boreholes of rows 2 are used as
"production wells", although it will be understood that various
alternate techniques can be used for bringing the heated tars to
the surface.
As in the case of oil shale, it will be recognized that electrical
heating can be advantageously performed to different degrees in
order to implement useful extraction of the organic resources from
the tar sand formations.
In a first version of the tar sand or heavy oil recovery technique,
electrical heating is applied to reduce the viscosity of the
in-place tars or heavy oils to a point where other known
complementary processes can be employed to recover the in-place
fuels. In such case, radio frequency electrical energy can be
applied to relatively uniformly heat a block of tar sands to a
temperature of about 150.degree. C. This, in effect, produces a
volume of low viscosity fluids in the tar sand matrix which is
effectively sealed around its periphery by the lower temperature
(impermeable or less permeable) cooler tar sands. Simple gravity
flow into producer holes or a pressurized drive, consistent with
FIG. 14, can be used to force the low viscosity fluids to the
surface using injection of hot fluids.
In a second version of the technique, useful fuels are recovered
from tar sand and heavy oil deposits by partially or completely
pyrolyzing the tars in situ. Electrical radio frequency energy is
applied in accordance with the principles of the invention to heat
a relatively large block of tar sand in situ to about 500.degree.
C. As the temperature of the tar sand increases above about
100.degree. C., the inherent moisture begins to change into steam.
A further increase in temperature to around 150.degree. C.
substantially reduces the viscosity of in-place tars or heavy oils.
As the pyrolysis temperature is approached, the higher volatiles
are emitted until complete pyrolysis of the in-place fuels is
accomplished. The tar sands nearest the electrodes will be heated
slightly more rapidly than the tar sands farther away, so regions
of relatively low viscosity and high permeability will be
progressively established outward from the electrodes. This permits
passage of the high volatiles and pyrolytic product vapors up the
boreholes for collection with or without a drive. A variation of
this second version would subsequently employ a water gas process,
as described above, to produce a low BTU gas from the remaining
pyrolytic carbon. Also, simple combustion of carbon residues can be
utilized in order to recover residual energy in the form of
sensible heat. It will be understood that various combinations or
sequences of the described steps can be performed, as desired.
Referring to FIG. 15, there is shown a schematic diagram which
illustrates how residual heat in the "spent" formations from which
constituents have already been extracted can be utilized for
pre-heating of the next block of the resource to be processed.
After the boreholes are formed in the new zone to be heat
processed, a system of pipes can be utilized to carry steamwater
mixtures which effectively transfers residual heat from the
just-processed zone to the next zone to be processed. In FIG. 15,
the relatively cool raw resource bed to be processed is illustrated
by the block 151, and the spent hot resource is represented by the
block 152. The water pumped into the block 152 via pump 153 and
feed pipe 157 becomes very hot steam which is circulated through
the pipes 159 to the block 151. The system is "closed loop" so that
after heat from the steam is expended in the block 151, it is
returned as cooler steam or condensate to the block 152 via return
pipe 158. It will be understood that the sequentially processed
zones may be adjacent zones to take advantage of thermal flow
outside a volume being processed. In particular, heat which flows
outside the volume being processed, which might normally be wasted,
can be utilized in preheating zones to be subsequently processed.
Thus, for example, rows defining zones in the formations being
processed can alternate with and "sandwich" zones to bo
subsequently processed so that heat which flows out of the zones
presently being processed can be, to a substantial extent, utilized
later. This technique, along with the use of residual heat in the
"spent" formations, as described in conjunction with FIG. 15, can
substantially reduce the amount of total input energy needed for
heat processing.
The present invention allows maximum extraction of desired organic
products while keeping pollution and waste accumulation to a
minimum and still being economically advantageous. Very little
mining, if any, is required and the pollution and waste aspects of
above ground retorting are, of course, absent. The invented
technique compares most favorably with those in situ techniques
that require combustion, since those techniques necessarily produce
hot flue gases that must be cleaned of particulates, sulfur, etc.
before release into the invironment. A further advantage is a
result of the relatively close control over the heating zone which
is a feature of the present invention and greatly reduces the
possibility of uncontrolled in situ combustion which can have
adverse safety and/or environmental effects.
The invention has been described with reference to particular
embodiments, but variations within the spirit and scope of the
invention will occur to those skilled in the art. For example, the
term "boreholes" as used herein is intended generically to include
any type of hole or slot in the formation formed by any suitable
means such as mechanical or water-jet drilling, pile driving, etc.,
as well as forms of mining or excavation. Also, the field confining
conductors of the present invention can be of any desired form,
including meshes, straps, or flexible foils, and will depend, to
some degree, upon the location and exposure of the particular
surface of the volume they confine. Further, it will be understood
that in addition to the resonant TEM type of lines described
herein, the confining structure can also take the form of
single-mode TE or TM in situ waveguides or multi-mode enclosed
cavities. In both instances, standing-wave correction, as
previously described, can be employed to substantially average over
time the electric field (and resultant heating) throughout the
confined volume, both electrical and mechanical techniques being
available as disclosed hereinabove. The excitation frequency can
also be varied during operation. In the case of a cavity,
appropriate drifts or adits can be mined to obtain access to
drilling locations (e.g. as illustrated in FIG. 7) so that
conductors can be positioned to define surfaces that completely
confine a volume to be heated. The resultant "in situ cavity" would
be somewhat similar in operation to a microwave oven (but with
radio frequency energy being utilized). Mode mixing can be
achieved, for example, by utilizing a multiplicity of electric
and/or magnetic dipoles at different locations on the walls or
within the cavity and sequentially exciting them to obtain
different modes to achieve substantially uniform heating of the
confined volume. Alternatively, conductors can be inserted and
withdrawn from a series of boreholes, as previously described. The
cavity approach is advantageous due to the absence of geometrical
constraints pertaining to achieving cutoff of potentially radiating
wave energy. This means that larger blocks of the resource can be
processed at once.
Further, it will be understood that non-resonant confining
structures can be utilized, if desired. For example, FIG. 17 is a
simplified diagram illustrating how a nonresonant confining
structure can be utilized in conjunction with a "sandwich" type of
processing technique that utilizes thermal flow from spent regions.
Three "loops" designated as loop 170A, 170B, and 170C, are
illustrated, each loop including, for example, a pair of tri-plate
lines of the type illustrated in FIG. 4. However, in this instance
the central row of each tri-plate line is not intentionally
truncated. Instead, connecting lines designated by reference
numerals 171A, 171B and 171C are employed, this being done by
inserting appropriate horizontal conductors from a mined tunnel.
Switches 181-187 are provided and are intially positioned as shown
in FIG. 17. In operation, the loops are first connected in series
and the switch 181 is coupled to the RF source 179. Wave energy is
introduced into the first tri-plate line of loop 170A and travels
around the loop and is then connected via switch 183 to loop 170B,
and so on. Dielectric heating of the hydrocarbonaceous formations
is achieved, with the electric field being progressively
attenuated. Accordingly, the loop 170A is heated more than the loop
170B which is heated more than the loop 170C, etc. When the
hydrocarbonaceous deposit of loop 170A has been heated to a desired
degree, switches 181 and 183 are switched so that loop 170A is no
longer energized and loop 170B is now heated to the greatest
extent. This procedure is continued until the alternate layers of
hydrocarbonaceous formations are fully heated to the extent
desired. After a suitable period of time, typically weeks or
months, for the heat from the spent regions to transfer into the
between-loop formations, the between-loop formations can be
processed in similar manner.
As previously noted, the invention is applicable to various types
of hydrocarbonaceous deposits, and variations in technique,
consistent with the principles of the invention, will be employed
depending upon the type of resource being exploited. For example,
in the case of coal, the electrical properties of the material
indicates that the lower portion of the radio frequency spectrum,
for example of the order of 100 KHz, will be useful. Further, it
will be understood that as heat processing of a particular resource
progresses, the properties of the resource can change and may
render advantageous the modification of operating frequency for
different processing stages.
Applicants have observed that the raw materials under consideration
can tend to exhibit different dielectric properties at different
temperatures. As a consequence, it may be desirable to modify
electrical parameters to match the characteristics of the AC power
source to the characteristics of the field exciting structure whose
properties are influenced by the different dielectric properties of
the raw materials. A variable matching network, such as is
represented by block 451 (in dashed line) of FIG. 4A, can be used
towards this end.
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