U.S. patent number 3,661,423 [Application Number 05/010,871] was granted by the patent office on 1972-05-09 for in situ process for recovery of carbonaceous materials from subterranean deposits.
This patent grant is currently assigned to Occidental Petroleum Corporation. Invention is credited to Donald E. Garret.
United States Patent |
3,661,423 |
Garret |
May 9, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
IN SITU PROCESS FOR RECOVERY OF CARBONACEOUS MATERIALS FROM
SUBTERRANEAN DEPOSITS
Abstract
Subterranean carbonaceous deposits, such as oil shale, are
conditioned for in-situ recovery of carbonaceous values by limited
undercutting over a large area to leave an overlaying deposit
supported by a multiplicity of pillars, if necessary, then removing
the pillars and expanding the overlaying deposit to fill the entire
void with particles of a uniform size, porosity and permeability.
Communication is then established with the upper level of the
expanded deposit and a high temperature gaseous media which will
liquify or vaporize the carbonaceous values is introduced in a
manner which causes the released values to flow downward for
collection at the base of the expanded deposit. Convenient media
are hot flue gases created by igniting the upper level of the
expanded carbonaceous deposit and forcing a flow of hot gases
downward through the expanded deposit.
Inventors: |
Garret; Donald E. (Claremont,
CA) |
Assignee: |
Occidental Petroleum
Corporation (Los Angeles, CA)
|
Family
ID: |
21747812 |
Appl.
No.: |
05/010,871 |
Filed: |
February 12, 1970 |
Current U.S.
Class: |
299/2;
299/13 |
Current CPC
Class: |
E21B
43/247 (20130101) |
Current International
Class: |
E21B
43/16 (20060101); E21B 43/247 (20060101); E21b
043/26 () |
Field of
Search: |
;299/2,4,5,13 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Purser; Ernest R.
Claims
What is claimed is:
1. A process for the recovery of carbonaceous values from
subterranean deposits which comprises the steps of:
a. establishing communication with the base of the subterranean
carbonaceous deposit;
b. undercutting at least at the base of said carbonaceous deposit
to remove from about 5 to about 25 percent of the volume of said
deposit, to provide an overlaying deposit supported by a plurality
of pillars;
c. removing said pillars;
d. expanding said overlaying carbonaceous deposit to provide
particulate mass of uniform particle size, porosity and
permeability, said particulate mass having a void volume
approximately equal to the volume of the carbonaceous deposit
removed;
e. providing at least one communicating conduit to the upper level
of said expanded deposit;
f. heating said expanded deposit to liquify and vaporize said
carbonaceous values by introducing and maintaining a downward
positive pressure of a high temperature gaseous media
therethrough;
g. collecting liquified carbonaceous values at the base of said
deposit for recovery.
2. A process as claimed in claim 1 in which from about 5 to 15
percent of the volume of said deposit is removed.
3. A process as claimed in claim 1 in which the pillars are
explosively removed.
4. A process as claimed in claim 1 in which the overlaying
carbonaceous deposit is explosively expanded.
5. A process as claimed in claim 1 in which the carbonaceous
deposit is selected from the group consisting of oil shale, oil
tars, oil sands, tar sands, gilsonite, lignite, low rank coal, and
bituminous coal.
6. A process as claimed in claim 1 in which the undercutting is
below the base of the carbonaceous deposit to provide a thin
intermediate overlayer and an intermediate layer is perforated to
allow the drainage of free carbonaceous values from said deposit
and the deposit expanded to provide a particulate mass of uniform
size, porosity and permeability for introduction of a high
temperature gaseous media through a communicating conduit.
7. A process as claimed in claim 1 in which the deposit provides
gaseous, carbonaceous values which are collected by an upward flow
to the conduit communicating with the upper level of the expanded
deposit.
8. A process as claimed in claim 1 in which the high temperature
gaseous media is selected from the group consisting of hot gases
and steam.
9. A process as claimed in claim 8 in which the hot gases are
generated by steps of igniting the upper level of the expanded
deposit and maintaining a uniform zone of combustion across the
expanded deposit said combustion zone propagating downward through
said deposit by maintaining a positive pressure supply of at least
a source of oxygen.
10. A process as claimed in claim 9 in which the source of oxygen
is air.
11. A process as claimed in claim 9 in which the residual deposit
is subsequently leached with a leachant selected from the group
consisting of water, acids, and bases and the enriched solution
withdrawn at the base for recovery of mineral content.
12. A process as claimed in claim 9 in which the residual
carbonaceous materials resulting from the combustion of a portion
of the deposit are utilized for the production of carbonaceous
gaseous values.
13. A process as claimed in claim 1 in combination with the
additional steps of:
a. accumulating the removed carbonaceous deposit in an underground
mined-out chamber until full;
b. continuously introducing to the filled mined-out area a high
temperature gaseous media to liquify and vaporize the contained
carbonaceous values;
c. collecting the liquified carbonaceous values at the base of the
mined-out chamber for recovery.
14. A process as claimed in claim 13 in which the exhausted
mined-out chamber is then emptied.
15. A process for the production of oil shale which comprises steps
of:
a. establishing communication with the base of a subterranean shale
deposit;
b. undercutting at the base of the shale deposit to remove 5 to
about 25 percent of the volume of said deposit to provide an
overlaying deposit supported by a plurality of pillars;
c. removing said pillars;
d. expanding the overlaying shale deposit to a particulate mass of
uniform size, porosity and permeability, said particulate mass
having a void volume approximately equal to the amount of shale
deposit removed;
e. providing at least one connecting conduit through the upper
level of said expanded deposit;
f. igniting the upper level of said shale deposit and maintaining a
flow of a source of oxygen and deposited pressure to cause a flow
of flue gas downwardly through said expanded mass of particles to
release the oil contained therein for downward flow to the base of
said deposit;
g. collecting the shale oil at the base of said deposit for
recovery.
16. A process as claimed in claim 15 in which from about 5 to about
15 percent of the volume of the shale deposit is removed.
17. A process as claimed in claim 15 in which the deposit is
subsequently leached with a leachant selected from the group
consisting of water, acids and bases to form an enriched solution
which is withdrawn at the base for recovery of mineral content.
18. A process for the recovery of carbonaceous values from
subterranean deposits which comprises the steps of:
a. establishing communication with the base of the portion of a
subterranean carbonaceous deposit to be retorted;
b. undercutting at least at said base of said carbonaceous deposit
to remove from about 5 to about 25 percent of the volume of said
deposit, to provide an overlaying deposit supported by a plurality
of pillars;
c. removing said pillars;
d. explosively expanding at least a portion of said overlaying
carbonaceous deposit to a height predetermined by placement of
explosive charges to provide a mass of packed carbonaceous solids
filling the undercut volume and extending to the predetermined
height so as to provide a mass of carbonaceous solids of desired
permeability and having a void volume approximately equal to the
volume of the deposit removed;
e. providing at least one communicating conduit to the upper level
of said expanded deposit;
f. heating said expanded deposit to liquefy and vaporize said
carbonaceous values by introducing and maintaining a downward
positive pressure of a high temperature gaseous media
therethrough;
g. collecting liquefied carbonaceous values at said base of said
deposit for recovery;
h. accumulating the removed carbonaceous deposit in an earthen
depression as an expanded particulate mass;
i. covering said expanded deposit with a mantle;
j. continuously introducing to the mantled deposit a high
temperature gaseous media to liquefy and vaporize the contained
carbonaceous values; and
k. collecting the liquefied carbonaceous values at the base of the
depression for recovery.
19. A process for the recovery of carbonaceous values from
subterranean deposits which comprises the steps of:
a. establishing communication with the base of the portion of a
subterranean carbonaceous deposit to be retorted;
b. undercutting at least at said base of said carbonaceous deposit
to remove from about 5 to about 25 percent of the volume of said
deposit, to provide an overlaying deposit supported by a plurality
of pillars;
c. removing said pillars;
d. explosively expanding at least a portion of said overlaying
carbonaceous deposit to a height predetermined by placement of
explosive charges to provide a mass of packed carbonaceous solids
filling the undercut volume and extending to the predetermined
height so as to provide a mass of carbonaceous solids of desired
permeability and having a void volume approximately equal to the
volume of the deposit removed;
e. providing at least one communicating conduit to the upper level
of said expanded deposit;
f. heating said expanded deposit to liquefy and vaporize said
carbonaceous values by introducing and maintaining a downward
positive pressure of a high temperature gaseous media
therethrough;
g. collecting liquified carbonaceous values at the base of said
deposit for recovery.
20. A process as claimed in claim 19 in which from about 5 to 15
percent of the volume of said deposit is removed.
21. A process as claimed in claim 20 in which the pillars are
explosively removed.
22. A process as claimed in claim 19 in which the overlaying
carbonaceous deposit is explosively expanded.
23. A process as claimed in claim 19 in which the carbonaceous
deposit is selected from the group consisting of oil shale, oil
tars, oil sands, tar sands, gilsonite, lignite, low rank coal, and
bituminous coal.
24. A process as claimed in claim 19 in which the high temperature
gaseous media is selected from the group consisting of hot gases
and steam.
25. A process as claimed in claim 24 in which the hot gases are
generated by steps of igniting the upper level of the expanded
deposit and maintaining a uniform zone of combustion across the
expanded deposit said combustion zone propagating downward through
said deposit by maintaining a positive pressure supply of at least
a source of oxygen.
26. A process as claimed in claim 25 in which the source of oxygen
is air.
27. A process as claimed in claim 19 in which the undercutting is
below the base of the carbonaceous deposit to provide a thin
intermediate overlayer and an intermediate layer is perforated to
allow the drainage of free carbonaceous values from said expanded
deposit by introduction of a high temperature gaseous media through
a communicating conduit.
28. A process as claimed in claim 19 in which the deposit provides
gaseous media through a communicating conduit.
29. A process as claimed in claim 19 in which the deposit provides
gaseous, carbonaceous values which are collected by an upward flow
to the conduit communicating with the upper level of the expanded
deposit.
Description
BACKGROUND OF THE INVENTION
Immense reserves of subterranean carbonaceous deposits are known to
exist throughout the world. In some, the contained values are
commercially recoverable. In many, such as oil shale deposits,
recovery is not competitive with natural petroleum or gas sources.
Over the years, extensive development projects have been conducted
to devise economic methods of recovering values from such deposits.
One method applied to oil shale generally involves the mining of
the oil shale, transporting the shale to the surface, crushing and
grinding it to correct size and passing it through a retort to
volatilize the oil content, then discarding the remaining shale.
This procedure is expensive and has many inherent technical
problems and is economically unattractive.
In-situ retorting has been proposed using three general approaches.
Conventional fracturing techniques, such as high explosives, have
been proposed to establish communication between adjacent wells
drilled into a formation. Pressure drop is high and utilization of
the complete formation extremely difficult. Nuclear explosions have
also been proposed to create a cavity or chimney by vaporization of
part of the formation with attendant by breaking of the rock to
fill the space created. The resultant chimney serves as a reactor
through which reactant gases are circulated. This approach is
applicable only to deep formations, namely, formations having at
least 1,000 feet of overburden.
The third approach is that described in U.S. Pat. Nos. 3,001,776
and 3,434,757. A tunnel, or cavity, is formed under the oil shale
deposit. The resultant roof support is then removed and the
overlying shale allowed to cave. Following this initial caving,
explosives are detonated in the remaining overlying shale to cause
more caving, to yield a large volume of rubble in a loosely filled
cavern area. Neither of these two schemes provide mechanisms to
control particle size of the rubbled shale and as a consequence gas
flow through the rubble is uneven. In addition, U.S. Pat. No.
3,434,757 calls for retort gas flow in a horizontal direction which
makes even retort gas distribution virtually impossible. The
resultant economics of such schemes are poor and in-situ recovery
of oil from shale has not been commercially exploited.
SUMMARY OF THE INVENTION
It has been found that carbonaceous values can be economically
recovered from subterranean deposits by a controlled expansion of
the deposit over a broad area to form a mass of particulate solids
of essentially uniform particle size, porosity, and permeability;
establishing communication with the upper level of the expanded
deposit and introducing a suitable high temperature gaseous media
which will cause the expanded particles to release their
carbonaceous values by liquification or vaporization.
This is accomplished by undercutting the deposit to remove only
from about 5 to about 25 percent of the deposit and leaving an
overlaying deposit supported by a multiplicity of supporting
pillars, if necessary, for roof support. The pillars are then
removed, preferably by blasting, to yield an initial mass of
particles of uniform particle size. The overlaying deposit is then
expanded to fill the undercut area with a mass of particulate
carbonaceous bearing particles of uniform size, porosity and
permeability having a void volume approximately equal to the volume
of the deposit removed. Communication is then established with the
upper level of the expanded deposit and a suitable high temperature
gaseous media introduced which will cause the expanded deposit to
release the carbonaceous values as a liquid and/or vapor by a
downward flow of the gaseous media. The liquified released
carbonaceous values are then recovered from the base of the
expanded deposit.
In the case of the recovery of hydrocarbon values such as shale
oil, the gaseous media preferably used are hot flue gases which
pass downward through the expanded deposit. Flue gases can be
conveniently generated by ignition of the upper level of the
expanded deposit and maintaining a combustion zone which propagates
downwardly through the expanded shale by maintaining a supply of a
source of oxygen, usually air, to the combustion zone. As the
expanded deposit is uniform in character the hot flue gases flow
uniformly and result in a maximum conversion of the kerogen to
shale oil which collect at the base and is recovered by pumping to
the surface. The creation of particles of uniform size, porosity
and permeability further prevents the formation of voids and
channels which hinder total recovery and provides a uniform
pressure drop through the entire mass of particles.
DRAWINGS
FIG. 1 is a side view of an oil bearing shale formation.
FIG. 2 is an illustration of an undercutting of the oil bearing
shale formation.
FIG. 3 is a plan view of the undercutting showing numerous support
pillars.
FIG. 4 is an illustration of the expanded oil bearing shale
formation recovery operation.
FIG. 5 is a plan view of the expanded oil bearing shale formation
recovery operation.
FIG. 6 is a plan of a small retorting operation for progressively
mining a large deposit.
DESCRIPTION
According to the present invention a subterranean carbonaceous
deposit, such as an oil shale deposit, is conditioned for recovery
of the carbonaceous values by undercutting a limited height of the
deposit over a broad area, removing the undercut fraction to give
porosity to the overlying deposit when uniformly expanded. The
expanded fraction will have the desired low pressure drop
characteristics needed for an economical retorting operation. The
area undercut should be as large as the formation will allow to
reduce unit cost, which in favorable locations should be about an
acre in area. Formation thickness should be as great as possible,
also, to reduce unit cost, normally 100 to 200 feet. Pillars should
be left in a normal mining pattern although for smaller areas, no
pillars, or other roof supporting methods need be employed. The
pillars are removed and the overlaying deposit progressively,
preferably explosively, expanded to form a mass of particulate
solids of uniform size, porosity and permeability, having a void
volume essentially equal to the volume of the deposit removed.
Communication is then established with the ceiling of the expanded
carbonaceous deposit and a gaseous media which will cause the
deposit to vaporize the carbonaceous values is forced downwardly
through the deposit. The released vapors are condensed on the
cooler formation below. The liquid droplets agglomerate on the same
formation and most of the total released carbonaceous values are
pumped from the base of the formation as liquid. The gaseous media
is returned to the surface by its own initial pressure. Additional
hydrocarbon recovery from the gaseous media may be made using
conventional cooling and collecting equipment. A preferred media
are hot flue gases generated by igniting the upper level of the
mass of expanded particles and uniformly forced downward through
the mass of the particles by a progressive downward burning
maintained by the forced supply of a source of oxygen, typically
air. The hot flue gases retort the mass of particles and cause the
release of the carbonaceous values which collect at the floor for
pumping to the surface.
The nature of the high temperature gaseous media is not narrowly
critical. It must, however, be sufficiently hot to liquify and/or
vaporize the carbonaceous values. In addition to in-situ generated
flue gases, there may be employed an externally heated gas stream,
steam as well as flue gases recycled from the base of the expanded
deposit or supplied from an adjacent retorting operation. Where the
hot gases are generated by in-situ combustion, recycling the flue
gases provides a convenient means to modulate combustion
temperature by diluting the supplied air.
While the practice of this invention is applicable to the recovery
of any carbonaceous values from subterranean deposit, its practice
may be conveniently illustrated in terms of the recovery of oil
from shale.
Accordingly, and with reference first to FIG. 1, a typical oil
shale formation includes an overburden zone 10 typically in order
of 200 to 3,000 feet in depth and a mineable grade of oil shale 12
of varying thickness, although most often in the neighborhood of
about 50 to about 300 feet thick, and lower base rock 14. Although
an outcrop, as in the case of a canyon deposit, may allow sideward
access to the shale, more typically access to the oil shale is
established by the formation of one or more shafts 16 to base rock
14.
With reference now to FIG. 2, after communication with base rock 14
has been established, a tunnel 18 from each shaft to the deposit is
established and an undercut 20 excavated to the length of the area
to be mined and, with reference now to FIG. 3, broadened to the
width of the area to be mined leaving a plurality of small support
pillars 22. The area mined should be conveniently large, sometimes
in the order of an acre or more. Care should be taken to generally
provide that the floor of the mined-out area will be flat or
inclined in the direction of tunnel 18 to assure no impediment to
the flow of oil in the direction of shaft 16 during the subsequent
retorting operation. Only a limited height of the deposit is
excavated, generally only from about 5 to about 25 percent,
preferably from about 5 to about 15 percent, of the total deposit
volume.
The removed shale may be treated, where desired, for recovery of
its oil at the surface in conventional retorts, or in underground
retorts such as might be prepared in mined-out areas. It may also
be conveniently dumped as an expanded particulate mass, in a
convenient canyon or like depression, covered with a suitable thin
overburden layer and the values extracted in a manner similar to
that set forth herein for the original deposit. Alternately, the
removed shale can be deposited in a mined-out chamber underground.
After retorting the mined-out chamber may be emptied or simply
closed. By such means, although mining costs are incurred in the
initial creation of the mined-out area, the values recovered from
the removed shale will substantially offset the mining costs but
without adding the costs of grinding, crushing and disposing as is
the current common practice.
After the area has been suitably mined, there is preferably placed,
again with reference to FIGS. 2 and 3, explosive charges 24 in
columns 22 and charges 26 in the shale overlayer.
The charges are programmed to fire explosively to destroy columns
22 followed by progressive explosive expansion of the overlay by
employing charges 26, generally, in a progressively sequential
manner from the bottom up.
A barricade 28 is then placed at the entrance to the mined-out area
to seal the mined-out area from tunnel 18 and shaft 16. With
reference now to FIGS. 2, 3, 4 and 5, once the tunnel is sealed the
pillars are explosively broken to yield a uniform rubble. Charges
26 are progressively exploded to expand the overlaying shale to
fill the void created by the undercut to establish, thereby, a mass
of particles of uniform size, porosity and permeability, having a
total void volume approximately equal to the amount of shale
removed in the undercut.
Expansion is preferably accompanied by a slight doming of the
overburden 10 to create dome 30, which serves to improve the
support provided by the overburden.
As illustrated in FIGS. 4 and 5, the net deposit is a plurality of
uniform particles spread throughout the mined-out area and
overlaying deposit area. Following explosive expansion of the
shale, communication is established with the top of the expanded
oil shale deposit by drilling at least one and preferably a
plurality of communicating conduits 32 to the top of the expanded
shale.
Alternatively, and easily convenient, a tunnel (not shown) may be
established at the top of the expanded shale, for introduction of
the media used for recovery of carbonaceous values in the
deposit.
Following expansion of the shale, the exhaust barrier 28 is removed
and exhaust conduit 34, and oil flow conduit 36 are installed,
preferably by employing shaft 16 and tunnel 18. A source of oxygen,
typically air, from compressor 38 is then provided to conduits 32.
To establish a flow of oil, from the shale, the upper level of
expanded shale deposit is ignited using an initial supply of fuel
and air to the top of the shale deposit through conduits 32. The
source of oxygen, typically air, is supplied at a pressure
sufficient to overcome the inherent pressure drop through the
conduits and the shale deposit, to establish a positive downward
flow of hot gases. Once the upper level of the shale is ignited,
the supply fuel may be discontinued as the shale deposit will
inherently provide a sufficient source of fuel to generate the hot
flue gases for retorting, The flue gases generated by combustion
flow downwardly through the shale and serve to retort the oil from
the expanded mass of particles. As they are of a uniform size,
porosity and permeability, uniform flow of flue gases will be
obtained as channels which would normally preclude flue gases from
reaching certain areas of the shale prematurely are avoided. In
passing through the shale, the flue gases cause the shale to
release its oil as a carbonaceous value which drains to the floor
and is conveniently pumped out through conduit 36. Depending on
point in time, the cross-sectional appearance of the expanded shale
deposit will generally have a burned out zone 40, a burning zone
42, and a retort zone 46, where oil is released from the shale. As
previously mentioned, the released oil flows downwardly concurrent
with the flow of hot flue gases and collects at the floor 48 for
recovery through conduit 36, with flue gases exiting through
conduit 34.
Because deposit is uniformly expanded, to a desired, predetermined
porosity, the air pressure required will be generally low and as a
consequence only a thin overburden is required to contain the
pressure. This offers, the advantage, as previously indicated, in
that the shale removed during the tunneling and mining operations,
can be deposited in a canyon or other depression as an expanded
particulate mass covered with only a thin overburden and processed
for recovery of the oil in the manner set forth herein. It offers
the additional economic advantage of minimum compression costs.
Once recovery is complete, air flow through the formation may be
continued to both cool the deposit and to heat the air which can be
pumped as the retorting hot gas stream by compressors to adjacent
areas being mined in a similar manner. Any gases generated during
retorting may be discarded, used to pre-heat an adjacent area being
processed, or the values contained therein, such as methane and
hydrocarbons, recovered. Where hydrogen is generated it may be used
directly for hydrogenation or stabilization of recovered oil.
Although in-situ generated flue gases are a most convenient media
for use in extracting hydrocarbons, such as oil, it will be
appreciated that other generated hot gases, such as those generated
in adjacent mined out areas may be pumped through the shale, with
or without combustion, to force the deposit to release its
carbonaceous values.
In the alternative, and depending upon the carbonaceous material to
be recovered, there may also be used as the media oxygen, steam,
water and the like, which can be forced through the expanded
deposit under positive pressure to cause the release of
carbonaceous values contained therein.
Although the process of this invention has been generally described
in terms of recovery of oil from shale it will be readily
appreciated that it is fully adaptive to the recovery of values
from other subterranean carbonaceous deposits. For instance, many
petroleum and gas bearing reserves are known in which porosity is
too low to allow a reasonable flow of the contained values to a
producing well or the like, in that the source has been extracted
to its commercial limit or in the alternative low recovery has
resulted because of the high viscosity of contained values.
Examples of such deposits include among others oil sands such as
Athabasca tar sands, gilsonite, lignite, low rank coal, bituminous
coal and like carbonaceous materials which, at present, are
generally uneconomically recoverable.
The carbonaceous bearing deposit may be mined in the manner set
forth above if the nature of the deposit is not unduly hazardous to
excavation. If it is, the undercut may be conveniently made in the
base rock slightly below the formation and the base rock perforated
to connect with the deposit. Any free carbonaceous fluids or
gaseous materials may then be allowed to flow through the
perforations for recovery and the formation then expanded in the
manner described above by breaking the interlayer between the
undercut and expanding the overlaying deposit to allow in-situ
retorting. In the alternative, the same result may be accomplished
by undercutting directly into formation using casements to protect
workers during the excavation operations. Perforations could then
be used to allow withdrawal of any free fluid carbonaceous values
or gases followed by expansion in the manner set forth above, for
recovery of carbonaceous values.
In some instances, a deposit may be operating a gas producer by
employing limited air, or an alternating with steam or water as the
extracting media.
In other instances, the expanded deposit may be first treated
pyrolytically in the manner set forth above for the production of
hydrocarbon values and residual coke treated as a gas producer by
the introduction of a source of oxygen, such as air.
In some instances, where deposit has some porosity, the amount of
undercutting required would be minimal as the amount of expansion
necessary for suitable processing would not be as great.
In each instance, as in the case of oil shale, the mined out
material can be retorted at the surfaces or deposited in a canyon
or mined out area covered with a thin mantle, and treated in the
manner described above.
With some deposits, maximum recovery of contained values can also
be realized by finally leaching the processed deposit for any
remaining values using media such as percolating water, acids,
bases and the like, to form an enriched solution which collects at
the floor and which can be pumped to the surface for recovery of
extracted ore values.
With reference to FIG. 6, the following example will serve to
illustrate this invention by describing the formation and retorting
of a single, relatively small, oil shale zone which may be operated
in successive stages in mining a large shale deposit.
The periphery 50 shown is a desired boundary or retort wall left
intact when the oil shale inside the zone is undercut and
explosively broken. The four small pillars 22 comprising
approximately 15 percent of the cross-sectional area of the retort
represent pillars left standing at the bottom of the oil shale bed.
Drifts 52, at the same depth as the pillars, allow access for
mining and ventilation during the mining operation. The bottom 15
feet of a 100 foot thick shale oil formation is mined through
access drifts 52. Mined material is removed to a main shaft and
dumped in a below-ground cavity near the outcrop for additional
retorting. Mining is complete when the only remaining structure in
the resulting cavern consists of the pillars 22, as shown and when
the retort floor is graded sufficiently to allow drainage toward
drifts 52. A small entry retorting air tunnel, and distribution
tunnels (not shown) are also formed above the shale bed.
Drill holes are next placed in the shale in a detailed firing
pattern through the pillars to obtain the desired particle size
distribution and permeability upon blasting. Explosives are loaded
into these holes, and barricades are placed in drifts 52. Blasting
is initiated by first exploding the pillars and then, sequentially
from the bottom up, detonating the explosives within the shale bed.
While the time interval between the detonations may vary from one
shale bed to another, it is kept short enough to eliminate free
collapse of the roof. In this manner the particle size and
subsequent permeability are controlled by the specific detonations
and the formation of large unbroken blocks of oil shale is
minimized. The result of this caving technique is a retort
uniformly packed, not loosely filled, with relatively small oil
shale particles or rubble, and with a controlled porosity based
upon the amount of undercutting.
Following the explosive caving, both product recovery and air
feeding facilities are installed. This may include repairing the
seals at the drifts. For the small retort of this example, a single
tunnel is sufficient to handle the total gas flow into the retort,
using flue gas from an adjacent older retort to pre-heat the
formation.
Gas flow through the retort is initiated by forcing compressed air
with or without flue gas through the central air tunnel through the
retort and out through heat and product recovery systems. If
preheating is not sufficient, start-up fuel is injected into the
inlet air and ignited. The resultant flue gases heat the top of the
bed and initiate the retorting process. When the top shale reaches
300.degree. F. to 400.degree. F. it will sustain combustion without
the start-up fuel which is then discontinued. Retorting proceeds as
the heat front descends through the bed causing decomposition of
the kerogen to yield the shale oil which is carried down through
the bed with the moving gases. Residual carbon left on the shale is
burned with the incoming oxygen, providing the heat for continued
retorting. Retorting is completed when the bottom of the bed
reaches around 900.degree.F., usually with a total gas flow of less
than 20,000 SCF/ton of oil shale. Only the amount of air necessary
for the heat balance is used, usually less than 10,000 SCF/ton
dependent upon efficiency of heat recovery. Gas velocity during the
retort is 1 to 4 SCF/min./ft..sup.2 retort cross-sectional area.
Oil recovery from the total formation, excluding the loss in the
barrier walls has been found to be from 75 to 85 percent for oil
greater than 15 gal.ton.
While the Example is illustrative for oil shale operation, other
carbonaceous deposits such as the thick seams of subbituminous
coals can be similarly explosively caved to give controlled
particle size and permeability. Gasification or liquefaction of the
resulting rubble may be by various techniques such as reaction with
air and steam, oxygen and steam, and the like.
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