U.S. patent number RE36,012 [Application Number 08/710,997] was granted by the patent office on 1998-12-29 for accelerator-based methods and apparatus for measurement-while-drilling.
This patent grant is currently assigned to Schlumberger Technology Corporation. Invention is credited to Paul Albats, S. Zema Chowdhuri, Benoit Couet, Michael L. Evans, Jacques M. Holenka, William A. Loomis, Keith A. Moriarty, Bradley A. Roscoe, William R. Sloan, Kenneth E. Stephenson, Jerome A. Truax, Wolfgang P. Ziegler.
United States Patent |
RE36,012 |
Loomis , et al. |
December 29, 1998 |
Accelerator-based methods and apparatus for
measurement-while-drilling
Abstract
Measurement-while-drilling apparatus includes a 14 MeV neutron
accelerator, a near-spaced neutron detector which primarily senses
source neutrons and whose output is proportional to source
strength, one or more intermediately-spaced epithermal neutron
detectors eccentered against the drill collar wall and primarily
responsive to formation hydrogen concentration, and a third
far-spaced radiation detector, either gamma ray or neutron,
primarily responsive to formation density. The
intermediately-spaced and far-spaced detector outputs, normalized
by the near-spaced detector output, are combined to provide
measurements of porosity, density and lithology and to detect gas.
A thermal neutron detector and/or a gamma ray detector may also be
provided at intermediate spacings to provide additional information
of interest, such as standoff measurements and spectral analysis of
formation composition. Tool outputs are related to the angular or
azimuthal orientation of the measurement apparatus in the
borehole.
Inventors: |
Loomis; William A. (Ridgefield,
CT), Stephenson; Kenneth E. (Cambridge, GB2),
Truax; Jerome A. (Brookfield, CT), Ziegler; Wolfgang P.
(Ridgefield, CT), Chowdhuri; S. Zema (Bloomington, IN),
Couet; Benoit (Bethel, CT), Evans; Michael L. (Missouri
City, TX), Albats; Paul (Ridgefield, CT), Roscoe; Bradley
A. (Ridgefield, CT), Holenka; Jacques M. (Missouri City,
TX), Moriarty; Keith A. (Houston, TX), Sloan; William
R. (Missouri City, TX) |
Assignee: |
Schlumberger Technology
Corporation (Ridgefield, CT)
|
Family
ID: |
23191622 |
Appl.
No.: |
08/710,997 |
Filed: |
September 25, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
307894 |
Sep 16, 1994 |
05539225 |
Jul 23, 1996 |
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Current U.S.
Class: |
250/269.4;
250/269.5 |
Current CPC
Class: |
G01V
5/101 (20130101); G01V 5/104 (20130101); G01V
5/107 (20130101) |
Current International
Class: |
G01V
5/10 (20060101); G01V 5/00 (20060101); G01V
005/10 () |
Field of
Search: |
;250/269.4,269.1,269.5,269.6 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Ellis, Darwin V., "Well Logging for Earth Scientists", Elsevier
Science Publishing Co., Inc., pp. 204-212, 418,425. .
Richard Odom et al., "A New 1.625" Diameter Pulsed Neutron Capture
and Inelastic/Capture Spectral Combination System Provides Answers
in Complex Reservoirs, Paper O, Transactions of the SPWLA 35th
Annual Logging Symposium, Jun. 19-22, 1994. .
"Advances in Openhole Well Logging" by R. D. Felder, SPE, Exxon
Exploration Co., Aug. 1994 JPT, pp. 693-701..
|
Primary Examiner: Westin; Edward P.
Assistant Examiner: Hanig; Richard
Attorney, Agent or Firm: Jeffery; Brigitte L. Garrod; David
Smith; Keith
Claims
We claim:
1. Measurement-while-drilling apparatus for measuring properties of
earth formations surrounding a borehole being drilled by a drill
bit at the end of a drill string, comprising:
an elongated tubular drill collar in said drill string;
a high energy neutron accelerator in said drill collar;
a first neutron detector in said drill collar at a first spacing
from the accelerator in the lengthwise direction of the drill
collar, said first neutron detector having an output that is
primarily proportional to the accelerator neutron flux;
a second neutron detector in said drill collar at a second, farther
spacing from the accelerator in the lengthwise direction of the
drill collar, said second neutron detector being sensitive to
epithermal neutrons and having an output that is primarily
responsive to the hydrogen concentration of the surrounding earth
formation and only secondarily responsive to the density of the
surrounding earth formation;
a third radiation detector in said drill collar at a third, still
farther spacing from the accelerator in the lengthwise direction of
the drill collar, said third detector having an output that is more
responsive to the density of the surrounding earth formation and
less responsive to the hydrogen concentration of the surrounding
earth formation than is the second detector;
means for recording the respective outputs of said first, second
and third detectors as a function of borehole depth
and means for determining a parameter related to the formation
density from the respective outputs.
2. The apparatus of claim 1, wherein:
said second neutron detector is located closely adjacent the
interior wall of the drill collar; and
said second neutron detector is back-shielded against neutrons
incident thereon from the borehole.
3. The apparatus of claim 2, further comprising means defining a
neutron window in the drill collar immediately adjacent to said
second neutron detector.
4. The apparatus of claim 3, wherein the neutron-window defining
means comprises a body of relatively low-scattering cross section
material in the drill collar.
5. The apparatus of claim 4, wherein said body of relatively
low-scattering cross section material is composed of titanium.
6. The apparatus of claim 5, wherein said titanium body is sheathed
in boron.
7. The apparatus of claim 4, wherein:
the exterior surface of the drill collar is surrounded by a layer
of neutron absorbing material in the region of the second detector;
and
said layer of neutron-absorbing material has an opening formed
therein at the location of said body of relatively low-scattering
cross section material.
8. The apparatus of claim 7, wherein said neutron-window defining
means comprises a plurality of spaced-apart transverse layers of
neutron-absorbing material in the drill collar in the region of the
second detector.
9. The apparatus of claim 4, wherein said neutron-window defining
means further comprises a plurality of spaced-apart
lengthwise-extending layers of neutron absorbing material in the
drill collar in the region of the second detector.
10. The apparatus of claim 2, further comprising means for
processing the output of said second neutron detector to derive a
measurement of the epithermal neutron slowing down time of the
surrounding earth formation.
11. The apparatus of claim 10, wherein said processing means
further derives a standoff-corrected measurement of the porosity of
the surrounding earth formation.
12. The apparatus of claim 11, wherein said processing means
further derives a measurement of standoff.
13. The apparatus of claim 1, wherein said first neutron detector
comprises an epithermal neutron detector shielded on all sides
thereof except the side facing the neutron accelerator with neutron
moderating-absorbing material.
14. The apparatus of claim 1, wherein said first neutron detector
comprises an MeV range neutron detector shielded on all sides
thereof except the side facing the neutron acceleration with a
high-Z material.
15. The apparatus of claim 14, wherein said first neutron detector
is a .sup.4 He detector.
16. The apparatus of claim 1, wherein said third detector comprises
a gamma ray detector.
17. The apparatus of claim 1, wherein said third detector is an MeV
range neutron detector.
18. The apparatus of claim 17, wherein said third detector is a
.sup.4 He detector.
19. The apparatus of claim 16 or 17, further comprising an
intervening neutron shield located between said neutron detector
and said third radiation detector.
20. The apparatus of claim 1, further comprising a gamma ray
detector located at an intermediate spacing in the lengthwise
direction of the drill collar between said first and third
detectors.
21. The apparatus of claim 20, wherein said gamma ray detector is
located at substantially the same distance from the accelerator in
the lengthwise direction of the drill collar as in said second
detector.
22. The apparatus of claim 16 or 20, further comprising means for
spectrally analyzing the output of said gamma ray detector to
obtain information concerning the lithology of the surrounding
earth formation.
23. The apparatus of claim 1 wherein:
a drilling fluid channel is located within said drill collar to one
side of the longitudinal axis thereof; and
the accelerator and the first neutron detector are eccentered to
the other side of the drill collar longitudinal axis and are
substantially coaxially aligned with one another.
24. The apparatus of claim 23, wherein:
the second neutron detector is located closely adjacent the inner
wall of the drill collar; and
the third radiation detector is substantially coaxially aligned
with the accelerator and the first neutron detector.
25. The apparatus of claim 1, wherein the lengthwise spacing
between the second neutron detector and the accelerator is
substantially twice the low-energy epithermal neutron slowing down
length (L.sub.epi).
26. The apparatus of claim 1, further comprising at least one
thermal neutron detector located at an intermediate spacing in the
lengthwise direction of the drill collar between the first and
third detectors.
27. The apparatus of claim 26, further comprising means for
processing the output of said thermal neutron detector to derive a
measurement of at least one of standoff and the formation
macroscopic cross section for capture of thermal neutrons.
28. The apparatus of claim 1, further comprising a plurality of
said second epithermal neutron detectors located at substantially
the same lengthwise position in the drill collar and spaced apart
circumferentially of the drill collar to provide enhanced angular
or azimuthal resolution.
29. The apparatus of claim 1, wherein said second detector is
located within a recess formed in the wall of the drill collar and
is back-shielded against borehole neutrons by a neutron
moderating-absorbing material.
30. The apparatus of claim 1, further comprising means for
recording said detector outputs as a function of the angular
orientation of the drill collar within the borehole.
31. The apparatus of claim 1, further comprising means for
recording said detector outputs as a function of the azimuthal
orientation of the drill collar within the borehole.
32. The apparatus of claim 1, wherein:
said first neutron detector is shielded against formation-origin
neutrons by a high-Z material; and
said second and third detectors are shielded against source
neutrons transported along the drill collar by a neutron
moderating-absorbing material.
33. The apparatus of claim 1, further comprising means for
combining the outputs of said first, second and third detectors to
derive an indication of at least one of the porosity, density and
lithology of or the presence of gas in the surrounding earth
formation.
34. The apparatus of claim 33, wherein:
said third detector comprises a neutron detector;
said first and third detector outputs are combined to derive a
measurement of at least one of the high energy neutron slowing down
length (L.sub.h) and the low-energy neutron slowing down length
(L.sub.epi);
the lengthwise spacing between the second detector and the
accelerator is substantially twice the low-energy neutron slowing
down length (L.sub.epi);
said first and second detector outputs are combined to derive a
measurement of hydrogen index; and
said at least one L.sub.h measurement or L.sub.epi measurement and
said hydrogen index measurement are cross plotted to obtain
information of at least one of the porosity and lithology of the
surrounding earth formation.
35. The apparatus of claim 33, wherein:
said third detector comprises a neutron detector;
the lengthwise spacing between said second detector and the
accelerator is substantially twice the low-energy neutron slowing
down length (L.sub.epi);
the outputs of the second and third detectors normalized by the
output of the first detector; and
the normalized outputs of the second and third detectors are cross
plotted by said combining means to provide information of at least
one of porosity, lithology and the presence of gas in the
surrounding earth formation.
36. The apparatus of claim 33, wherein the output combining means
combines said first detector output with the outputs of said second
detector and said third detector by normalizing the outputs of said
second and third detectors with said first detector output.
37. The apparatus of claim 36, wherein the combining means combines
the normalized outputs of said second and third detectors by cross
plotting said normalized outputs.
38. The apparatus of claim 36, wherein said combining means, in
accordance with a first predetermined empirical relationship,
derives a value of the hydrogen index from the normalized second
detector output and, in accordance with a second predetermined
empirical relationship, derives a value of the neutron slowing down
length from the normalized third detector output and said value of
the hydrogen index.
39. A method for measuring properties of earth formations
surrounding a borehole being drilled by a drill bit at the end of a
drill string, comprising:
providing a high energy neutron accelerator in said drill
string;
providing a first neutron detector in said drill string at a first
spacing from the accelerator in the lengthwise direction of the
drill string, said first neutron detector having an output that is
primarily proportional to the accelerator neutron flux;
providing a second neutron detector in said drill string at a
second, farther spacing from the accelerator in the lengthwise
direction of the drill string, said second neutron detector being
sensitive to epithermal neutrons and having an output that is
primarily responsive to the hydrogen concentration of the
surrounding earth formation and only secondarily responsive to the
density of the surrounding earth formation;
providing a third radiation detector in said drill string at a
third, still farther spacing from the accelerator in the lengthwise
direction of the drill string, said third detector having an output
that is more responsive to the density of the surrounding earth
formation and less responsive to the hydrogen concentration of the
surrounding earth formation than is said second detector; and
combining the outputs of said first, second and third detectors to
derive an indication of at least density of the surrounding earth
formation.
40. The method of claim 39, wherein said combining step comprising
combining said first detector output with the outputs of said
second detector and said third detector by normalizing the outputs
of said second and third detectors with said first detector
output.
41. The method of claim 40, wherein said combining step further
comprises cross plotting the normalized outputs of said second and
third detectors.
42. The method of claim 41, wherein the normalized outputs cross
plotted are inverse normalized outputs.
43. The method of claim 40, wherein said combining step, in
accordance with a first predetermined empirical relationship,
derives a value of the hydrogen index from the normalized second
detector output and, in accordance with a second predetermined
empirical relationship, derives a value of the neutron slowing down
length from the normalized third detector output and said value of
the hydrogen index.
44. The method of claim 43, wherein said combining step further
comprises combining said hydrogen index value and said slowing down
length value to obtain information of at least one of porosity,
lithology, and the presence of gas in the surrounding earth
formation.
45. The method of claim 39, wherein:
said third detector comprises a neutron detector;
said first and third detector outputs are combined to derive a
measurement of at least one of the high energy neutron slowing down
length (L.sub.h) and the low-energy slowing down length
(L.sub.epi);
the lengthwise spacing between the second detector and the
accelerator is substantially twice the low-energy neutron slowing
down length (L.sub.epi);
the first and second detector outputs are combined to derive a
measurement of hydrogen index; and
said at least one L.sub.h measurement or L.sub.epi measurement and
said hydrogen index measurement are cross plotted to obtain
information of at least one of the porosity and lithology of the
surrounding earth formation.
46. The method of claim 39, wherein:
said third detector comprises a neutron detector;
the lengthwise spacing between said second detector and the
accelerator is substantially twice the low-energy neutron slowing
down length (L.sub.epi);
the outputs of the second and third detectors are normalized by the
output of the first detector; and
the normalized outputs of the second and third detectors are cross
plotted to provide information of at least one of porosity,
lithology and the presence of gas in the surrounding earth
formation.
47. The method of claim 39, wherein the combining step
comprises:
combining the normalized outputs of said second and third detectors
to derive values of the hydrogen index and the high-energy neutron
slowing down length (L.sub.h) or the low-energy neutron slowing
down length (L.sub.epi) for the surrounding earth formation;
combining said values of the hydrogen index and L.sub.h or
L.sub.epi, in accordance with a predetermined relationship relating
changes in the measured values of L.sub.h or L.sub.epi to changes
in bulk density for a calibration formation of known bulk density,
hydrogen index and elemental composition, to obtain information of
the bulk density of the surrounding earth formation.
48. The method of claim 39, wherein the combining step
comprises:
determining the hydrogen index and the neutron slowing down length
of the surrounding earth formation;
determining the difference between said slowing down length and the
neutron slowing down length of a calibration formation of
substantially the same hydrogen index and known bulk density;
and
combining said neutron slowing down time difference with the
density-slowing down length sensitivity ratio for the calibration
formation to obtain a measurement of the bulk density of the
surrounding earth formation.
49. Measurement-while-drilling apparatus for measuring properties
of earth formations surrounding a borehole being drilled by a drill
bit at the end of a drill string, comprising:
an elongated tubular drill collar in said drill string;
a neutron accelerator in said drill collar for irradiating the
surrounding earth formations with high energy neutrons;
at least one radiation detector in said drill collar spaced from
the accelerator in the lengthwise direction of the drill collar for
detecting radiation resulting from said neutron irradiation and
generating an output in response to said detected radiation, the
spacing being such that the radiation resulting from said neutron
irradiation is influenced by the density of the formations; and
means for recording the output of said at least one detector as a
function of at least one of borehole depth and azimuthal
orientation within the borehole and means for determining a
parameter related to the density of the earth formation.
50. A method for measuring the properties of earth formations
surrounding a borehole being drilled by a drill bit at the end of a
drill string, comprising:
providing a neutron accelerator in said drill string for
irradiating the earth formations with high energy neutrons;
providing at least one radiation detector in said drill string
spaced from the accelerator in the lengthwise direction of the
drill string for detecting radiation resulting from said neutron
irradiation of the earth formations and for generating an output in
response to said detected radiation, the spacing being such that
the radiation resulting from said neutron irradiation is influenced
by the density of the formations; and
recording the output of said at least one detector as a function of
at least one of borehole depth and azimuthal orientation in the
borehole and means for determining a parameter related to the
density of the earth formation. .Iadd.51. An apparatus for
measuring properties of earth formations surrounding a borehole,
comprising:
a) a housing;
b) a high energy neutron accelerator in the housing for irradiating
the formations from within the borehole;
c) a first neutron detector in said housing at a first spacing from
the accelerator in the lengthwise direction of said housing, said
first neutron detector having an output that is primarily
proportional to the accelerator neutron flux;
d) a second neutron detector in said housing at a second, farther
spacing from the accelerator in the lengthwise direction of said
housing, said second neutron detector being sensitive to epithermal
neutrons and having an output that is primarily responsive to the
hydrogen concentration of the surrounding earth formation and only
secondarily responsive to the density of the surrounding earth
formation;
e) a third radiation detector in said housing at a third, still
farther spacing from the accelerator in the lengthwise direction of
said housing, said third detector having an output that is more
responsive to the density of the surrounding earth formation and
less responsive to the hydrogen concentration of the surrounding
earth formation than is the second detector;
f) means for recording the respective outputs of said first, second
and third detectors; and
g) means for determining a parameter related to the formation
density from the respective outputs..Iaddend..Iadd.52. The
apparatus of claim 51, wherein:
said neutron detector is located closely adjacent the interior wall
of said housing; and
said second neutron detector is back-shielded against neutrons
incident thereon from the borehole..Iaddend..Iadd.53. The apparatus
of claim 52, further comprising means defining a neutron window in
said housing immediately adjacent to said second neutron
detector..Iaddend..Iadd.54. The apparatus of claim 53, wherein the
neutron-window defining means comprises a body of relatively
low-scattering cross section material in said
housing..Iaddend..Iadd.55. The apparatus of claim 54, wherein said
body of relatively low-scattering cross section material is
composed of titanium..Iaddend..Iadd.56. The apparatus of claim 55,
wherein said titanium body is sheathed in boron..Iaddend..Iadd.57.
The apparatus of claim 54, wherein:
the exterior surface of said housing is surrounded by a layer of
neutron absorbing material in the region of the second detector;
and
said layer of neutron-absorbing material has an opening formed
therein at the location of said body of relatively low-scattering
cross section material..Iaddend..Iadd.58. The apparatus of claim
57, wherein said neutron-window defining means comprises a
plurality of spaced-apart transverse layers of neutron-absorbing
material in said housing in the region of the second
detector..Iaddend..Iadd.59. The apparatus of claim 54, wherein said
neutron-window defining means further comprises a plurality of
spaced-apart lengthwise-extending layers of neutron absorbing
material in said housing in the region of the second
detector..Iaddend..Iadd.60. The apparatus of claim 52, further
comprising means for processing the output of said second neutron
detector to derive a measurement of the epithermal neutron slowing
down time of the surrounding earth formation..Iaddend..Iadd.61. The
apparatus of claim 60, wherein said processing means further
derives a standoff-corrected measurement of the porosity of the
surrounding earth formation..Iaddend..Iadd.62. The apparatus of
claim 61, wherein said processing means further derives a
measurement of
standoff..Iaddend..Iadd. 3. The apparatus of claim 51, wherein said
first neutron detector comprises an epithermal neutron detector
shielded on all sides thereof except the side facing the neutron
accelerator with neutron moderating-absorbing
material..Iaddend..Iadd.64. The apparatus of claim 51, wherein said
first neutron detector comprises an MeV range neuron detector
shielded on all sides thereof except the side facing the neutron
acceleration with a high-Z material..Iaddend..Iadd.65. The
apparatus of claim 64, wherein said first neutron detector is a
.sup.4 He detector..Iaddend..Iadd.66. The apparatus of claim 51,
wherein said third detector comprises a gamma ray
detector..Iaddend..Iadd.67. The apparatus of claim 51, wherein said
third detector is an MeV range neutron detector..Iaddend..Iadd.68.
The apparatus of claim 67, wherein said third detector is a .sup.4
He detector..Iaddend..Iadd.69. The apparatus of claim 66 or 67,
further comprising an intervening neutron shield located between
said neutron detector and said third radiation
detector..Iaddend..Iadd.70. The apparatus of claim 51, further
comprising a gamma ray detector located at an intermediate spacing
in the lengthwise direction of the housing between said first and
third detectors..Iaddend..Iadd.71. The apparatus of claim 70,
wherein said gamma ray detector is located at substantially the
same distance from the accelerator in the lengthwise direction of
the housing as is said second detector..Iaddend..Iadd.72. The
apparatus of claim 66 or 67, further comprising means for
spectrally analyzing the output of said gamma ray detector to
obtain information concerning the lithology of the surrounding
earth formation..Iaddend..Iadd.73. The apparatus of claim 51,
wherein the length wise spacing between the second neutron detector
and the accelerator is substantially twice the low-energy
epithermal neutron slowing down length
(L.sub.epi)..Iaddend..Iadd.74. The apparatus of claim 51, further
comprising at least one thermal neutron detector located at an
intermediate spacing in the lengthwise direction of the housing
between the first and third detectors..Iaddend..Iadd.75. The
apparatus of claim 74, further comprising means for processing the
output of said thermal neutron detector to derive a measurement of
at least one of standoff and the formation macroscopic cross
section for capture of thermal neutrons..Iaddend..Iadd.76. The
apparatus of claim 51, further comprising means for recording said
detector outputs as a function of the angular orientation of the
housing within the borehole..Iaddend..Iadd.77. The apparatus of
claim 51, further comprising means for recording said detector
outputs as a function of the azimuthal orientation of the housing
within the borehole..Iaddend..Iadd.78. The apparatus of claim 51,
wherein:
said first neutron detector is shielded against formation origin
neutrons by a high-Z material; and
said second and third detectors are shielded against source
neutrons transported along the housing by a neutron
moderating-absorbing
material..Iaddend..Iadd.79. The apparatus of claim 51, further
comprising means for combining the outputs of said first, second,
and third detectors to derive an indication of at least one of the
porosity, density and lithology of or the presence of gas in the
surrounding earth formation..Iaddend..Iadd.80. The apparatus of
claim 79, wherein:
said third detector comprises a neutron detector;
said first and third detector outputs are combined to derive a
measurement of at least one of the high energy neutron slowing down
length (L.sub.h) and the low-energy neutron slowing down length
(L.sub.epi);
the lengthwise spacing between the second detector and the
accelerator is substantially twice the low-energy neutron slowing
down length (L.sub.epi);
said first and second detector outputs are combined to derive a
measurement of hydrogen index; and
said at least one L.sub.h measurement or L.sub.epi measurement and
said hydrogen index measurement are cross plotted to obtain
information of at least one of the porosity and lithology of the
surrounding earth formation..Iaddend..Iadd.81. The apparatus of
claim 79, wherein:
said third detector comprises a neutron detector;
the lengthwise spacing between said second detector and the
accelerator is substantially twice the low-energy neutron slowing
down length (L.sub.epi);
the outputs of the second and third detectors normalized by the
output of the first detector; and
the normalized outputs of the second and third detectors are cross
plotted by said combining means to provide information of at least
one of porosity, lithology and the presence of gas in the
surrounding earth formation..Iaddend..Iadd.82. The apparatus of
claim 79, wherein the output combining means combines said first
detector output with the outputs of said second detector and said
third detector by normalizing the outputs of said second and third
detectors with said first detector output..Iaddend..Iadd.83. The
apparatus of claim 82, wherein the combining means combines the
normalized outputs of said second and third detectors by cross
plotting said normalized outputs..Iaddend..Iadd.84. The apparatus
of claim 83, wherein said combining means, in accordance with a
first predetermined empirical relationship, derives a value of the
hydrogen index form the normalized detector output and, in
accordance with a second predetermined empirical relationship,
derives a value of the neutron slowing down length from the
normalized third detector output and said valued of the hydrogen
index..Iaddend..Iadd.85. A method for measuring properties of earth
formations surrounding a borehole, comprising:
providing a high energy neutron accelerator for irradiating the
earth formations from within the borehole;
providing a first neutron detector at a first spacing from the
accelerator in the lengthwise direction of the borehole, said first
neutron detector having an output that is primarily proportional to
the accelerator neutron flux;
providing a second neutron detector at a second, farther spacing
from the accelerator in the lengthwise direction of the borehole,
said second neutron detector being sensitive to epithermal neutrons
and having an output that is primarily responsive to the hydrogen
concentration of the surrounding earth formation and only
secondarily responsive to the density of the surrounding earth
formation;
providing a third radiation detector at a third, still farther
spacing from the accelerator in the lengthwise direction of the
borehole, said third detector having an output that is more
responsive to the density of the surrounding earth formation and
less responsive to the hydrogen concentration of the surrounding
earth formation than is said second detector; and
combining the outputs of said first, second, and third detectors to
derive an indication of at least density of the surrounding earth
formation..Iaddend..Iadd.86. The method of claim 85, wherein said
combining step comprising combining said first detector output with
the outputs of said second detector and said third detector by
normalizing the outputs of said second and third detectors with
said first detector output..Iaddend..Iadd.87. The method of claim
86, wherein said combining step further comprises cross plotting
the normalized outputs of said second and third
detectors..Iaddend..Iadd.88. The method of claim 87, wherein the
normalized outputs cross plotted are inverse normalized
outputs..Iaddend..Iadd.89. The method of claim 40, wherein said
combining step, in accordance with a first predetermined empirical
relationship, derives a value of the hydrogen index from the
normalized second detector output and, in accordance with a second
predetermined empirical relationship, derives a value of the
neutron slowing down length from the normalized third detector
output and said value of the hydrogen index..Iaddend..Iadd.90. The
method of claim 89, wherein said combining step further comprises
combining said hydrogen index value and said slowing down length
value to obtain information of at least one of porosity, lithology,
and the presence of gas in the surrounding earth
formation..Iaddend..Iadd.91. The method of claim 85, wherein:
said third detector comprises a neutron detector;
said first and third detector outputs are combined to derive a
measurement of at least one of the high energy neutron slowing down
length (L.sub.h) and the low-energy slowing down length
(L.sub.epi);
the lengthwise spacing between the second detector and the
accelerator is substantially twice the low-energy neutron slowing
down length (L.sub.epi);
the first and second detector outputs are combined to derive a
measurement of hydrogen index; and
said at least one L.sub.h measurement or L.sub.epi measurement and
said hydrogen index measurement are cross plotted to obtain
information of at least one of the porosity and lithology of the
surrounding earth formation..Iaddend..Iadd.92. The method of claim
85, wherein:
said third detector comprises a neutron detector;
the lengthwise spacing between said second detector and the
accelerator is substantially twice the low-energy neutron slowing
down length (L.sub.epi);
the outputs of the second and third detectors are normalized by the
output of the first detector; and
the normalized outputs of the second and third detectors are cross
plotted to provide information of at least one of porosity,
lithology and the presence of gas in the surrounding earth
formation..Iaddend..Iadd.93. The method of claim 85, wherein the
combining step comprises:
combining the normalized outputs of said second and third detectors
to derive values of the hydrogen index and the high-energy neutron
slowing down length (L.sub.h) or the low-energy neutron slowing
down length (L.sub.epi) for the surrounding earth formation;
and
combining said values of the hydrogen index and (L.sub.h) or
(L.sub.epi) in accordance with a predetermined relationship
relating changes in the measured valued of (L.sub.h) or (L.sub.epi)
to changes in bulk density for a calibration formation of known
bulk density, hydrogen index and elemental composition, to obtain
information of the bulk density of the
surrounding earth formation..Iaddend..Iadd.94. The method of claim
85, wherein the combining step comprises:
determining the hydrogen index and the neutron slowing down length
of the surrounding earth formation;
determining the difference between said slowing down length and the
neutron slowing down length and the neutron slowing down length of
a calibration formation of substantially the same hydrogen index
and known bulk density; and
combining said neutron slowing down time difference with the
density-slowing down length sensitivity ration for the calibration
formation to obtain a measurement of the bulk density of the
surrounding earth formation..Iaddend..Iadd.95. An apparatus for
measuring properties of earth formations surrounding a borehole,
comprising:
a housing;
a neutron accelerator in said housing for irradiating the
surrounding earth formations with high energy neutrons;
at least one radiation detector in said housing spaced from the
accelerator in the lengthwise direction of the housing for
detecting radiation resulting from said neutron irradiation and
generating an output in response to said detected radiation, the
spacing being such that the radiation resulting from said neutron
irradiation is influenced by the density of the formations; and
means for recording the output of said at least one detector as a
function of at least one of borehole depth and azimuthal
orientation within the borehole and means for determining a
parameter related to the density of the earth
formation..Iaddend..Iadd.96. A method for measuring the properties
of earth formations surrounding a borehole, comprising:
providing a neutron accelerator for irradiating the earth
formations with high energy neutrons from within the borehole;
providing at least one radiation detector spaced from the
accelerator in the lengthwise direction of the borehole for
detecting radiation resulting from said neutron irradiation of the
earth formations and for generating an output response to said
detected radiation, the spacing being such that the radiation
resulting from said neutron irradiation is influenced by the
density of the formations; and
recording the output of said at least one detector as a function of
at least one of borehole depth and azimuthal orientation in the
borehole and determining a parameter related to the density of the
earth formation..Iaddend.
Description
DESCRIPTION
1. Field of the Invention
The present invention relates generally to the investigation of
subsurface earth formations contemporaneously with the drilling of
a borehole therethrough and, more specifically, to methods and
apparatus for making neutron-accelerator based measurements while
drilling. In their broadest aspects, certain of the techniques
disclosed also relate to wireline logging as well.
2. Background of the Invention
The measurement of the porosities of subsurface earth formations
surrounding a well borehole by means of the attenuation of neutron
flux with distance from the neutron source is well known in
wireline logging. Epithermal logging tools, in particular, are
sensitive to the hydrogen density or concentration in a formation.
As hydrogen is generally found in formation fluids, hydrogen
concentration is related to the amount of pore space, and thus the
porosity, of the formation. For a given porosity, however, an
increase in matrix density (keeping the same matrix chemical
composition) can cause an epithermal neutron detector count rate
(for a source-to-detector spacing of 60 cm for example) to
decrease. This change in count rate is in the same direction as
would occur if the porosity increased for a given matrix density.
Thus a neutron porosity measurement by itself cannot unambiguously
determine the porosity of a formation of unknown composition.
It is conventional in wireline logging, therefore, to make bulk
density measurements of a formation of interest by running a second
tool, based on Compton scattering of gamma rays from electrons,
over the same depth interval as the neutron porosity tool. An
increase in matrix density also causes a decrease in the detector
count rate in the density tool. On the other hand, if the porosity
increases for a given matrix density, the density tool detector
count rate increases. Changes in matrix density and porosity thus
have complimentary effects on neutron porosity and
Compton-scattering density tools, which effects can be offset by
cross plotting the responses of the two tools. By use of such cross
plots, the physics can be untangled and changes in matrix density
and composition (lithology) can be determined. Because the
inclusion of gas in the matrix pore spaces also affects the neutron
porosity and density tool responses, it is possible in certain
circumstances to detect the presence of gas by means of
neutron/density cross plots.
Although such wireline porosity and density logging tools afford
much useful information concerning subsurface formations, they are
necessarily employed only after the borehole has been drilled and
the drill string has been removed, which may be hours or even days
after the borehole has been formed. As a result, the formations and
the borehole may have undergone changes that mask or obscure
important petrophysical properties under investigation. For
example, both the invasion of drilling fluid into the formation and
the build-up of mudcake on the borehole wall can adversely affect
many logging measurements, including both the gamma ray bulk
density measurement and the neutron porosity measurement. Both
measurements are also affected by mudcake density, as well as by
any sloughing or caving of the borehole wall that might have
occurred. Further disadvantages of wireline tools include the loss
of drilling time and the expense and delay of tripping the drill
string so as to enable the wireline tool to be lowered into the
borehole. It would be quite advantageous, therefore, if the density
and neutron porosity measurements, as well as other measurements of
interest, could be made during the drilling operation itself.
Efforts have been made in the prior art to provide nuclear (gamma
ray density or neutron porosity) formation evaluation while
drilling; see, for example, U.S. Pat. No. 4,596,926, U.S. Pat. No.
4,698,501, U.S. Pat. No. 4,705,944, U.S. Pat. No. 4,879,463 and
U.S. Pat. No. 4,814,609. The conventional bulk density measurement
technique, however, requires a source of gamma rays, typically a
.sup.137 Cs isotopic source. The conventional neutron porosity
measurement technique likewise employs an isotopic chemical source,
such as AmBe. Such radioactive chemical sources have obvious
disadvantages from a radiation safety viewpoint. This is of
particular concern in measurement-while-drilling applications,
where operating conditions make both the loss of a source more
likely and its retrieval more difficult than in wireline
operations. Indeed, the aforementioned measurement-while-drilling
prior art patents have focused in substantial part on preventing
the loss or, if lost, the recovery of such chemical sources.
Although accelerator-based wireline porosity tools have recently
been developed, see, for example, U.S. Pat. No. 4,760,252 to Albats
et al., such tools are not directly transposable to
measurement-while-drilling applications because of the perturbing
effects on the tool responses of the large amounts of steel and
drilling fluid present in those applications. Moreover, there
currently is no practical and economical accelerator-based
alternative to the .sup.137 Cs gamma ray source for density
logging. A need exists, therefore, for an accelerator-based
measurement-while-drilling tool which would eliminate the
requirement for the radioactive chemical sources of conventional
neutron porosity and bulk density tools.
SUMMARY OF THE INVENTION
The foregoing and other requirements of the prior art are met, in
accordance with the invention, by the provision of
measurement-while-drilling apparatus and methods which include a
high-energy (preferably 14 MeV) neutron accelerator in a drill
collar section of a drill string and at least one radiation
(neutron or gamma ray) detector spaced from the accelerator for
measuring the radiation resulting from the neutron irradiation of
the surrounding earth formations. In a preferred embodiment, a
near-spaced neutron detector for monitoring neutron source flux, an
intermediately-spaced epithermal neutron detector that is primarily
responsive to formation hydrogen concentration and a far-spaced
detector that is more responsive to formation density than is the
epithermal neutron detector are provided in the drill collar. The
near-detector output is used to normalize the other detector
outputs for source strength fluctuation. The normalized
intermediately-spaced epithermal neutron detector output and the
normalized far-spaced detector output are combined, in a manner
conceptually similar to the conventional neutron porosity-density
cross plot, to obtain measurements of formation porosity, bulk
density and lithology and/or to detect gas. The measurements are
made and recorded as a function of borehole depth and angular or
azimuthal orientation in the borehole.
The near-spaced detector is preferably an epithermal neutron
detector shielded by a neutron moderating-absorbing material to be
substantially insensitive to formation-origin neutrons.
Alternatively, it may comprise an MeV neutron detector, such as a
.sup.4 He detector or a liquid scintillator neutron detector,
shielded by a high-Z material. The intermediately-spaced epithermal
neutron detector may be one of a number of similarly spaced
detectors forming a detector array. The array may include a
plurality of like epithermal detectors spaced circumferentially
about the inner wall of the drill collar to provide enhanced
horizontal resolution. One or more gamma ray detectors and/or
thermal neutron detectors may also be included in the array. If
desired, the array detectors may be vertically spaced for improved
vertical resolution. The far-spaced detector is preferably a gamma
ray detector, but also may comprise a high-energy (>0.5 MeV)
neutron detector, e.g., a .sup.4 He or liquid scintillator
detector. Alternatively, both a far-spaced gamma ray and a
far-spaced neutron detector may be provided. Where a liquid
scintillator is used, it could be configured to detect both
neutrons and gamma rays.
The neutron accelerator and the near-spaced detector are preferably
coaxially aligned and eccentered to one side of the drill collar to
accommodate the drilling fluid channel on the other side of the
drill collar. To enhance sensitivity to the formation, the array
detectors are preferably eccentered against the inner wall of the
drill collar and back-shielded against borehole and drill
collar-transported neutrons. The far-spaced detector(s) is
preferably coaxial with the accelerator and the near-spaced
detector. It, too, is shielded against neutrons streaming along the
borehole and the drill collar.
A neutron transparent window is preferably provided opposite each
neutron detector in the array to further enhance formation
sensitivity and to increase the depth of investigation. A preferred
construction of the neutron windows includes a low-scattering cross
section material, such as titanium, sheathed in boron or other
neutron-absorbing material to minimize neutron leakage into the
drill collar. An external neutron-absorbing layer, formed with
openings at the locations of the neutron windows, may also be
provided to further reduce neutron flow into the drill collar. As
an alternative neutron window construction, transverse and/or
longitudinal layers of neutron-absorbing material may be provided
in the drill collar to attenuate longitudinal and/or
circumferential neutron flow therein.
In addition to the aforementioned cross plot technique, the
intermediately-spaced detector outputs and the far-spaced detector
output may also be separately processed, if desired, to obtain
other information of interest. For example, measurements of
porosity and standoff may be derived from the slowing down time
curve generated by the array epithermal neutron detector(s) and
information as to the chemical composition of the formation may be
obtained from a spectral analysis of gamma ray energy spectra
recorded at the array gamma ray detector. Such a spectral analysis
may alternatively be based on the output of the far-spaced detector
where that detector detects gamma rays. The thermal neutron
detector output is useful in determining the formation macroscopic
capture cross section and in measuring standoff. The thermal
neutron macroscopic capture cross section, or its correlative the
thermal neutron decay time constant, may also be determined from
the gamma ray detector output. These additional measurements are
useful alone or in interpreting the basic cross plot
presentation.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects, features and advantages of the invention may be
further understood from the following description of representative
embodiments thereof, taken in conjunction with the accompanying
drawings, in which:
FIG. 1 is a schematic diagram, partly in block form, of one
embodiment of a measurement-while-drilling apparatus constructed in
accordance with the invention and including a drill string
suspended from a rotary drilling platform;
FIG. 2 is a vertical cross-sectional view, partly in schematic
form, of one embodiment of the downhole measurement subassembly
including the neutron accelerator and associated radiation
detectors;
FIG. 3 is a horizontal cross-sectional view taken along the line
3--3 in FIG. 2, showing the preferred location of the near-spaced
detector relative to the drill collar;
FIG. 4 is a horizontal cross-sectional view taken along the line
4--4 in FIG. 2, showing one configuration of the array detectors
and the associated neutron windows relative to the drill
collar;
FIG. 5 is a partial horizontal cross-sectional view, showing
another configuration of an array epithermal neutron detector and
its associated neutron window;
FIG. 6 is a partial vertical cross-sectional view of another
embodiment of the downhole measurement subassembly, showing an
alternative embodiment of a neutron window;
FIG. 7 is an external view taken along the line 7--7 in FIG. 6,
showing the external configuration of the neutron window of FIG.
6;
FIG. 8 is an external view similar to FIG. 7, showing the external
configuration of another embodiment of a neutron window;
FIG. 9 is a cross plot of the inverse normalized flux for a
near-spaced epithermal neutron detector vs. the normalized inverse
flux for a far-spaced gamma ray or neutron detector, as determined
from Monte Carlo modelling of the accelerator-based tool of FIG.
2;
FIG. 10 is a cross plot of hydrogen index vs. inverse flux at
different neutron and gamma ray energies and source/detector
spacings in several standard lithologies, as determined from Monte
Carlo modelling of the accelerator-based tool of FIG. 2;
FIG. 11 is a cross plot of inverse eV or MeV slowing down length
vs. inverse eV and MeV neutron flux for a far-spaced detector in
several standard lithologies; and
FIG. 12 is a cross plot of hydrogen index vs. neutron slowing down
length at both eV and MeV neutron energy ranges in three standard
lithologies;
FIG. 13 is a cross plot of the normalized inverse epithermal
neutron flux at an array detector vs. the normalized inverse MeV
flux at a far-spaced detector in three standard lithologies;
FIG. 14A is a surface representation of the epithermal neutron
density-slowing down length sensitivity ratio as a function of
chemical element and hydrogen index of porous sandstone;
FIG. 14B is a surface representation of the MeV neutron
density-slowing down length sensitivity ratio as a function of
chemical element and hydrogen index of porous sandstone;
FIG. 15A is a projection of the surface representation of FIG.
14A;
FIG. 15B is a projection of the surface representation of FIG. 14B;
and
FIG. 16 is a cross plot of hydrogen index vs. the average
density-slowing down length sensitivity ratio for a partially
gas-saturated formation and a partially kaolinite clay-bearing
formation.
DETAILED DESCRIPTION
The present invention has particular utility in
measurement-while-drilling applications, and such an application is
illustrated in FIG. 1 of the drawings. In that regard, and unless
otherwise specified, measurement-while-drilling (also known as
measuring-while-drilling and logging-while-drilling) as used herein
is intended to include the recording of data and/or the making of
measurements in an earth borehole, with the drill bit and at least
some of the drill string in the borehole, during drilling, pausing
and/or tripping. It will be understood, however, that certain
aspects of the invention will have application to wireline logging
as well.
As shown in FIG. 1, a platform and derrick 10 are positioned over a
borehole 12 that is formed in the earth by rotary drilling. A drill
string 14 is suspended within the borehole and includes a drill bit
16 at its lower end. The drill string 14 and the drill bit 16
attached thereto are rotated by a rotating table 18 (energized by
means not shown) which engages a kelly 20 at the upper end of the
drill string. The drill string is suspended from a hook 22 attached
to a travelling block (not shown). The kelly is connected to the
hook through a rotary swivel 24 which permits rotation of the drill
string relative to the hook. Alternatively, the drill string 14 and
drill bit 16 may be rotated from the surface by a "top drive" type
of drilling rig.
Drilling fluid or mud 26 is contained in a mud pit 28 adjacent to
the derrick 10. A pump 30 pumps the drilling fluid into the drill
string via a port in the swivel 24 to flow downward (as indicated
by the flow arrow 32) through the center of drill string 14. The
drilling fluid exits the drill string via ports in the drill bit 16
and then circulates upward in the annulus between the outside of
the drill string and the periphery of the borehole, as indicated by
the flow arrows 34. The drilling fluid thereby lubricates the bit
and carries formation cuttings to the surface of the earth. At the
surface, the drilling fluid is returned to the mud pit 28 for
recirculation. If desired, a directional drilling assembly (not
shown) with a mud motor having a bent housing or an offset sub
could also be employed.
Mounted within the drill string 14, preferably near the drill bit
16, is a bottom hole assembly (indicated generally by the reference
numeral 36), which includes subassemblies for making measurements,
processing and storing information and for communicating with the
earth's surface. Preferably, the bottom hole assembly is located
within several drill collar lengths of the drill bit 16. In the
illustrated bottom hole arrangement of FIG. 1, a stabilizer collar
section 38 is shown immediately above the drill bit 16, followed in
the upward direction by a drill collar section 40, another
stabilizer collar section 42 and another drill collar section 44.
This arrangement of drill collars and stabilizer collars is
illustrative only, and other arrangements may of course be used.
The need for or desirability of the stabilizer collars will depend
on drilling conditions.
In the embodiment shown in FIG. 1, the components of the downhole
measurement subassembly are preferably located in the drill collar
section 40 above the stabilizer collar 38. Such components could,
if desired, be located closer to or farther from the drill bit 16,
such as, for example, in either stabilizer collar section 38 or 42
or the drill collar section 44.
The bottom hole assembly 36 also includes a telemetry subassembly
(not shown) for data and control communication with the earth's
surface. Such apparatus may be of any suitable type, e.g., a mud
pulse (pressure or acoustic) telemetry system as disclosed in U.S.
Pat. No. 5,235,285 (hereby incorporated by reference), which
receives output signals from the data measuring sensors and
transmits encoded signals representative of such outputs to the
surface where the signals are detected, decoded in a receiver
subsystem 46 and applied to a processor 48 and/or a recorder 50.
The processor 48 may comprise any suitably programmed digital or
analog computer, and the recorder 50 preferably comprises a
conventional recorder-plotter for making the usual visual and/or
magnetic data record as a function of borehole depth. A surface
transmitter subsystem 52 may also be provided for establishing
downward communication with the bottom hole assembly 36, as
disclosed, for example, in the aforementioned U.S. Pat. No.
5,235,285.
The bottom hole assembly 36 preferably also includes conventional
acquisition and processing electronics (not shown) comprising a
microprocessor system (with associated memory, clock and timing
circuitry, and interface circuitry) capable of timing the operation
of the accelerator and the data measuring sensors, storing data
from the measuring sensors, processing the data and storing the
results, and coupling any desired portion of the data to the
telemetry components for transmission to the surface.
Alternatively, the data may be stored downhole and retrieved at the
surface upon removal of the drill string. Suitable downhole
circuitry for these purposes is described in U.S. Pat. No.
4,972,082 and U.S. Pat. No. 5,051,581, the disclosures of which are
hereby incorporated by reference. To facilitate electrical
connections and signal transmission between the measurement
subassembly, the data acquisition and processing subassembly, and
the data telemetry subassembly, these components are preferably
located adjacent to each other in the drill string. Where this is
not feasible, the data communications system of the aforementioned
U.S. Pat. No. 5,235,285, which provides for both local downhole
communication over short distances and downhole-to-surface
communication, may be utilized. Power for the downhole electronics
may be provided by battery or, as known in the art, by a downhole
turbine generator powered by the drilling fluid.
A preferred embodiment of the downhole measurement subassembly is
shown in FIGS. 2-4, where the drill collar section 40 is shown as
surrounding a stainless steel tool chassis 54. The drill collar may
be of any suitable size, e.g. having an 8" OD with a 5" ID. Formed
in the chassis 54 to one side of the longitudinal axis thereof, as
best seen in FIGS. 3 and 4, is a longitudinally extending mud
channel 56 for conveying the drilling fluid downward through the
drill string. Eccentered to the other side of the chassis 54 are a
neutron accelerator 58, its associated control and high voltage
electronics package 60 and a coaxially aligned, near-spaced
detector 62. The accelerator is preferably a D-T type (14 MeV)
source as is known in the art.
In accordance with the invention, the near-spaced detector 62
should be primarily responsive to accelerator output with minimum
formation influence. To that end, the detector 62 may comprise an
epithermal neutron detector, e.g., a .sup.3 He proportional
counter, which is located close to the accelerator without
intervening high density shielding. The sensitive volume of the
detector 62 is clad in cadmium or other high thermal neutron
capture cross section material (not shown) to raise the detection
threshold to epithermal levels. The detector 62 is also surrounded,
preferably on all surfaces except that adjacent to the accelerator
58, by a shield 64 of combined neutron moderating-neutron absorbing
material, such as boron carbide (or other 1/v type absorber)
distributed in an epoxy (or other hydrogenous material) binder
("B4CE"). More detailed information concerning the structure and
function of the shielding for such a near-spaced .sup.3 He detector
is set out in U.S Pat. No. 4,760,252, the pertinent portions of
which are hereby incorporated by reference.
Alternatively, the near detector 62 may be a higher energy (MeV)
detector, such as a .sup.4 He detector, surrounded by tungsten,
heavimet or other high-Z shielding to both shield the detector from
the formation and multiply the number of non-formation neutrons
incident upon the detector. The multiplying effect is due to the
large (n, 2n) and (n, 3n) cross section of the high-Z material,
which converts 14 MeV source neutrons into two or three neutrons
below approximately 6 MeV, where the .sup.4 He scattering cross
section is large. Thus, the high-Z shielding not only decreases the
sensitivity of the near-detector signal to formation scattered
neutrons, it also effectively attenuates the source (14 MeV)
neutron flux along the tool.
If, as described below, the farther-spaced neutron detectors are
shielded in a B4CE (or like moderating-absorbing) material, the
slowing down power of the hydrogen in the B4CE can be used to
further reduce the energy of the neutrons while the absorbing power
of the boron serves to attenuate the low energy neutron flux. The
ordering of the shielding materials, high-Z material near the
neutron source and the B4CE (or like) material following, is
critical, as the reverse order is ineffective to shield high energy
neutrons.
Whether the near-spaced detector 62 is an eV detector or an MeV
detector, the combined effect of the detection energy, placement
and shielding of the near detector should be such as to render the
detector output relatively insensitive to formation porosity and
primarily proportional to the neutron flux from the accelerator.
The output of the near detector 62 may then be used to normalize
other detector outputs for source strength fluctuation.
Located longitudinally adjacent to the near-spaced detector 62 is a
plurality or array of detectors 66a, 66b, 66c and 66d. The array
includes at least one, and preferably more than one, epithermal
neutron detector and at least one gamma ray detector. One or more
thermal neutron detectors may optionally be included. As
illustratively depicted in FIG. 4, there are two epithermal
detectors 66a and 66b, one thermal neutron detector 66c and one
gamma ray detector 66d. A different number or mix of detectors may
be provided if desired.
The principal purpose of the epithermal neutron detectors 66a, 66b
is to measure the epithermal neutron flux in the formation at a
spacing sufficiently close to the neutron source to minimize, or at
least significantly reduce, the effect on the detector outputs of
the heavier formation elements, such as oxygen, silicon, carbon,
calcium, etc., which dominate bulk density, and to maximize, or at
least significantly enhance, the influence of formation hydrogen on
the detector outputs. So positioned, the epithermal neutron
detector response will depend primarily on the hydrogen index with
only a residual lithology effect. To enhance sensitivity to the
formation, the epithermal detectors 66a, 66b, which may be .sup.3
He proportional counters, are preferably located closely adjacent
the drill collar wall and back-shielded, as shown at 68a and 68b,
to reduce borehole neutron sensitivity. The shielding material is
preferably the same as that described previously in connection with
the near detector 62, i.e. cadmium cladding and B4CE. As described
more fully below, neutron-transparent windows 70a and 70b are
preferably formed in the drill collar to further enhance detector
sensitivity and to provide a greater depth of investigation.
As illustrated in FIG. 4, the epithermal neutron detectors 66a, 66b
and the associated windows 70a, 70b are preferably spaced apart
circumferentially of the drill collar 40 for enhanced angular or
azimuthal resolution. Any desired circumferential spacing of the
detectors may be used. Although the detectors 66a, 66b are shown at
the same longitudinal spacing from the accelerator 58, one or more
additional detectors could be provided at different longitudinal
spacings for enhanced vertical resolution. Circumferentially and
horizontally spaced detector arrays, as well as further details
concerning the configuration of the individual detectors and their
shielding, are described in more detail in U.S. Pat. No. 4,760,252
and U.S. Pat. No. 4,972,082, which are hereby incorporated by
reference. The high spatial resolution of the slowing down time
measurement, as described in the '082 patent, makes the azimuthal
measurement of slowing down time in accordance with the present
invention of particular interest and value.
It is to be noted that the source/detector spacings described in
U.S. Pat. No. 4,760,252 and U.S. Pat. No. 4,972,082 are for
wireline tools. Somewhat longer spacings should be provided in a
measurement-while-drilling tool to account for the fact that the
detectors are looking at the formation through the drill
collar.
The thermal neutron detector 66c may likewise be a .sup.3 He
proportional counter shielded, as at 68c, similarly to the
epithermal detectors 66a, 66b, except that the cadmium cladding is
omitted on the formation side to render the detector sensitive to
formation thermal neutrons. A neutron transparent window 70c may be
provided in the drill collar 44 adjacent to the thermal detector
66c. Additional thermal neutron detectors may be provided as needed
to obtain the desired horizontal and/or vertical resolution. The
output signals from the thermal neutron detector(s) 66c may be
processed as described in the incorporated portions of U.S. Pat.
No. 4,760,252 to derive a thermal neutron porosity measurement
and/or in accordance with the disclosure of U.S. Pat. No.
5,235,185, hereby incorporated by reference, to derive measurements
of formation sigma and standoff.
The gamma ray detector 66d may comprise any suitable type detector,
such as NaI, BGO, CsI, anthracene, etc., but preferably is a
cerium-activated gadolinium orthosilicate (GSO) detector as
disclosed in U.S. Pats. No. 4,647,781 and U.S. Pat. No. 4,883,956,
both of which are hereby incorporated by reference. As disclosed in
those patents, the GSO detector is preferably surrounded by boron
to reduce the influence of thermal and epithermal neutrons on the
detector response. Also, a tungsten or other high density shield
(not shown) may be placed between the accelerator 58 and the GSO
detector 66d to reduce the flux of high energy neutrons incident on
the detector.
Although not shown, it will be understood that appropriate timing
and control circuitry will be provided to operate the accelerator
58 in a pulsed mode and to gate the detector 66d as needed
selectively to detect inelastic and/or capture gamma rays. The
energy detection range is preferably broad, e.g. from 0.1 to 11
MeV. A principal purpose of the detector 66d is to provide
inelastic and/or capture gamma ray energy spectra and energy window
count rates. In particular, the energy spectra can be spectrally
analyzed to derive information concerning the elemental composition
of the formations under investigation. The preferred technique for
analyzing the spectral data from the gamma ray detector 66d to
obtain the elemental spectroscopy and lithology information is
described in the copending, commonly-owned U.S. patent application
Ser. No. 08/221,158 for "Methods and Apparatus for Determining
Formation Lithology by Gamma Ray Spectroscopy", filed on Mar. 31,
1994 by B. A. Roscoe. The disclosure of the Roscoe application is
hereby incorporated by reference.
Briefly, in accordance with the Roscoe disclosure, inelastic
scattering gamma ray spectra are analyzed by a least squares
spectral fitting process to determine the relative elemental
contributions thereto of chemical elements postulated to be present
in an unknown earth formation and contributing to the measured
spectra from the formation. The relative inelastic yields for
silicon, calcium and magnesium are calibrated to provide
straightforward estimates of the respective elemental
concentrations for those elements and of the volumetric fractions
of the elements or associated rock types, such as sandstone,
limestone and dolomite, in the formation. The ratio of the relative
inelastic yields for magnesium and calcium provides an indication
of the degree of dolomitization of a formation. Based on the
calibrated inelastic yields for silicon and/or calcium, calibrated
estimates of the elemental yields from measured thermal neutron
capture gamma ray spectra may also be determined, from which
further information concerning formation lithology may be
derived.
Measurements of the epithermal neutron slowing down time and tool
standoff from the borehole wall may be derived from the outputs of
the epithermal neutron detectors 66a, 66b. Because the large amount
of steel present in the drill collar 40 and chassis 54 acts as a
long lifetime storage sink for neutrons, the sensitivity of the
detectors 66a, 66b to epithermal neutron slowing down time is
substantially reduced. To measure epithermal neutron slowing down
time while drilling, therefore, it is important to properly locate
the detectors 66a, 66b relative to the drill collar 40, to provide
properly constructed neutron windows 70a, 70b and to properly
back-shield the detectors 66a, 66b. As shown in FIG. 4 and as noted
above, the sensitive volumes of the detectors 66a, 66b are
preferably mounted in the tool chassis 54 closely adjacent the
inner wall of the drill collar 40 and immediately opposite the
respective neutron windows 70a, 70b in the drill collar. Each
detector is also preferably back-shielded (with B4CE or the like)
on both ends and on all sides except the side facing the drill
collar. The windows 70a, 70b are preferably made of titanium or
other high-strength, low-scattering cross section material which is
sheathed in boron. To further reduce neutron entry into the drill
collar 40, a boron carbide layer 72 with holes to match the
locations of the windows 70a, 70b is preferably provided on the
exterior of the drill collar 40 in the region of the detectors.
Modelling and experimental data have shown that the sensitivity of
the epithermal neutron slowing down time curves to porosity from
detectors positioned, shielded and windowed in this way is greater
than for detectors without windows or external boron shielding.
As alternatively shown in FIG. 5, it is possible to enhance
detector sensitivity still further by placing the detectors 74 in
the drill collar 40 itself, with boron carbide back-shielding 76
and an external boron carbide layer 72 with matching holes as in
FIG. 4. This combination, though feasible, exposes the detectors to
greater risk of damage during drilling and also requires machining
of the drill collar to form the detector receptacles.
As an alternative to the use of boron-sheathed neutron transparent
windows 70a, 70b, as shown in FIG. 4, the slowing down time and
count rate sensitivity of the epithermal neutron detectors 66a, 66b
can be enhanced by providing transverse layers of boron or other
high-absorption cross section material in the drill collar 40 in
the region of the epithermal neutron detectors. This is illustrated
in FIGS. 6-8. FIG. 6 depicts an epithermal neutron detector 78
eccentered against the drill collar wall and back-shielded as in
FIG. 4. A plurality of transverse boron carbide layers 80 are
embedded in the collar wall, where they act as a "venetian blind"
to permit neutron travel transversely across the collar to the
detector while blocking neutron flow along the collar. FIG. 7
illustrates the external pattern of boron carbide layers 80 of FIG.
6. An alternative pattern of boron carbide layers 82 is shown in
FIG. 8. This pattern serves to minimize neutron flow through the
collar in both longitudinal and circumferential directions without
interfering with transverse flow. The boron carbide layers 80 and
82, therefore, function essentially as a neutron window for the
epithermal or thermal neutron detectors.
The use of neutron absorbing layers as shown in FIGS. 6-8 has been
found to be particularly important for reducing neutron flow in low
scattering cross section material, such as titanium, which is
desirable as a drill collar material in measurement-while-drilling
applications because of its relative transparency to neutrons but
which, because of its lower density, does not attenuate the
component of neutron transport parallel to or circumferentially of
the drill collar to the same extent as does steel. For further
effectiveness, the boron carbide layers could also be included in
the tool chassis 54 on the accelerator side, or on both sides, of
the neutron detectors.
With reference again to the overall measurement subassembly
configuration shown in FIG. 2, a far-spaced detector 84 is located
downstream of the array detectors 66a-66d with an intervening
neutron shield 86. The detector 84 and shield 86 are preferably
coaxial with the accelerator 58. In accordance with the invention,
the far-spaced detector 84 is selectively positioned relative to
the neutron source so as to be sensitive to MeV energy neutrons
(or, preferably, MeV neutron-induced gamma rays) that penetrate to
relatively far distances in the formation. As the transport of MeV
energy neutrons has reduced sensitivity to formation hydrogen
content and enhanced sensitivity to the density of heavier
formation elements, as compared to KeV-ev energy neutrons, the
response of the detector 84 will be strongly influenced by
formation bulk density and, because of the close relation between
density and matrix type, formation lithology.
Preferably, the detector 84 comprises a GSO gamma ray detector as
described in the aforementioned U.S. Pats. No. 4,647,782 and U.S.
Pat. No. 4,883,956, although any suitable type, such as anthracene,
NaI, BGO, CsI, etc., may be used so long as acceptable count rate
statistics and energy resolution are achieved. The preferred energy
detection range is from 0.1 MeV to 11 MeV. Alternatively, a neutron
detector sensitive to MeV range neutrons, e.g. >0.5 MeV., may be
used. The preferred neutron detectors are a .sup.4 He type or a
liquid scintillator type.
Where a gamma ray detector is employed as the far-spaced detector
84, the intervening shield 86 is preferably B4CE or like neutron
moderating-absorbing material. If an MeV neutron detector is used,
the shield 86 is preferably a high-Z material such as tungsten,
except where the near-spaced detector 62 is also a .sup.4 He (or
other MeV detector) shielded by high-Z material. In the latter
case, the shielding 86 should also be B4CE or the like to take full
advantage of the aforementioned neutron moderating effect of the
high-Z shielding material 64 surrounding the near-spaced detector
62.
Although the far-spaced detector 84 may be either a gamma ray
detector or an MeV neutron detector, a gamma ray detector is
preferred because gamma rays have better sensitivity to gas than do
neutrons in some situations, thereby facilitating the
identification of gas-bearing formations. Also, as described above
in connection with the array gamma ray detector 66d, the use of a
gamma ray detector permits a spectral analysis to be made to obtain
information of the elemental composition and lithology of the
formation. Such a spectral analysis may be made at both, or only
one, of the array detector 66d and the far-spaced detector 84. The
output of either (or both) gamma ray detector could additionally be
employed to derive measurements of the formation macroscopic
capture cross section for thermal neutrons (.SIGMA.) or its
correlative the thermal neutron decay time constant (.tau.). Any of
the known techniques for deriving .SIGMA. or .tau. may be used for
this purpose. Also, where the far-spaced detector 84 is a gamma ray
detector, the array gamma ray detector could be omitted if space or
other considerations dictate.
A second far-spaced detector (not shown) may be provided if
desired. If so, it preferably is located coaxially with and closely
adjacent to the detector 84. If the detector 84 is a gamma ray
detector, the second far-spaced detector is preferably a neutron
detector and vice versa.
Although not specifically shown, it will be understood that the
above-described detectors include all amplification, pulse shaping,
power supply and other circuitry required to generate output
signals representative of the radiation detected. All such
circuitry is well known in the art.
The signals from the several detectors provided in the tool may be
processed in various ways to obtain the desired petrophysical
information. As mentioned, the output of the near-spaced detector
62 is proportional to the neutron source output and is used
principally to normalize the other detector output signals for
source strength fluctuation.
The outputs of the array epithermal neutron detectors 66a, 66b are
mostly sensitive to hydrogen index, and thus porosity, and,
according to one feature of the invention, are used in combination
with the output of the far-spaced detector 84 to derive information
as to formation density, porosity and lithology and to detect gas.
The basic signal processing solution uses the neutron flux A1
(count rate from detector 66a or 66b) normalized by the count rate
N1 from the near-spaced detector 62, i.e., (A1/N1) (-1), and the
similarly normalized inverse count rates (F1g/N1) (-1) or (F1n/N1)
(-1) from the far-spaced gamma ray or MeV neutron detector 84,
respectively. As will be described, these quantities may be used in
several ways to determine the hydrogen index HI, the slowing down
lengths (eV or MeV) and the lithology of the formation. As a
further feature of the invention, the slowing down lengths and the
hydrogen index may be used to derive the bulk density of the
formation Finally, the determination of bulk density may be
improved by lithological information obtained from the near gamma
spectroscopy detector 66d.
The simplest use of the inverse normalized fluxes is to cross plot
them. Such a cross plot is shown in FIG. 9, where the inverse
fluxes have been obtained from Monte Carlo modelling of the
accelerator-based tool depicted in FIG. 2. The cross plot of FIG. 9
is conceptually similar to the neutron-density cross plot that is
conventionally used in wireline logging for lithology and porosity
determination based on the responses of the standard bulk density
and neutron porosity tools. See, for example, Ellis, Well Logging
for Earth Scientists, Elsevier, 1987, pp. 420-421. The
interpretation of the cross plot of FIG. 9 is likewise similar to
that of the conventional neutron-density cross plot. Inverse count
rates are plotted in FIG. 9 so that higher porosities will appear
at the upper right of the plot and lower porosities at the lower
left, as in the conventional neutron-density cross plot.
As shown in FIG. 9, there are curves 88, 90, 92 representing the
porosity trends for the standard sandstone, limestone and dolomite
lithologies. A measured point 94 (Flgm, Alm) can be plotted on this
cross plot and its porosity and lithology can be interpolated as in
the neutron-density cross plot, as represented by the dashed line
96. Shaly lithologies and gas bearing formations appear in the same
relative positions on the cross plot of FIG. 9 as they do in the
neutron-density cross plot, although their exact positions may
differ systematically from those in the neutron-density cross
plot.
One embodiment of the invention is a solution where the normalized
inverse fluxes for the array and far detectors are used to derive
the hydrogen index HI and the eV slowing down length L.sub.epi or
the MeV slowing down length L.sub.h. Specifically, this technique
is based on the output of an array epithermal neutron detector 66a,
66b, which is mostly sensitive to hydrogen index but has a residual
lithology effect, and the output of the far-spaced MeV detector 84
(either gamma ray or neutron), which is sensitive to hydrogen index
as well as the MeV or eV slowing down lengths. The count rate
signals from both detectors are normalized by the output of the
near-spaced detector 62. For purposes of this technique, the
far-spaced detector 84 could be selected to be sensitive to eV
range neutrons, but fluxes in the MeV range are preferred because
they are less sensitive to hydrogen index. Hence, the precision of
the hydrogen index measurements is less important with MeV fluxes
than it is with eV fluxes.
FIGS. 10 and 11 illustrate the responses of an array epithermal
neutron detector, far-spaced eV and MeV neutron detectors and an
MeV far-spaced gamma ray detector for a Monte Carlo modeled mock-up
of the accelerator-based tool depicted in FIG. 2. The Monte Carlo
simulation gives the flux of inelastic gamma rays in the far gamma
ray detector. In what follows, reference to gamma ray fluxes means
inelastic gamma rays. These may be separated from capture gamma
rays by well known neutron accelerator pulsing techniques.
FIG. 10 shows the detector responses in a number of different
formations versus hydrogen index. As may be seen, the array
epithermal neutron detector responds mainly to hydrogen index, as
all of the data fall nearly on a single curve with little lithology
variation. The far-spaced gamma ray detector and eV and MeV neutron
detectors show considerable lithology and density dependence as
well as dependence on hydrogen index.
FIG. 11 plots the calculated inverse flux of eV and MeV neutrons at
the far-spaced detector versus the respective slowing down lengths.
These data show that formation slowing down length is the most
important variable affecting the far neutron flux. Similarly,
inspection of Monte Carlo data shows that eV slowing down length is
the most important variable affecting the far inelastic gamma ray
count.
It may be shown that the respective fluxes plotted in FIGS. 10 and
11 may be well fit by a combination of slowing down length
(L.sub.epi for the eV slowing down length and L.sub.h for the MeV
slowing down length) and hydrogen index (HI). The following
illustrative models have been constructed for the array epithermal
neutron detector and the preferred far-spaced MeV detector based on
results from simulation programs, but could be constructed from
experimental results if desired.
Array epi thermal detector model:
Far-spaced MeV detector model:
It will be understood that other models may be employed, with the
object of providing the best match to the data.
Given a set of array detector and far-spaced detector flux
measurements, it is straightforward to solve the model equations
(4) and (5) and obtain derived values of the hydrogen index and the
inverse slowing down length.
Further techniques for cross plotting to derive hydrogen index (or
porosity) and slowing down length are described below.
One such technique is based on the substantial difference in the
n,p scattering cross section of formation constituents for neutrons
below approximately 1 MeV as compared to that for neutrons above 1
MeV. For neutrons below approximately 1 MeV, the n,p scattering
cross section is large and is due principally to elastic scattering
with hydrogen nuclei. Consequently, the neutron slowing down length
is strongly dependent on hydrogen concentration for neutrons with
an initial energy of 1 MeV or lower. For neutrons above 1 MeV, on
the other hand, the n,p scattering cross section decreases rapidly
and becomes comparable with elastic scattering from heavier matrix
elements, such as oxygen, silicon, calcium, etc. Elastic scattering
from the heavier matrix elements, however, is relatively
ineffective in slowing neutrons to low energies. Non-elastic
reactions with matrix elements (mostly inelastic scattering (n,p)
and (n,.alpha.) reactions) are much more effective at removing
neutrons from the high energy region. Thus the neutron slowing down
length for high energy neutrons (14 Mev-->1 MeV) exhibits
increased sensitivity to matrix density and chemical composition
and is only weakly dependent on hydrogen index (porosity). On the
other hand, the low energy (<1 MeV--epithermal) slowing down
length is primarily sensitive to hydrogen concentration.
Simple diffusion theory predicts a radial fall-off of high-energy
neutron flux .phi..sub.h with distance r from the neutron source
according to: ##EQU1## where S is the source strength,
.SIGMA..sub.rh is the macroscopic cross section for the removal of
neutrons from the 1-14 MeV energy range, and L.sub.h is the high
energy slowing down length.
Given two measurements of the >1 MeV neutron flux at different
source/detector spacings r.sub.1 and r.sub.2, a direct measurement
of L.sub.h can be made: ##EQU2## where .phi..sub.h (r.sub.1) and
.phi..sub.h (r.sub.1) are the >1 MeV neutron flux measurements
at distances r.sub.1 and r.sub.2, respectively.
The epithermal neutron flux .phi..sub.epi follows a similar law in
one group diffusion theory: ##EQU3## where .SIGMA..sub.rs is the
macroscopic cross section for the removal of neutrons from the 14
MeV-->epithermal range and L.sub.epi is the length for neutron
slowing from 14 MeV to 0.5 eV (the cadmium cutoff).
Although L.sub.epi has some dependence on the matrix, the flux
dependence of these variations vanishes at a source/detector
spacing of 2 L.sub.epi. Also, the source factor S can be eliminated
by normalizing the 1 MeV flux measurement with a like measurement
at a short source/detector spacing. Thus, with an epithermal
neutron detector, e.g. detector 66a in FIG. 2, and two spaced 1 MeV
detectors, e.g. detectors 62 and 84 in FIG. 2, measurements can be
made of both porosity (hydrogen index) and L.sub.epi and/or
L.sub.h. A cross plot of these measurements can then determine
porosity and matrix type and identify gas, as illustrated in FIG.
12.
In an alternative presentation similar to that of FIG. 9, the
inverse neutron count rate from the far-spaced MeV detector
(detector 84 in FIG. 2), normalized by the count rate of a
near-spaced MeV detector (detector 62 in FIG. 2), is cross-plotted
against the inverse neutron count rate from an epithermal neutron
detector at a 2 Ls spacing (array detector 66a in FIG. 2),
normalized by the count rate from the near-spaced MeV detector.
Such a cross plot is shown in FIG. 13 for a near MeV detector
spacing of 20 cm, an epithermal neutron detector spacing of 30 cm
and a far MeV detector spacing of 60 cm. As there indicated, the
three curves correspond to the three major rock matrices, dolomite
(2.87 g/cc), limestone (2.71 g/cc) and sandstone (2.64 g/cc). As
expected, the near/far ratio and the near/array ratio provide
almost independent measures of matrix type and porosity,
respectively. The interpretation of this cross plot to obtain
porosity and lithology and to detect gas is as described above in
connection with FIG. 9.
The measured slowing down length L.sub.epi or L.sub.h and the
measured hydrogen index HI may be used to derive the bulk density
of a formation. One technique for this purpose is described below.
An alternative technique is described in the commonly-owned,
copending U.S. patent application Ser. No. 08/006,903, which will
issue as U.S. Pat. No. 5,349,184 on Sep. 20, 1994, which is hereby
incorporated by reference.
In the following discussion, the term slowing down length may refer
to either L.sub.epi or L.sub.h ; for the actual examples L.sub.epi
is used. Starting from a standard formation such as porous
limestone or sandstone for which the slowing down length, the
hydrogen index (the same as the porosity) and the bulk density are
all known, the ratio between small changes in the bulk density of
the standard formation and the resultant small changes in its
slowing down length is calculated. This ratio is referred to as the
density-slowing down length sensitivity ratio. The ratio can be
used, under assumptions to be described, so that a small change in
slowing down length will allow calculation of a small change in
bulk density which can be added to the bulk density of the standard
formation to determine the bulk density of the measured formation.
Since the invention measures slowing down lengths and hydrogen
index, the slowing down length difference can be calculated from
the measured slowing down length of an unknown formation and that
of the standard porous formation having the same hydrogen index as
the measured hydrogen index for the unknown formation.
The calculation of the slowing down length of a formation of known
elemental composition may be done by analytic or Monte Carlo
methods. A suitable analytic method is described by A. Kreft,
"Calculation of the Neutron Slowing Down Length in Rocks and
Soils", Nukleonika, Vol. 19, 145-156, 1974; "A Generalization of
the Multigroup Approach for Calculating the Neutron Slowing Down
Length", Inst. of Nuclear Physics and Techniques (Cracow) Report
32/I, 1972, which are hereby incorporated by reference.
The following is an example of the calculation of the
density-slowing down length sensitivity ratio. Using a computer
code implementation of Kreft's method, the epithermal neutron
slowing down length of a standard formation, say 30 pu limestone
(hydrogen index, HI=0.3; bulk density, 2.197 g/cc; 0.033 g/cc
hydrogen, 0.228 g/cc carbon, 1.176 g/cc oxygen, 0.760 g/cc calcium)
is calculated to be 13.27 cm. The epithermal slowing down length of
a similar formation like the first but with the addition of 0.05
g/cc of a common formation element such as aluminum (hydrogen
index, HI=0.3, bulk density=2.247 g/cc) is calculated to be 13.08
cm. The difference between the resulting slowing down lengths is
dL.sub.epi =-0.19 cm. This difference results from the difference
in the input aluminum densities of 0.05 g/cc. The density-slowing
down length sensitivity ratio is the percentage change in slowing
down length: -0.19/13.27=-1.43% divided by the percentage change in
density, 2.27%, the ratio being -0.63. FIGS. 14A and 14B show these
ratios for epithermal and MeV slowing down lengths, respectively,
as surface functions of the hydrogen index of porous sandstone and
of the element whose density changes relative to the porous
sandstone elemental composition. Projections of these surfaces are
shown in FIGS. 15A and 15B. The density-slowing down sensitivity
ratios calculated for porous limestone are very similar to those
for sandstone.
An average density-slowing down length sensitivity ratio may be
calculated for any formation having the same hydrogen index as a
standard porous formation. This ratio is a weighted average over
the sensitivity ratios for each element whose density differs. The
weighting is proportional to the density difference for each
element. FIG. 16 shows the epithermal ratio for two typical
formations as a function of hydrogen index. The formations are
partially gas saturated formations (0.2 g/cc methane replaces
water) or formations in which kaolinite clay (formula A14Si4018H8,
density 2.54 g/cc) partially replaces the standard formation and
are calculated relative to porous sandstone. The average ratio is
highly insensitive to whether gas or clay is involved. Thus for a
given measured hydrogen index of say 0.30 (30 pu porosity), the
density-slowing down length sensitivity ratio is -0.63 to an
accuracy of 10%.
The key feature of the density-slowing down length sensitivity
ratio is that it is insensitive to the element causing the change
in density, unless that element is carbon, or in the case of the
epithermal slowing down length L.sub.epi only, sodium and chlorine.
Thus for many formations common to well logging, such as those
above, the density-slowing down length sensitivity ratio is
accurately known. This ratio may be applied to the percentage
difference in slowing down length of a measured formation relative
to a standard formation (such as porous sandstone of the same
hydrogen index), to calculate the percentage difference of the
density of the measured formation relative to the standard
formation. The percentage difference leads in turn to the density
of the measured formation. Provided that the measured formation is
not too different from the standard formation in terms of the
amount of carbon (or chlorine and sodium for epithermal slowing
down length), the calculated bulk density of the measured formation
will be accurate.
As a further refinement of the invention, knowledge of the
elemental composition of the formation gained from gamma
spectroscopy can be used to refine the calculation of the
appropriate density sensitivity ratio.
Because the measurement subassembly rotates along with the drill
string 14, provision is made for making the aforementioned
measurements as a function of the angular or azimuthal orientation
of the tool as the tool turns during drilling. Various methods and
apparatus are known in the art for that purpose. For example, U.S.
Pat. No. 5,091,644, hereby incorporated by reference, discloses an
azimuthal measuring system in which the borehole cross section is
divided into two or more segments, e.g., quadrants. As the tool
rotates, it passes through the borehole segments.
Each time it passes a segment boundary, a counter is incremented,
pointing to the next segment. This allows the data, e.g. gamma ray
or neutron count rates, to be segregated according to the
respective segments which each detector was traversing when the
measurements were made. In this way, plural angular or azimuthal
measurements can be made at each depth level. The separate
segmental measurements may be combined, to provide an average
measurement for the depth level, or they may be processed
separately, as, for example, where borehole conditions, such as a
washout, indicate that one or more of the segmental measurements is
unreliable.
In the commonly-owned, copending U.S. patent application Ser. No.
08/183,089 for "Logging While Drilling Method and Apparatus for
Measuring Formation Characteristics as a Function of Angular
Position Within a Borehole", filed Jan. 14, 1994, by J. M. Holenka
et al., improved methods and apparatus are disclosed for making
neutron porosity, bulk density and other measurements as the tool
rotates in the borehole and relating them to the azimuthal position
of the tool. The measurements are made in angular distance segments
which preferably are quadrants, but which may be greater or less
than four in number and need not be of equal angular distance. The
angular segments are measured from the down vector of the
measurement-while-drilling tool. The down vector is preferably
derived by first determining an angle .phi. between a vector to
earth's north magnetic pole, as referenced to the cross sectional
plane of the measuring-while-drilling tool and a gravity down
vector as referenced in such plane. To that end, orthogonally
arranged magnetometers may be provided to continuously determine
the angle .phi.. Alternatively, surveys may be performed
periodically by the measuring-while-drilling tool when drilling is
halted to add drill pipe to the drill string. The Holenka et al.
disclosure is applicable both to tools with and without stabilizer
collars. The disclosure of the Holenka et al. application is
likewise incorporated herein by reference.
Although the invention has been described and illustrated herein by
reference to exemplary embodiments thereof, it will be understood
by those skilled in the art that such embodiments are susceptible
of variation and modification without departing from the inventive
concepts disclosed. All such variations and modifications,
therefore, are intended to be included within the spirit and scope
of the appended claims.
* * * * *