U.S. patent application number 11/940236 was filed with the patent office on 2008-11-06 for method for characterizing shear wave formation anisotropy.
This patent application is currently assigned to Schlumberger Technology Corporation. Invention is credited to Tom Bratton, J. Donald, John Walsh.
Application Number | 20080273422 11/940236 |
Document ID | / |
Family ID | 37631462 |
Filed Date | 2008-11-06 |
United States Patent
Application |
20080273422 |
Kind Code |
A2 |
Donald; J. ; et al. |
November 6, 2008 |
METHOD FOR CHARACTERIZING SHEAR WAVE FORMATION ANISOTROPY
Abstract
A method of characterizing shear wave anisotropy in a formation
includes obtaining crossed-dipole waveforms from a borehole
penetrating the formation over a range of depths and frequencies,
determining far-field slowness in a fast-shear and slow-shear
direction using a low-frequency portion of the crossed-dipole
waveforms, and determining near-wellbore slowness in the fast-shear
and slow-shear directions using a high-frequency portion of the
crossed-dipole waveforms. The method also includes marking a
selected depth of the formation as having intrinsic anisotropy if
at the selected depth the far-field slowness in the fast-shear
direction is less than the far-field slowness in the slow-shear
direction and the near-wellbore slowness in the fast-shear
direction is less than the near-wellbore slowness in the slow-shear
direction. The selected depth is marked as having stress-induced
anisotropy if the far-field slowness in the fast-shear direction is
less than the far-field slowness in the slow-shear direction and
the near-wellbore slowness in the fast-shear direction is greater
than the near-wellbore slowness in the slow-shear direction.
Inventors: |
Donald; J.; (Highlands
Ranch, CO) ; Bratton; Tom; (Littleton, CO) ;
Walsh; John; (Houston, TX) |
Correspondence
Address: |
OSHA . LIANG L.L.P. / SLB
1221 MCKINNEY STREET
SUITE 2800
HOUSTON
TX
77010
UNITED STATES
713-228-8600
713-228-8778
lord@oshaliang.com
|
Assignee: |
Schlumberger Technology
Corporation
5599 San Felipe, Suite 1700
Houston
TX
77056
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20080106975 A1 |
May 8, 2008 |
|
|
Family ID: |
37631462 |
Appl. No.: |
11/940236 |
Filed: |
November 14, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11/196,907 |
Aug 4, 2005 |
|
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11940236 |
Nov 14, 2007 |
|
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Current U.S.
Class: |
367/40 |
Current CPC
Class: |
G01V 2210/626 20130101;
G01V 1/284 20130101; G01V 1/50 20130101 |
Class at
Publication: |
367/040 |
International
Class: |
G01V 1/30 20060101
G01V001/30 |
Claims
1.-17. (canceled)
18. A system configured to characterize shear wave anisotropy in a
formation, comprising: a logging tool configured to: obtain
crossed-dipole waveforms from a borehole penetrating the formation
over a range of depths and frequencies; and a surface unit
operatively connected to the logging tool and configured to:
determine far-field slowness in a fast-shear direction and
slow-shear direction using a low-frequency portion of the
crossed-dipole waveforms; determine near-wellbore slowness in the
fast-shear direction and slow-shear direction using a
high-frequency portion of the crossed-dipole waveforms; select a
depth in the formation; characterize the depth of the formation as
having intrinsic anisotropy when at the depth the far-field
slowness in the fast-shear direction is less than the far-field
slowness in the slow-shear direction and the near-wellbore slowness
in the fast-shear direction is less than the near-wellbore slowness
in the slow-shear direction; and characterize the depth of the
formation as having stress-induced anisotropy when at the depth the
far-field slowness in the fast-shear direction is less than the
far-field slowness in the slow-shear direction and the
near-wellbore slowness in the fast-shear direction is greater than
the near-wellbore slowness in the slow-shear direction.
19. The system of claim 18, wherein the surface unit is further
configured to: determine the fast-shear direction prior to
determining the far-field slowness and the near-wellbore slowness,
wherein the slow-shear direction is orthogonal to the fast-shear
direction.
20. The system of claim 19, wherein determining the fast-shear
direction comprises Alford Rotation processing of the
crossed-dipole waveforms.
21. The system of claim 19, wherein determining the fast-shear
direction comprises parametric inversion of the crossed-dipole
waveforms.
22. The system of claim 18, wherein obtaining crossed-dipole
waveforms comprises firing a plurality of dipole sources located on
the logging tool to generate dipole acoustic signals which are
transmitted into the formation.
23. The system of claim 22, wherein obtaining crossed-dipole
waveforms further comprises firing the plurality of dipole sources
at different azimuthal positions in the borehole.
24. The system of claim 22, wherein obtaining crossed-dipole
waveforms further comprises detecting dipole acoustic signals from
the formation using a plurality of dipole receivers located on the
logging tool.
25. The system of claim 24, wherein a first set of the dipole
receivers selected from the plurality of dipole receivers are
inline with a first one of the plurality of dipole sources and a
second set of the dipole receivers selected from the plurality of
dipole receivers are inline with a second one of the plurality of
dipole sources.
26. The system of claim 22, wherein a first one of the plurality of
dipole sources fires at a low frequency and a second one of the
plurality of dipole sources fires at a high frequency.
27. The system of claim 26, wherein the low frequency is in a range
from approximately 1 to 3 kHz.
28. The system of claim 26, wherein the high frequency is in a
range from approximately 4 to 7 kHz.
29. The system of claim 26, wherein the low frequency and the high
frequency are selected such that dispersion crossover would be
detectible if dispersion curves were generated from the
crossed-dipole waveforms.
30. The system of claim 26, wherein the high frequency is selected
to probe into the formation a radial distance of approximately
one-half the borehole diameter.
31. The system of claim 26, wherein the low frequency is selected
to probe into the formation a radial distance of approximately two
to three times the borehole diameter.
32. The system of claim 18, wherein determining far-field slowness
involves processing the crossed-dipole waveforms using
slowness-time-coherence.
33. The system of claim 18, wherein determining near-wellbore
slowness involves processing the crossed-dipole waveforms using
slowness-time coherence.
34. A system configured to characterize shear wave anisotropy in a
formation, comprising: a logging tool configured to: obtain
crossed-dipole waveforms from a borehole penetrating the formation
over a range of depths and frequencies; and a surface unit
operatively connected to the logging tool and configured to:
determine far-field slowness in a fast-shear direction and
slow-shear direction using a low-frequency portion of the
crossed-dipole waveforms; determine near-wellbore slowness in the
fast-shear direction and slow-shear direction using a
high-frequency portion of the crossed-dipole waveforms; select a
depth in the formation; and characterize the depth as having
isotropic anisotropy when at the depth the far-field slowness in
the fast-shear direction is substantially the same as the far-field
slowness in the slow-shear direction.
35. The system of claim 34, wherein the surface unit is further
configured to: characterize the depth as having isotropic
anisotropy when at the depth the near-wellbore slowness in the
fast-shear direction is substantially the same as the near-wellbore
slowness in the slow-shear direction.
36. A system configured to characterize shear wave anisotropy in a
formation, comprising: a logging tool configured to: obtain
crossed-dipole waveforms from a borehole penetrating the formation
over a range of depths and frequencies; and a surface unit
operatively connected to the logging tool and configured to:
determine far-field slowness in a fast-shear direction and
slow-shear direction using a low-frequency portion of the
crossed-dipole waveforms; determine near-wellbore slowness in the
fast-shear direction and slow-shear direction using a
high-frequency portion of the crossed-dipole waveforms; select a
depth in the formation; and characterize the depth as having
isotropic anisotropy when at the depth the far-field slowness in
the fast-shear direction is substantially the same as the far-field
slowness in the slow-shear direction and the near-wellbore slowness
in the fast-shear direction is substantially the same as the
near-wellbore slowness in the slow-shear direction.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/196,907, filed Aug. 4, 2005, and entitled
"Method For Characterizing Shear Wave Formation Anisotropy," in the
names of J. Adam Donald, Tom R. Bratton, and John Walsh.
BACKGROUND OF THE INVENTION
[0002] The invention relates generally to formation evaluation
using borehole sonic logging. More specifically, the invention
relates to a method for distinguishing between intrinsic and
stress-induced anisotropy in an anisotropic formation.
[0003] A formation is said to be anisotropic if the value of a
property of the formation varies with direction of measurement. A
formation has shear wave anisotropy if shear wave velocity in the
formation varies with azimuth. Shear wave anisotropy can be
detected in a formation using a crossed-dipole sonic log obtained
from a borehole penetrating the formation. The crossed-dipole sonic
log is generated by measuring velocities of two orthogonal dipole
modes in the formation. Two forms of shear wave anisotropy are
considered herein: intrinsic and stress-induced. Intrinsic shear
wave anisotropy may arise from intrinsic structural effects, such
as layering of shale in a deviated borehole or aligned fractures,
and tectonic stresses. Stress-induced shear wave anisotropy arises
from the redistribution of the far-field horizontal stresses around
the borehole. Existing crossed-dipole sonic log indicates
anisotropic zones of the formation but does not indicate the
dominant underlying cause of the anisotropy. However,
distinguishing between intrinsic and stress-induced anisotropy is
important. Intrinsic anisotropy, specifically fracture anisotropy,
plays an important role in drilling, production, and completion
strategies. For example, it is desirable that boreholes are placed
in the formation such that they intersect as many fractures as
possible. Stress-induced anisotropy plays an important role in
completion strategies. For example, perforations oriented
perpendicular to minimum stress direction can be used to optimize
hydraulic fracture design.
[0004] Plona et al. describe a method for distinguishing between
intrinsic and stress-induced anisotropy in a formation using a
crossed-dipole sonic log. (Plona T. J., et al., "Using Acoustic
Anisotropy," paper presented at 41.sup.st SPWLA Symposium: June
2000). The method exploits the fact that stress-induced dipole
anisotropy in slow formations exhibits flexural mode dispersion
crossover whereas intrinsic dipole anisotropy does not. (Plona T.
J., et al., "Stress-Induced Dipole Anisotropy: Theory, Experiment
and Field Data," paper RR, presented at 40.sup.th SPWLA Symposium
'99). The method includes obtaining crossed-dipole waveforms from a
borehole. Alford Rotation is applied to the crossed-dipole
waveforms to identify the fast-shear direction. Flexural dispersion
curves, i.e., slowness versus frequency curves, are obtained by
processing the rotated waveforms for dipole polarizations parallel
and normal to the fast-shear and slow-shear directions using a
modified matrix pencil algorithm. The slow-shear direction is
orthogonal to the fast-shear direction. Slowness, measured in
microseconds per foot, is the amount of time for a wave to travel a
certain distance. FIGS. 1A and 1B show dispersion curves for an
intrinsic anisotropic formation and a stress-induced anisotropic
formation, respectively. The dispersion curves are generally
parallel for an intrinsic anisotropic formation and cross for a
stress-induced anisotropic formation. Although not shown in the
figures, dispersion curves coincide for an isotropic formation.
[0005] The Plona et al. method of distinguishing between intrinsic
and stress-induced anisotropy requires interpretation of individual
dispersion curves, which may not be efficient or practical for
large data sets. A continuous method of distinguishing between
intrinsic and stress-induced anisotropy would be useful to
efficiently diagnose the cause of anisotropy.
SUMMARY OF THE INVENTION
[0006] In one aspect, the invention relates to a method of
characterizing shear wave anisotropy in a formation which comprises
obtaining crossed-dipole waveforms from a borehole penetrating the
formation over a range of depths and frequencies, determining
far-field slowness in a fast-shear and slow-shear direction using a
low frequency portion of the crossed-dipole waveforms, determining
near-wellbore slowness in the fast-shear and slow-shear directions
using a high-frequency portion of the crossed-dipole waveforms,
marking a selected depth of the formation as having intrinsic
anisotropy if at the selected depth the far-field slowness in the
fast-shear direction is less than the far-field slowness in the
slow-shear direction and the near-wellbore slowness in the
fast-shear direction is less than the near-wellbore slowness in the
slow-shear direction, and marking a selected depth of the formation
as having stress-induced anisotropy if at the selected depth the
far-field slowness in the fast-shear direction is less than the
far-field slowness in the slow-shear direction and the
near-wellbore slowness in the fast-shear direction is greater than
the near-wellbore slowness in the slow-shear direction.
[0007] Other features and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIGS. 1A and 1B illustrate dispersion curves for different
media.
[0009] FIGS. 2A and 2B are flowcharts illustrating a method of
characterizing shear wave formation anisotropy according to one
embodiment of the invention.
[0010] FIGS. 3A and 3B illustrate a setup for acquiring
crossed-dipole waveforms from a borehole.
[0011] FIG. 4 illustrates near-wellbore and far-field regions for a
borehole.
[0012] FIG. 5 shows a log obtained from Alford Rotation processing
of crossed-dipole waveforms.
[0013] FIG. 6A shows crossed-dipole waveforms obtained at a
selected depth in a borehole penetrating a slow formation.
[0014] FIG. 6B shows a contour plot of slowness vs. time for the
crossed-dipole waveforms of FIG. 6A.
[0015] FIG. 6C shows a log obtained from STC processing of
crossed-dipole waveforms.
DETAILED DESCRIPTION OF THE INVENTION
[0016] The invention will now be described in detail with reference
to a few preferred embodiments, as illustrated in accompanying
drawings. In the following description, numerous specific details
are set forth in order to provide a thorough understanding of the
invention. However, it will be apparent to one skilled in the art
that the invention may be practiced without some or all of these
specific details. In other instances, well-known features and/or
process steps have not been described in detail in order to not
unnecessarily obscure the invention. The features and advantages of
the invention may be better understood with reference to the
drawings and discussions that follow.
[0017] FIG. 2A is a flowchart illustrating a method of
characterizing shear wave formation anisotropy according to one
embodiment of the invention. The method includes acquiring
crossed-dipole waveforms from a borehole penetrating a formation as
a function of frequency and depth in the borehole (200). The method
further includes determining the fast-shear direction or azimuth
(202). Methods for determining the fast-shear direction include,
but are not limited to, Alford Rotation and parametric inversion of
the crossed-dipole waveforms. The slow-shear direction is
orthogonal to the fast-shear direction. The method further includes
determining far-field slowness in the fast-shear and slow-shear
directions (204). The method further includes determining
near-wellbore slowness in the fast-shear and slow-shear directions
(206). For a selected interval of the formation, the method
includes distinguishing between intrinsic and stress-induced
anisotropy by comparing the far-field and near-wellbore slownesses
in the fast-shear and slow-shear directions (208). If the interval
of the formation has intrinsic anisotropy, the shear slownesses in
the fast-shear and slow-shear directions will be consistent from
the near-wellbore to the far-field (i.e., parallel dispersion
curves). If the interval of the formation has stress-induced
anisotropy, the shear slownesses in the fast-shear and slow-shear
directions will not be consistent from the near-wellbore to the
far-field (i.e., crossing dispersion curves). The method of the
invention avoids advanced dispersion analysis by simply comparing
the far-field and near-wellbore slownesses for the fast-shear and
slow-shear directions in the time domain.
[0018] FIG. 3A illustrates a setup for acquiring crossed-dipole
waveforms from a borehole 300 penetrating a subterranean formation
302. The crossed-dipole waveforms are acquired as a function of
frequency and depth in the borehole 300. It should be noted that
only the parts of the setup relevant to the understanding of the
invention are shown and described. The borehole 300 may be a
vertical hole or a deviated hole and is filled with fluid or
drilling mud. A logging tool 304 is disposed in the borehole 300.
For measurement purposes, the logging tool 304 may be conveyed to a
desired depth in the borehole 300 in a number of ways, including,
but not limited to, on the end of a wireline, coiled tubing, or
drill pipe. For illustration purposes, the logging tool 304 is
shown at the end of a wireline 306. The length of the wireline 306
may provide an estimate of the depth of the logging tool 304 in the
borehole 300. The wireline 306 may also be used to provide
communication between the logging tool 304 and a surface system
307. The surface system 307 may include a processor which executes
an algorithm for characterizing shear wave formation anisotropy, as
outlined in FIGS. 2A and 2B.
[0019] The logging tool 304 can. be any tool that can provide
borehole shear slowness along two orthogonal directions, such as
one available under the trade name Dipole Shear Imager (DSI) tool
from Schlumberger. For illustration purposes, the logging tool 304
includes dipole sources 308, 310. The dipole sources 308, 310 are
in orthogonal relation to each other and may or may not be on the
same plane. The logging tool 304 may include an isolation joint 312
to prevent signals from the dipole sources 308, 310 from traveling
up the tool. The dipole sources 308, 310 may be any source suitable
for shear/flexural logging, such as piezoelectric ceramics made in
benders or cylindrical sections, magnetostrictive transducers, and
electrodynamic vibrators. In one embodiment, the dipole source 308
generates flexural waves at a relatively low frequency, and the
dipole source 310 generates flexural waves at a relatively high
frequency. The low and high frequencies are preferably chosen such
that if a dispersion crossover occurs it would be detectible.
However, this does not mean that a dispersion analysis is required
for practice of the invention. On the other hand, existing
dispersion curves can provide general information on radial
gradient of shear slowness, which can assist in selecting operating
frequencies of the dipole sources 308, 310. In general, long
wavelength/low frequency probes deep and short wavelengthigh
frequency probes shallow.
[0020] Preferably, the relatively low frequency of the dipole
source 308 is chosen such that the far-field region of the borehole
300 is probed. Preferably, the relatively high frequency of the
dipole source 310 is chosen such that the near-wellbore region of
the borehole 300 is probed. The depth of investigation is
proportional to the wavelength, which is a function of velocity and
frequency, i.e., .lamda.=v/f, where .lamda. is wavelength, V is
velocity, and f is frequency. Velocity and frequency depend on the
formation characteristics and borehole diameter. FIG. 4 illustrates
a near-wellbore region 400 and a far-field region 402 for a
borehole 404. Generally, the near-wellbore region 400 is about 1/2
borehole diameter, measured radially from the surface 404a of the
borehole 404. If the borehole diameter is 12 in., for example, then
the near-wellbore region 400 would be about 6 in. measured radially
from the surface 404a of the borehole 404. For many formations,
approximately 4-7 kHz would probe the near-wellbore region.
Generally, the far-field region 402 is about 2-3 borehole
diameters, measured radially from the surface 404a of the borehole
404. If the borehole diameter is 12 in., for example, then the
far-field region 402 would be about 24 in. to 36 in. measured
radially from the surface 404a of the borehole 404. For many
formations, approximately 1-3 kHz would probe the far-field region.
However, the invention is not limited to these frequency ranges.
For example, approximately 4-12 kHz could be used to probe the
near-wellbore region, and approximately 1-3.5 kHz could be used to
probe the far-field region.
[0021] Returning to FIG. 3A, the logging tool 304 includes a
plurality of spaced-apart receiver stations 314. As shown in FIG.
3B, each receiver station 314 includes four dipole receivers 314a,
314b, 314c, and 314d. The dipole receivers 314a, 314c form a pair
and are oriented inline with the dipole source 308 and orthogonal
to the dipole source 310, and the dipole receivers 314b, 314d form
a pair and are oriented inline with the dipole source 310 and
orthogonal to the dipole source 308. This arrangement allows
detection of flexural wave signals in the fast-shear and slow-shear
directions. The dipole receivers 314a, 314b, 314c, and 314d may be
any type of dipole transducer that detects pressure gradients or
particle vibrations, such as hydrophones, benders, and
electrodynamic transducers, and is sensitive in the frequency range
of the dipole sources (308, 310 in FIG. 3A). Although this figure
shows just four receivers, the receiver station could consist of
any number of receivers, for example eight receivers arranged
azimuthally with 45 degree separation, thus including the detection
of flexural wave signals from modal decomposition.
[0022] Returning to FIG. 3A, the logging tool 304 also includes an
electronics cartridge 316 which includes circuitry to power the
dipole sources 308, 310 and receiver stations 314 and to process
signals received at the receiver stations 314. Such processing may
include digitizing the separate waveforms received at the receiver
stations 314 and stacking the waveforms from multiple firings of
the dipole sources 308, 310. The electronics cartridge 316 may
further transmit the processed signals to the surface system 307 or
store the processed signals in a downhole memory tool (not shown),
in which case the data can be retrieved when the logging tool 304
is pulled to the surface.
[0023] In operation, the dipole sources 308, 310 emit dipole
acoustic signals which excite flexural wave frequencies in the
formation 302. The dipole receivers 314 detect dipole acoustic
signals from the formation 302. The logging tool 304 rotates in the
borehole 300 so that the dipole sources 308, 310 can be fired at
different azimuthal positions around the borehole 300. The
crossed-dipole waveforms recorded by the dipole receivers 314
generally have a multitude of arrivals, often including a
compressional arrival, a shear arrival, and a flexural mode
arrival. The flexural mode arrival dominates the borehole response
and is dispersive and is most suitable for processing. However,
other modes could be processed as well. Excitation of the borehole
300 at an arbitrary azimuthal orientation results in two shear
waves if anisotropy is present, one propagating as a fast-shear
wave and another propagating as a slow-shear wave.
[0024] Each crossed-dipole waveform received at one of the receiver
stations 314 has four components produced from inline and
orthogonal orientation of each receiver pair (314a, 314c and 314b,
314d in FIG. 3B) with each of the dipole sources 308, 310. The
method according to one embodiment of the invention includes
determining the fast-shear direction or azimuth from these
four-component crossed-dipole waveforms (202 in FIG. 2A). Methods
for determining the fast-shear direction include, but are not
limited to, Alford Rotation and parametric inversion of the
crossed-dipole waveforms. The slow-shear direction is simply
orthogonal to the fast-shear direction.
[0025] Alford rotation is described in, for example, Alford, R. M.,
1986, Shear data in the presence of azimuthal anisotropy: 56.sup.th
Annual International Meeting, Society of Exploration Geophysicists,
Expanded Abstracts, 476-479, and U.S. Pat. Nos. 4,803,666,
4,817,061, 5,025,332, 4,903,244, and 5,029,146, the contents of
which are incorporated herein by reference. Generally speaking,
Alford rotation involves choosing a number of rotation angles,
applying these rotation angles to the four-component crossed-dipole
waveform data, and finding an angle that minimizes the energy in
the mismatched components (or cross-line/off-line components).
[0026] FIG. 5 shows an example of a log produced from Alford
Rotation processing of crossed-dipole waveforms. The raw waveforms
are shown at 500. The difference between minimum and maximum
cross-line energy resulting from the mismatched components, which
is the end result of the Alford Rotation processing for determining
the fast-shear direction, is shown at 502. The fast-shear
direction, which is determined based on the minimization of the
cross-line components, is shown at 504. Track 506 represents the
raw waveforms 500 rotated into the fast-shear and slow-shear
directions. The slow-shear direction is orthogonal to the
fast-shear direction. Track 508 shows the difference between
fast-shear and slow-shear slowness of rotated waveforms. Track 510
shows the difference between fast and slow arrival times of rotated
waveforms. It should be noted that the slownesses are presented
only at low frequencies (1-3 kHz), but the invention involves
Alford Rotation of low- and high-frequency portions of the
crossed-dipole waveforms.
[0027] The method according to one embodiment of the invention
includes determining far-field slowness in the fast-shear and
slow-shear directions (204 in FIG. 2A). Far-field slowness in the
fast-shear and slow-shear directions can be determined from the
low-frequency portion of the rotated crossed-dipole waveforms
using, for example, Slowness-Time-Coherence (STC) analysis, also
known as semblance processing. STC involves identifying and
measuring the slowness and time arrival of coherent energy
propagating across an array of receivers. The technique includes
passing a narrow window across the waveforms received at the
receiver stations and measuring the coherence within the window for
a wide range of slowness and times of arrivals. STC is described
in, for example, Kimball, C. V., Shear slowness measurement by
dispersive processing of the borehole flexural mode: Geophysics,
Vol. 63, No. 2, p. 337-344. The same process can be used to
determine near-wellbore slowness in the fast-shear and slow-shear
directions (206 in FIG. 2A), except in this case STC is applied to
the high-frequency portion of the rotated crossed-dipole waveforms.
FIG. 6A depicts crossed-dipole waveforms at a depth X50 in a
borehole penetrating a slow formation, taken with an eight-receiver
array, with 0.5 ft (0.152 m) spacing between the receivers. FIG. 6B
shows a contour plot of slowness versus time for the crossed-dipole
waveforms shown in FIG. 6A. The slowness versus time is obtained
from STC processing of the crossed-dipole waveforms. FIG. 6C shows
a log produced by STC processing of crossed-dipole waveform data
for the borehole of FIG. 6A for depths X30 to X90. The track 600
represents slowness as a function of depth.
[0028] Once the far-field and near-wellbore slownesses are
determined, the process for distinguishing between intrinsic and
stress-induced anisotropy is quite simple. As previously mentioned,
this involves comparing the far-field and near-wellbore slownesses
in the fast-shear and slow-shear directions (208 in FIG. 2A). The
test is illustrated in FIG. 2B. A depth of the formation is
selected (208a). For the selected depth, if the fast-shear slowness
in the far-field (low frequency) is less than the slow-shear
slowness in the far-field (208b) and if the fast-shear slowness in
the near-wellbore (high frequency) is less than the slow-shear
slowness in the near-wellbore (208c), then the formation at the
selected depth is marked as having intrinsic anisotropy. For the
selected depth, if the fast-shear slowness in the far-field (low
frequency) is less than the slow-shear slowness in the far-field
(208b) and if the fast-shear slowness in the near-wellbore (high
frequency) is greater than the slow-shear slowness in the
near-wellbore (208d), then the formation at the selected depth is
marked as having stress-induced anisotropy. It follows from the
above that the selected interval of the formation is isotropic if
the fast-shear slowness and slow-shear slowness in the far-field
are the same and if the fast-shear slowness and slow-shear slowness
in the near-wellbore are the same. The method may also include
marking a selected depth of the formation as having isotropic
anisotropy.
[0029] The invention typically provides the following advantages.
The method allows continuous processing of crossed-dipole waveform
data to characterize shear wave formation anisotropy. Shear wave
formation anisotropy can be characterized without advanced
dispersion analysis. The fast-shear and slow-shear slownesses in a
stressed-induced anisotropic zone are proportional to the minimum
and maximum horizontal stress, which allows for quantification of
these stresses. This allows for three-dimensional stress inversion
modeling for reservoir stimulation, drilling optimization, and
hydraulic fracture stimulation.
[0030] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
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