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Anisotropy analysis

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[1]Formation anisotropy—the directional variation of physical properties—can be the result of either of the following processes:

  • Depositional processes (intrinsic)
  • Tectonic processes (stress-induced)

Formation anisotropy is evidenced through variations in:

  • Permeability
  • Rock strength
  • Fractures
  • Borehole failure

Acoustic logging can provide insight on these variations that assist in anisotropy analysis.

Formation anisotropy

In acoustic/seismic terms, intrinsic anisotropy is structural in nature and is commonly seen as transverse isotropy (TI or vertical transverse isotropy, VTI), in which properties differ in the vertical or horizontal planes, such as in shales or thinly bedded intervals.[2] Stress-induced anisotropy is known as azimuthal anisotropy (or horizontal transverse isotropy, HTI), in which acoustic parameters in a vertical borehole vary with azimuthal orientation, such as the case of fractures parallel to the borehole.

Analyses of in-situ anisotropy (primarily stress induced) are made using direct or derived shear-wave velocity and provide the magnitude and azimuth of anisotropy (i.e., direction of the maximum and minimum horizontal stresses as well as an indication of their difference). Anisotropy analysis has been widely used in solid-earth seismology, geothermal studies, and more recently, in exploration geophysics (see summaries in Crampin and Chastin[3] and Helbig and Thomsen[4]).

In the petroleum industry, these results are used in well design and well placement:

  • For optimum reservoir drainage[5]
  • To detect and characterize faults and fractures in openhole and cased hole[6]
  • To predict borehole instability and sand production,[7]
  • For optimizing the design and evaluation of well completions (perforations and hydraulic fracturing) [See section below on Crossed-dipole anisotropy analysis]

Compressional-, shear- and Stoneley-wave properties are each affected, to some degree, by the presence and type of formation anisotropy. While the shear-wave response to azimuthal anisotropy (see the section below on Crossed-dipole anisotropy analysis) and Stoneley waves to VTI anisotropy is well known, the effect on compressional-wave energy is less well characterized.[8][9][10][11] In some situations, information from additional measurements, such as borehole images, dip logs, or both, may be necessary for relating the measured anisotropy to geological features.

Borehole acoustic azimuthal and VTI anisotropy analysis is an advance made possible by the recent introduction of new inversion methods.[12][13][14][15][16][17][18][19] These methods use the crossed-dipole shear to derive azimuthal anisotropy and the Stoneley wave to derive TI anisotropy in slow formations, or a combination of these modes in deviated wells. A reasonable shear velocity can be derived using inversion techniques with low-frequency Stoneley-wave dispersion which is sensitive to the horizontal shear (in contrast to the dipole’s sensitivity to the vertical shear).[20][21][22]

Several new applications have been made possible by anisotropy analysis:

  • Identification of formation alteration using dipole-shear dispersion[23][14][24][25]
  • Stress estimation,[16]
  • Distinguishing between intrinsic and stress-induced anisotropy using dispersion crossover[26][27][28][29]

Anisotropy analysis can also be conducted using shear-wave parameters derived from Stoneley-wave dispersion.[19][30]

Crossed-dipole anisotropy analysis

In anisotropic media, shear waves (both monopole refracted and dipole flexural) split into orthogonally polarized components having different velocities. This is known as[31][32][33][34]:

  • Shear-wave splitting
  • Shear-wave birefringence
  • Shear-wave velocity anisotropy

The difference in fast and slow shear-wave slowness provides a measure of the magnitude of anisotropy. Shear-wave splitting is useful for evaluating:

  • Fractures
  • Faults
  • Bedding planes that intersect the borehole at an angle
  • Unbalanced tectonic stresses perpendicular to the borehole

Because the flexural waves induced by dipole-acoustic sources have a directional component, the use of mutually perpendicular (crossed) pairs of dipole transmitters and receivers can detect and measure dipole shear-wave splitting.[35][36][37][38] In isotropic formations, both receiver components (X and Y) will measure the same shear-wave arrival time. However, in an azimuthally anisotropic formation, the shear wave measured at the X-component of the receiver pair will be different from the one measured at the Y-component; one is called the fast shear component, the other, the slow shear component (Fig. 1). To measure the fast and slow shear-wave slowness, four components are measured (Fig. 2):

  • Two inline components, X-X and Y-Y
  • Two crossline components, XY and YX

The dipole flexural shear mode is affected by a variety of factors including formation anisotropy. Significant borehole ellipticity (the result of borehole failure or breakouts)[39] and high relative inclination between the borehole and formation[1][40] may result in erroneous interpretation of dipole-derived anisotropy and must be accounted for during data processing.[41] Additionally, the presence of shale anisotropy in high-angle and horizontal wells, can significantly influence compressional velocity, which must be corrected for this effect.[42]

Initially, mathematical rotation methods originally developed for use in processing surface seismic data[43] were used to determine the fast-wave direction together with the slowness values for the fast and slow shear wave. More recently, inversion methods[44] are being applied to the four data sets to simultaneously determine the azimuth and magnitude of the anisotropy (Fig. 3).

An anisotropy map (Fig. 4) combines the derived average anisotropy and its azimuth to generate an azimuthal image. This display facilitates interpretation by allowing the analyst to quickly assess depth intervals of interest by looking at the brightness, direction, and continuation of the features on the map. The map also facilitates comparison with borehole-image logs. Rose diagrams (Track 3) provide an accurate indication of the fast shear azimuth over each labeled depth interval. The integration of monopole and dipole measurements yields improved estimates of[22][45]:

  • Anisotropy
  • Magnitude of anisotropy
  • Permeability

Horizontal stress and hydraulic fracturing

Fractures, both natural and hydraulically induced, develop in relation to regional or localized stress patterns and play a major role in optimizing production and reservoir drainage. An estimate of the magnitude and azimuth of the horizontal stresses surrounding a borehole is needed for accurately placing wells to take advantage of existing fracture patterns and for artificially inducing fracture patterns during well completion through hydraulic stimulation.[5][46]

The stresses operating on a rock formation are described by a triaxial coordinate system that consists of:

  • Two principal horizontal stresses, σx and σy
  • A vertical stress component, σz, which is the overburden.

In a borehole, these downhole stresses are expressed as radial components at the borehole wall:

  • The vertical component σz
  • The radial component σr
  • The tangential component σθ
  • The (azimuthal) shear component, σ

Unbalanced-formation-stress components produce distortion around the borehole (stress-induced anisotropy) that results in shear-wave splitting. The azimuth of the fast-shear wave parallels the direction of maximum horizontal stress and the azimuth of the slow-shear wave parallels the direction of the minimum horizontal stress. Hydraulic-fracture azimuth is parallel to the direction of maximum horizontal stress.[47]

Hydraulic well stimulation consists of perforating an interval, then packing and pressuring these perforations to create fractures behind casing to allow increased production. Crossed-dipole anisotropy logging can estimate the vertical extent of the formation-stress fracture along the borehole and its azimuth in the formation (Fig. 5).[48][16][17][18][38][49][50][51][52][53][54]

References

  1. 1.0 1.1 De, G.S. and Schmitt, D.P. 2005. Issues With Shear-wave Azimuthal Anisotropy in Highly Deviated Wells. Presented at the Offshore Technology Conference, Houston, Texas, 2-5 May. OTC-17647-MS. http://dx.doi.org/10.4043/17647-MS
  2. Wang, Z. 2002. Seismic Anisotropy in Sedimentary Rocks, Part 1—A Single-Plug Laboratory Method; Part 2—Laboratory Data. Geophysics 67 (5): 1415–1440. http://dx.doi.org/10.1190/1.1512787
  3. Crampin, S. and Chastin, S. 2003. A Review of Shear Wave Splitting in the Crack-Critical Crust. Geophysical J. Intl. 155: 221–240.
  4. Helbig, K. and Thomsen, L. 2005. 75-plus Years of Anisotropy in Exploration and Reservoir Seismics: A Historical Review of Concepts and Methods. Geophysics 70 (6): 9ND–23ND. http://dx.doi.org/10.1190/1.2122407
  5. 5.0 5.1 Franco, J.L.A., de la Torre, H.G., Ortiz, M.A.M. et al. 2005. Using Shear-Wave Anisotropy To Optimize Reservoir Drainage And Improve Production In Low-Permeability Formations in the North of Mexico. Presented at the SPE Annual Conference and Technical Exhibition, Dallas, Texas, 9–12 October. SPE-96808-MS. http://dx.doi.org/10.2118/96808-MS
  6. Klimentos, T. 2003. Shear-Wave Anisotropy Applications for Perforation Strategy and Production Optimization in Oil Bearing Porous Sands, paper LL. Trans., 2003 Annual Logging Symposium, SPWLA, 1–12.
  7. Klimentos, T., Farghaly, A., and Qleibo, M. 2003. Finding Faults with Shear-Wave Anisotropy, paper F. Trans., 2003 Annual Logging Symposium, SPWLA, 1–12.
  8. Furre, A.-K. and Brevik, I. 1998. Characterization of Angle Dependency in Sonic Logs, paper BH 2.1. Expanded Abstracts, 1998 Annual Meeting Technical Program, SEG, 292–295.
  9. Hornby, B., Howie, J., and Ince, D. 1999. Anisotropy Correction for Deviated Well Sonic Logs—Application to Seismic Well Tie, paper BH/RP 4.7. Expanded Abstracts, 1999 Annual Meeting Technical Program, SEG, 112–115.
  10. Wang, Z. 2001. Seismic Anisotropy in Sedimentary Rocks, paper RP 2.4. Expanded Abstracts, 2001 Annual Meeting Technical Program, SEG, 1740–1743.
  11. Sun, Y. et al. 2003. Effects of Stress-Induced Anisotropy on Monopole and Dipole Logging, paper RBG P1.2. Expanded Abstracts, 2003 Annual Meeting Technical Program, SEG, 1282–1285.
  12. Koster, K. et al. 1994. Applied Production Geophysics Using Shear-Wave Anisotropy—Production Applications for the Dipole Shear Imager and the Multicomponent VSP, paper DP1.1. Expanded Abstracts, 1994 Annual Meeting Technical Program, SEG, 233–235.
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  17. 17.0 17.1 Badri, M., Sousa, S., and Klimentos, T. 2000. Shear Anisotropy Applications in Production Optimization, Western Desert, Egypt, paper RPB 1.5. Expanded Abstracts, 2000 Annual Meeting Technical Program, SEG, 1695–1698.
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  19. 19.0 19.1 Tang, X.M. 2003. Determining Shear-Wave Transverse Isotropy from Borehole Stoneley Waves. Geophysics 68 (1): 118–126. http://dx.doi.org/10.1190/1.1543199
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  23. Plona, T. et al. 2002. Mechanical Damage Detection and Anisotropy Evaluation Using Dipole Sonic, paper F. Trans., 2002 Annual Logging Symposium, SPWLA, 1–14.
  24. Murray, D., Plona, T., and Valero, H.P. 2004. Case Study of Borehole Sonic Dispersion Curve Analysis, paper BB. Trans., 2004 Annual Logging Symposium, SPWLA, 1–14.
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  26. Plona, T.J., Winkler, K.W., Sinha, B.K. et al. 1998. Measurement of Stress Direction and Mechanical Damage Around Stressed Boreholes Using Dipole and Microsonic Techniques. Presented at the SPE/ISRM Rock Mechanics in Petroleum Engineering, Trondheim, Norway, 8-10 July 1998. SPE-47234-MS. http://dx.doi.org/10.2118/47234-MS
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  49. Cipolla, C.L., Liu, D., and Kyte, D.G. 1994. Practical Application of In-Situ Stress Profiles. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 25-28 September 1994. SPE-28607-MS. http://dx.doi.org/10.2118/28607-MS
  50. Cipolla, C.L. 1996. Hydraulic Fracture Technology in the Ozona Canyon and Penn Sands. Presented at the Permian Basin Oil and Gas Recovery Conference, Midland, Texas, 27-29 March 1996. SPE-35196-MS. http://dx.doi.org/10.2118/35196-MS
  51. De, G.S., Winterstein, D.F., Johnson, S.J. et al. 1998. Predicting Natural or Induced Fracture Azimuths From Shear-Wave Anisotropy. SPE Res Eval & Eng 1 (4): 311-318. SPE-50993-PA. http://dx.doi.org/10.2118/50993-PA
  52. Garg, A., Desai, A.M., and Towler, B.F. 1997. Horizontal Stresses in Anisotropic Formation and Prediction of Hydraulic Fracture Direction. Presented at the SPE Western Regional Meeting, Long Beach, California, 25-27 June 1997. SPE-38342-MS. http://dx.doi.org/10.2118/38342-MS
  53. Aquila, F.J., Barajas, J.S., Mesa, H. et al. 2003. Using Cross Dipole Sonic Anistropy Data to Improve Reservoir Understanding in the Southern/Marine Areas of Mexico. Presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, 5-8 October 2003. SPE-84204-MS. http://dx.doi.org/10.2118/84204-MS
  54. Barajas, J.S., Patino, A.H., Garcia, E.R. et al. 2004. Case History - Cased Hole Dipole Sonic Applications in Mexico. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 26-29 September 2004. SPE-90703-MS. http://dx.doi.org/10.2118/90703-MS

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See also

Fracture identification with acoustic logging

Hydraulic fracturing

PEH:Acoustic_Logging