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Overpressure prediction using acoustic logging

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Abnormal pressure (overpressure) conditions in the subsurface can pose significant drilling hazards if not detected. This article discusses how acoustic logs can help to identify overpressure situations.

Importance of identifying overpressure

Abnormal pressure is defined as any departure from normal hydrostatic pressure at a given depth.[1] Abnormal subsurface pressures, either overpressure (geopressure) or underpressure, are encountered in hydrocarbon basins throughout the world in all lithologies, from all geologic ages, and at all depths.[2]

Early and reliable detection of geopressure is vital to avoid or mitigate potential drilling and safety hazards, e.g.:

  • Shallow water flow
  • Blowouts
  • Shale instability

During drilling, advanced warning of approaching geopressuring enables the mud weight to be adjusted to avoid well and reservoir damage and to determine casing points. This is a particular concern in deepwater wells in which the pressure difference; i.e., the operating window, between the hydrostatic gradient and the fracture gradient can be very narrow.

Causes of overpressure

Geopressuring in hydrocarbon reservoirs may result from a variety of geologic and tectonic processes.[2][3][4] Borehole-acoustic detection methods using compressional and shear slowness can identify abnormally pressured zones before they are drilled and can quantify pressure gradients. These methods are used in[5][6][3][7][8][9][10][11][12][13][14][15][16]:

  • Conventional borehole logging (wireline and logging while drilling [LWD])
  • New seismic-while-drilling techniques
  • More recently, surface seismic data

Undercompaction is the primary mechanism for creating overpressure, particularly in deltaic basins in which high rates of deposition commonly prevent the escape of pore water trapped in shales. Undercompacted shales have higher acoustic transit times (i.e., higher apparent porosity) than normally pressured shales at the same depth.[17][18][19][20]

Detecting overpressure with acoustic methods

With the onset of overpressuring, a semi-logarithmic plot of acoustic slowness with depth will diverge from a normal (hydrostatic) straight-line trend of decreasing slowness (increasing velocity) with depth (Fig. 1).

  • Fig. 1 – Semi-log plot illustrating acoustic detection of geopressure by use of shale slowness[17] (courtesy of SPE).

    Fig. 1 – Semi-log plot illustrating acoustic detection of geopressure by use of shale slowness[17] (courtesy of SPE).

The "normal," or hydrostatic, trend for the well, which may vary with different geologic provinces, is defined by plotting slowness values for shale beds (> 10-ft thickness) in the well. A constant overburden gradient of 1.0 psi/ft is generally assumed. The difference in acoustic slowness between the normal and abnormal trends can be converted to an equivalent fluid-pressure gradient and formation pressure for a given depth (Fig. 2). This method may not be applicable or effective in all areas or where undercompaction is not the mechanism for overpressuring.[21][22] More-general approaches to determination of pore pressure and fracture gradient use an effective-stress, rock-mechanics approach.[21][23][24]

  • Fig. 2 – Relationship between fluid-pressure gradient (FPG) and the acoustic slowness difference for U.S. Gulf Coast[17] (courtesy of SPE).

    Fig. 2 – Relationship between fluid-pressure gradient (FPG) and the acoustic slowness difference for U.S. Gulf Coast[17] (courtesy of SPE).

Recent investigations into the effects of pressure on shale porosity suggest that the relationship is more complex than previously thought. While additional study is necessary, the results to-date suggest that it may be necessary to reconsider or revise the well-log methods currently used in pore pressure and exhumation analysis (see Geological applications of acoustic logging).[16][25][26][27]

References

  1. ↑ Bruce, B. and Bowers, G. 2002. Pore Pressure Terminology. The Leading Edge 21 (2): 170–173. http://dx.doi.org/10.1190/1.1452607
  2. ↑ 2.0 2.1 Fertl, W.H., Chapman, R.E., and Holz, R.F. eds. 1994. Studies in Abnormal Pressure, 1-454. Amsterdam: Elsevier, Developments in Petroleum Science No. 38.
  3. ↑ 3.0 3.1 Bowers, G.L. 2002. Detecting High Pressure. The Leading Edge 21 (2): 174–177.
  4. ↑ Chilingar, G.V., Serebryakov, V.A. and Robertson, J.O. Jr. eds. 2002. Origin and Prediction of Abnormal Formation Pressures, 1-390. Amsterdam: Elsevier, Developments in Petroleum Science No. 50.
  5. ↑ Hsu, K. et al. 1997. Interpretation and Analysis of Sonic While Drilling Data in Overpressured Formations, paper FF. Trans., 1997 Annual Logging Symposium, SPWLA, 1–14.
  6. ↑ Underhill, W., Esmersoy, C., Hawthorn, A. et al. 2001. Demonstrations of Real-Time Borehole Seismic From an LWD Tool. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 30 September-3 October. SPE-71365-MS. http://dx.doi.org/10.2118/71365-MS
  7. ↑ Evans, B.J. 1999. Developments in Pore Pressure Prediction Using Seismic-While-Drilling Technology. Australian Petroleum Production and Exploration Assn. J. 39 (Part 1): 461–474.
  8. ↑ Walls, J., Dvorkin, J., Mavko, G. et al. 2000. Use of Compressional and Shear Wave Velocity for Overpressure Detection. Presented at the Offshore Technology Conference, Houston, Texas, 1-4 May. OTC-11912-MS. http://dx.doi.org/10.4043/11912-MS
  9. ↑ Badri, M.A., Sayers, C., Hussein, R.A. et al. 2001. Pore Pressure Prediction Data Using Seismic Velocities and Log Data in the Offshore Nile Delta, Egypt. Presented at the SPE Middle East Oil Show, Bahrain, 17-20 March 2001. SPE-68195-MS. http://dx.doi.org/10.2118/68195-MS
  10. ↑ Carcione, J.M. and Tinivella, U. 2001. The Seismic Response to Overpressure—A Modelling Study Based on Laboratory, Well and Seismic Data. Geophysical Prospecting 49 (5): 523–539.
  11. ↑ Huffman, A.R. 2001. The Future of Pore-Pressure Prediction Using Geophysical Methods. Presented at the Offshore Technology Conference, Houston, Texas, 30 April-3 May. OTC-13041-MS. http://dx.doi.org/10.4043/13041-MS
  12. ↑ Sayers, C.M. and Woodward, M.J. 2002. Seismic Pore-Pressure Prediction Using Reflection Tomography and 4-C Seismic Data. The Leading Edge 21 (2): 188–192. http://dx.doi.org/10.1190/1.1452611
  13. ↑ Citta, F. et al. 2004. Deepwater Hazard Avoidance in a Large Top-Hole Section Using LWD Acoustic Data. The Leading Edge 23 (6): 566–573. http://dx.doi.org/10.1190/1.1766236
  14. ↑ Althoff, G., Cornish, B., Varsamis, G. et al. 2004. New Concepts for Seismic Surveys While Drilling. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 26-29 September 2004. SPE-90751-MS. http://dx.doi.org/10.2118/90751-MS
  15. ↑ Mancilla-Castillo, J., Mendez-Hernández, E., and Santana-Fernández, J. 2003. Geopressure Evaluation from seismic data and its application for exploratory wells in Mexico. Presented at the Offshore Technology Conference, Houston, Texas, 5-8 May. OTC-15250-MS. http://dx.doi.org/10.4043/15250-MS
  16. ↑ 16.0 16.1 Storvoll, V., Bjorlykke, K., and Mondol, N.M. 2005. Velocity-Depth Trends in Mesozoic and Cenozoic Sediments from the Norwegian Shelf. AAPG Bulletin 89 (3): 359–381.
  17. ↑ 17.0 17.1 17.2 Hottmann, C.E. and Johnson, R.K. 1965. Estimation of Formation Pressures from Log-Derived Shale Properties. J Pet Technol 17 (6): 717-722. SPE-1110-PA. http://dx.doi.org/10.2118/1110-PA
  18. ↑ Ham, H.H. 1966. A Method of Estimating Formation Pressures from Gulf Coast Well Logs. Trans., Gulf Coast Assn. of Geological Soc. 16: 185–197.
  19. ↑ Japsen, P. 1999. Overpressured Cenozoic Shale Mapped from Velocity Anomalies Relative to a Baseline for Marine Shale, North Sea. Petroleum Geoscience 5: 321–336.
  20. ↑ Draou, A. and Osisanya, S.O. 2000. New Methods for Estimating of Formation Pressures and Fracture Gradients from Well Logs. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 1-4 October 2000. SPE-63263-MS. http://dx.doi.org/10.2118/63263-MS
  21. ↑ 21.0 21.1 Bowers, G.L. 1995. Pore Pressure Estimation From Velocity Data: Accounting for Overpressure Mechanisms Besides Undercompaction. SPE Drill & Compl 10 (2): 89–95. SPE-27488-PA. http://dx.doi.org/10.2118/27488-PA
  22. ↑ Swarbrick, R.E. 2001. Pore-Pressure Prediction: Pitfalls in Using Porosity. Presented at the Offshore Technology Conference, Houston, Texas, 30 April-3 May. OTC-13045-MS. http://dx.doi.org/10.4043/13045-MS ↑ Eaton, B.A. 1975. The Equation for Geopressure Prediction from Well Logs. Presented at the Fall Meeting of t
  23. ↑ Eaton, B.A. 1975. The Equation for Geopressure Prediction from Well Logs. Presented at the Fall Meeting of the Society of Petroleum Engineers of AIME, Dallas, 28 September–1 October. SPE 5544. http://dx.doi.org/10.2118/5544-MS
  24. ↑ Eaton, B.A. and Eaton, T.L. 1997. Fracture Gradient Prediction for the New Generation. World Oil 218 (10): 93–94 and 96–100.
  25. ↑ Lahann, R.W., Conoco, McCarty, D.K. et al. 2001. Influence of Clay Diagenesis on Shale Velocities and Fluid-Pressure. Presented at the Offshore Technology Conference, Houston, Texas, 30 April-3 May. OTC-13046-MS. http://dx.doi.org/10.4043/13046-MS
  26. ↑ Domnesteanu, P., McCann, C., and Sothcott, J. 2002. Velocity Anisotropy and Attenuation of Shale in Under- and Overpressured Conditions. Geophysical Prospecting 50 (5): 487–503.
  27. ↑ Dewhurst, D.N., Siggins, A.F., and Raven, M.D. 2002. Influence of Pore Pressure, Composition and Microstructure on the Acoustic Properties of Shales. Presented at the SPE/ISRM Rock Mechanics Conference, Irving, Texas, 20-23 October 2002. SPE-78197-MS. http://dx.doi.org/10.2118/78197-MS

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See also

Acoustic logging

Acoustic logging tools

Pore pressure prediction using seismic

Methods to determine pore pressure

Log analysis in shaly formations

PEH:Acoustic_Logging

Category

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