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Pore pressure prediction using seismic
Drilling engineers require estimates of the fluid pressures that they are likely to encounter in any given well to anticipate mud weights required to maintain optimal drilling rates and safety. In addition, the locations of anomalous pore-pressure regions are of interest in exploration because they often:
- Correlate with highly productive “sweet” spots in otherwise tight gas sands;
- Provide constraints on basin evolution;
- May correspond to density of open fractures, including bedding-plane fractures.
Seismic velocities for pore pressure prediction
Because seismic velocities correlate with effective pressure in the formation, sufficiently precise estimates of velocity obtained from seismic observations can be used to determine pore pressure. In the absence of dense well control, interval velocities derived from stacking velocities are used to estimate pore pressure. These interval velocities are compared with a general trend of velocities in the region (Fig. 1), and a pore pressure volume is developed for use by drilling engineers, as shown in Fig. 2. Acoustic impedance volumes obtained from seismic trace inversion can also be used to identify and detect anomalous pore pressure regions. In any case, calibration to local velocity-pressure profiles is required. Without this calibration, the pore-pressure indicator is relative rather than absolute, although some empirical relationships exist. The resolution of pore-pressure volumes obtained from seismic interval velocities is fairly coarse, compared to velocities used in detailed migration or tomography, which are somewhat more detailed, as shown by the example in Fig. 3. To meet the need for fine-scale predictions of pore-pressure ahead of the bit, new or improved methods for obtaining reverse VSP data, using the drill bit as a seismic source, and VSP data that uses logging-while-drilling techniques are being developed.
Fig. 1—Geopressure trend curve schematic examples. The left side shows a plot of the seismic interval velocity, determined from stacking velocities, as a function of depth and a smooth monotonically increasing trend line; the right side shows the pore-pressure interpretation from these data. Notice the overpressured zone where the velocity departs from the trend.
Fig. 2—Seismic data and pore pressure predictions. The top diagram shows an interpreted seismic line with faults and specific horizons indicated. The same faults and horizons are also shown on the bottom diagram with predicted pore pressure displayed in color. Notice that the major faults seem to act as pressure barriers in this case.
Fig. 3—Anomalous pore pressure regions resolved using tomography. The upper figure is a pore pressure volume determined from interval velocities that had been calculated from stacking velocities. The lower figure shows the volume after updating with tomographic velocities derived from methods related to pre-stack depth migration. (Original figure, from Sayers et al.,120 provides more detail using a color scale for pressure.)
- Surdam, R.C., Iverson, W., and Jiao, Z. 1996. Natural Gas Resource Characterization Study of the Mesaverde Group in the Greater Green River Basin, Wyoming: A Strategic Plan for the Exploitation of Tight Gas Sands. Final report, Contract No. GRI 96/0220, Gas Research Inst., Chicago.
- Osborne, M.J. and Swarbrick, M.E. 1997. Mechanisms for Generating Overpressure in Sedimentary Basins: A Re-evaluation. AAPG Bull. 81 (6): 1023.
- Japsen, P. 1998. Regional Velocity-Depth Anomalies, North Sea Chalk: A Record of Overpressure and Neogene Uplift and Erosion. AAPG Bull. 82 (11): 2031.
- Pisetski, V.B. 1999. The Dynamic Fluid Method: Extracting Stress Data from the Seismic Signal Adds a New Dimension to Our Search. The Leading Edge 18 (9): 1084.
- Kan, T-K., Kilsdonk, B., and West, C.L. 1999. 3D Geopressure Analysis in the Deepwater Gulf of Mexico. The Leading Edge 18 (4): 502.
- Sayers, C.M., Johnson, G.M., and Denyer, G. 2000. Predrill Pore Pressure Prediction Using Seismic Data. Presented at the IADC/SPE Drilling Conference, New Orleans, Louisiana, 23-25 February 2000. SPE-59122-MS. http://dx.doi.org/10.2118/59122-MS.
- Dutta, N., Gelinsky, S., Reese, M. et al. 2001. A New Petrophysically Constrained Predrill Pore Pressure Prediction Method for the Deepwater Gulf of Mexico: A Real-Time Case Study. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 30 September–3 October. SPE-71347-MS. http://dx.doi.org/10.2118/71347-MS.
- 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.
- 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.
- Miranda, F. et al. 1996. Impact of Seismic ‘While Drilling’ Technique on Exploration Wells. First Break 14 (2): 55.
- Kulkarni, R., Meyer, J.H., and Sixta, D. 1999. Are Pore-Pressure Related Drilling Problems Predictable? The Value of Using Seismic Before and while Drilling. Paper presented at the 1999 Society of Exploration Geophysicists Intl. Exposition and Annual Meeting, Houston, 31 October–5 November.
- 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.
Noteworthy papers in OnePetro
Brunel, C., Dunand, J.P., Rappin, D. et al. 3D Anisotropy Corrections For Pore Pressure Prediction. Presented at the 2004/1/1/.
Lindsay, R.O. and Ratcliff, D.W. Pore Pressure Prediction From 3-D Seismics. Presented at the 1997/1/1/. http://dx.doi.org/10.4043/8337-MS.
Sayers, C.M., Johnson, G.M., and Denyer, G. Pore Pressure Prediction From Seismic Tomography. Presented at the 2000/1/1/. http://dx.doi.org/10.4043/11984-MS.
Sayers, C.M., Johnson, G.M., and Denyer, G. Predrill Pore Pressure Prediction Using Seismic Data. Presented at the 2000/1/1/. http://dx.doi.org/10.2118/59122-MS.
Sayers, C.M. and Woodward, M.J. Enhanced Seismic Pore-Pressure Prediction. Presented at the 2001/1/1/. http://dx.doi.org/10.4043/13044-MS.
Sayers, C.M. and Woodward, M.J. Predrill Pore Pressure Prediction Using 4C Seismic Data. Presented at the 2001/1/1/.
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