Calibration of Effective Pressure Coefficient for the Gulf of Mexico
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Pore pressure prediction provides an important risk assessment in the oil and gas industry. It is widely used to estimate the seal integrity, reservoir pressure compartmentalization, hydrocarbon column height, and to design the optimum well mud weight and casing programs. This work ties pore pressure prediction to estimates of the effective pressure coefficient n. In many pore pressure predictions, the differential pressure rather than the effective pressure is estimated from velocity measurements as a value of 1 is assumed for n. In addition, the estimation of a Normal Compaction Trend (NCT) for shale velocity is compromised because of transitional disequilibrium at shallow depths. To compensate for this, we depth adjust the initial shale velocity NCT. Then, n is calibrated with well-log data in two different Gulf of Mexico (GOM) pressure regimes: compaction disequilibrium and unloading/clay digenesis processes. Values for n in abnormal pressure zones are approximately 0.83. Our pore pressure predictions based on the adjusted shale velocity NCT and the depth-calibrated n are consistent with measured mud weight profiles. For extending the results to areas where there is not sufficient well control to calibrate n, it can be inverted from Gassmann’s equation. S-wave velocity decreases more than P-wave velocity below the onset of abnormal pressure, which is only 90% of Greenberg-Castagna estimation. Overall this study develops a consistent method to predict pore pressure in the GOM by incorporating reliable NCT and variations in the pore pressure coefficient, n, that result from different degrees of consolidation.