To design well paths that minimize risk and avoid drilling hazards such as blowouts and stuck pipe, drilling engineers would like to have a quantitative understanding of the overpressure zones in the subsurface. Currently, pre-drill prediction of pore pressure is done using kinematically determined seismic velocity, which has a low resolving power in identifying various subsurface formations. In the rare examples where high-resolution velocity is used, the primary seismic input is inverted acoustic impedance. The acoustic impedance is converted into high-frequency velocity and density for effective stress and overburden stress computations. Both require transformation schemes, potentially causing additional uncertainty in pore-pressure prediction. In this paper, we present a method based directly on acoustic impedance. We thus avoid the additional, potentially error-prone step of converting impedance to velocity and density. We modify the methodology described in Rasolofosaon and Tonellot (2011). We call this the RT method in this paper. Using well log data, we first demonstrate that the RT method provides practically the same results as those using velocity and density data at the well location, and does it more efficiently. This leads us to suggest that the formation pore pressure itself can be written as a piece-wise continuous function of a single variable, acoustic impedance. This greatly simplifies the work steps in pore-pressure prediction methodology. This new method is then applied to well and seismic data in deepwater Gulf of Mexico (GoM) subsalt basins, predicting subsalt and salt-exit pore pressure. We compare the predicted results with measured pore-pressure data where available.

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A target-oriented, predrill, pore-pressure prediction method using seismically estimated acoustic impedance is applied to recently acquired and processed dual-coil data in the Green Canyon and Walker Ridge areas of the Gulf of Mexico. Three deepwater subsalt wells are used in the study. One was used as a calibration well, and the other two were used as blind wells for comparing prediction results with the measured pore-pressure and mud-weight data. All three wells are in the Green Canyon area, penetrating thick salt bodies. A new seismic-inversion method is used for inversion of seismic impedance. The pore-pressure method uses a direct transform of the inverse of acoustic impedance, with two adjustable parameters. The optimization of the parameters is done through an iterative process to match the pore-pressure gradient obtained from the well acoustic impedance with those of pore-pressure measurements and mud weights within a tolerable range of those data for the calibration well. The optimized parameters are then used to transform the seismic acoustic-impedance volume to pore-pressure gradient volume. The predicted pore pressure at the blind wells from the well impedance and seismic impedance match reasonably with well data.

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