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Integration of Geomechanical Modeling and Seismic Data to Predict Pore Pressure and Stress in Complex Subsurface Settings

Abstract

We couple field-scale geomechanical modeling with seismic velocity data to improve prediction of pore pressure and stress in a complex geologic setting. Most pore pressure prediction methods assume that sediments deform only by vertical, uniaxial strain, and therefore pore pressure is driven only by the vertical (overburden) stress. In complex settings, such as near faults, folds, and salt or where tectonic shortening or extension is present, lateral strains and stresses and their contribution to pore pressure are significant. In our Full-Effective-Stress (FES) seismic pore pressure prediction approach, we include the contribution of all stress components to pore pressure. Pore pressure is calculated as the difference of total and effective stresses. Total stresses are estimated from a geomechanical model, and effective stresses are estimated from seismic velocity using a new relationship that we develop between velocity and effective stresses. This relation is calibrated at a control well and then used to calculate effective stresses across the field. Total stresses estimated by the geomechanical model depend on the pore pressure field that is input into the model. We iterate the pore pressure prediction process until the predicted and the input pore pressure fields converge. This enables our integrated approach to explicitly provide an estimate of the full stress state that is consistent with the predicted pore pressures. We apply our method along with the standard, Vertical-Effective-Stress (VES) method to a salt basin beneath the Sigsbee Escarpment in the Mad Dog field, Gulf of Mexico. Salt and basin bathymetry are shown to substantially perturb the stress field, producing a large lateral stress in sediments on the side of the salt and downdip of the escarpment. The difference between pressures predicted by the VES and FES methods at a point increases as the stress state at that point differs more from the stress state at the same burial depth at the control well. The FES method predicts pore pressures measured along a subsalt well better than the VES method. The FES and VES methods predict significantly different minimum stress and drilling window along a vertical profile near salt. Our FES method could improve strategies for hydrocarbon search, drilling and well placement, and seismic imaging and processing.