Pore pressure prediction in orthotropic medium based on rock physics modeling of shale gas

Abstract

Pore pressure prediction is critical to reducing drilling costs and exploring hydrocarbons. Current seismic approaches for pore pressure prediction mainly initially establish the relation between vertical effective stress and physical parameters (velocity, resistivity or porosity), and then predict pore pressures with the known effective stress theory. The commonly used Eaton's equation requires the estimation of normal compaction trend (NCT), which cannot be measured but only be inferred. However, the conventional NCT is a curve which is able to describe the normal trend but is relatively rough, limiting the accuracy of pore pressure prediction. In this study, the shale rock physics modeling in orthotropic (OA) medium is designed to equate the NCT. The modeled and the measured P-wave velocity match well in normal pore pressure segments, but they differ in abnormal pore pressure segments. The modeled NCT is utilized to predict pore pressures, and the comparison with the measured pore pressures proves that the accuracy can be improved. Then P-wave velocity is inverted from azimuthal seismic data based on the reflectivity approximation of OA medium, and the pore pressure of the research area is predicted using the modeled P-wave velocity (the target play is a shale gas field in Sichuan basin of China). The prediction result is consistent with the characteristics of overpressure mechanism of hydrocarbon generation, indicating the effectiveness of rock physics modeling in pore pressure prediction.