Pore-Pressure Prediction in the Permian Basin Using Seismic Prestack Inversion

摘要

Although pore pressure is difficult to measure in low permeability rocks such as shale, it nevertheless has an enormous impact on drilling, hydrocarbon production and geomechanics applications such as hydraulic fracturing and wellbore stability analysis. Pore pressure is, thus, a key parameter for economic development of unconventional reservoirs, for which a predrill pore-pressure estimate is essential. In such reservoirs, changes in lithology occur over vertical depth intervals too small to be resolved using seismic velocities derived by means of normal moveout (NMO) analysis or kinematic inversion because these velocities typically have poor vertical resolution. By comparison, inversion of reflection amplitudes yields higher vertical resolution and greater sensitivity to reservoir properties, due to the influence of density in addition to that of velocity. We present an approach for predicting pore pressure based on P-impedance and the ratio V_P/V_S of the vertical P-velocity, V_P, to vertical S-velocity, V_S, obtained from inversion of seismic amplitude variation with offset (AVO) data. Employing P-impedance is analogous to using seismic velocity in conventional seismic-based pore-pressure prediction. However P-impedance from AVO inversion has higher vertical resolution and greater sensitivity than velocities from kinematics. Use of V_P/V_S allows partial compensation for the influence of variable lithology and mineralogy on P-impedance.