Seismic Pore Pressure Estimation In Unconventional Reservoirs

Summary Pore pressure impacts decisions related to drilling, hydraulic fracturing, and hydrocarbon production, so is a key parameter for economic development of unconventional reservoirs. Pore pressure is, however, difficult to measure in low-permeability rocks typical of unconventional shale resources. In environments like the Gulf of Mexico, where young Tertiary shales dominate the sedimentary column, pore-pressure prediction based on seismic interval velocities often gives good results. Unfortunately, changes in lithology in unconventional reservoirs often occur over vertical depth intervals too small to be resolved by seismic velocities, since these typically have poor vertical resolution. Prestack seismic inversion, however, provides estimates of P-impedance and the ratio of vertical P-velocity to vertical S-velocity (VP/VS) that have greater vertical resolution than seismic velocities obtained by kinematic analysis. This presentation shows how pore pressure may be estimated using P-impedance and VP/VS obtained from inversion of seismic amplitude variation with offset (AVO) data. Using P-impedance (product of P-velocity and density) is conceptually similar to using seismic velocity. However, it offers greater vertical resolution and is more sensitive to pore pressure changes since velocity and density tend to respond in the same direction to changes in pore pressure. P-impedance is, however, sensitive to changes in rock composition. Since VP/VS also varies with composition, a seismic-inversion-to pore-pressure transform based on both P-impedance and VP/VS allows the influence of variable lithology and mineralogy on predicted pore pressure to be reduced and a more reliable estimate of pore pressure to be obtained.