The purpose of this study is to provide a pore pressure prediction from seismic data targeting unconventional gas plays (i.e., the Qusaiba Member hot shale and the underlying Sarah Formation). The area of interest (AOI) located in NW Saudi Arabia, and is covered by1,600km2 of 3D pre-stack time migrated (PSTM) data. The pore pressure prediction workflow consists of three main steps: pre-stack seismic data conditioning, seismic velocity calibration with sonic and density logs from offset wells, calibrated effective stress vs. velocity relationship and stratigraphic model-based pore pressure prediction.
Three wells in the AOI are used to calibrate the interval velocities and densities prior to pore pressure prediction. In addition, formation pressures from the Repeat Formation Tester (RFT) tool, mud weights and drilling history are incorporated to constrain the predicted pore pressures. As a result, the pore pressure gradient variations in the Qusaiba Member hot shale and the Sarah Formation are mapped in the AOI. 2D seismic lines are used in this study to extrapolate pore pressure prediction results to satellite well locations outside the 3D seismic area. The Pore Pressure model predicts a pore pressure profile close to hydrostatic for most of the stratigraphic column with a small gradient of overpressure (~ 9.5 to 10.5 ppg) in the deeper portion of Qusaiba near to the base. Within the Qusaiba formation, the predicted pressure profile ranges from hydrostatic to a maximum of 11.0 ppg, with depth, transitioning to a hydrostatic profile to the flanks of the structure.
The resulting pore pressure maps for the Qusaiba Member hot shale and Sarah Formation highlight potential drilling hazards. Importantly, areas of higher pore pressure may ultimately represent potential areas of higher gas production (i.e., sweet spots). This work is the first application of pore pressure prediction in unconventional resources in Saudi Aramco, opening the door for more comprehensive efforts to predict pore pressures in other areas targeting shale gas and other unconventional plays in Saudi Arabia.
This article is a joint effort with my co-author Dr. Abdulfattah Aldajani (Saudi Aramco); and it was published at the 2013 SEG Annual Meeting, Houston, TX.