Online Journal for E&P Geoscientists

Mujica, Daniel L.^*1; Aldajani, Abdulfattah ²
(1) GPTSD, ARAMCO, Dhahran, Saudi Arabia. (2) Exploration, ARAMCO, Dhahran, Saudi Arabia.

The purpose of this work is to provide a pore pressure prediction workflow and analysis using 3D seismic data. The study area is about 1,600 km2 in size and covered by 3D prestack time migrated (PSTM) data that has been decimated to every 40th inline. The goal of this project is to predict the pore pressure for an unconventional gas play targeting Early Silurian and Late Ordovician sediments. The pore pressure prediction consists of three main steps: data conditioning, data calibration, and pore pressure prediction. Three wells in the area of interest (AOI) are used for calibration of interval velocities and densities.The calibration process includes the use of full log suites,pressure data,and other drilling information. Finally,the predicted 3D pore pressure volume is used to map the pore pressure in the study area and extended into other satellite areas using 2D seismic data. A residual normal move out (NMO) volume is computed from the PSTM gathers based on the use of AVO (Amplitude vs. Offset) attributes. The NMO residual field is used to update the instantaneous velocities and to derive a geologically constrained velocity volume calibrated with checkshot and sonic logs data from the wells. The corresponding comparison with the resulting velocity trend model from the seismic shows how accurately the seismic velocities resemble the log information across the target intervals. A blind test was performed using two recently drilled wells to validate the accuracy of the interval velocity prediction compared to the sonic log velocities.Subsequent integration of the seismic velocities with drilling data at the control wells was conducted by adopting a uniaxial effective stress model with two major compaction trend lines. This model predicted a pore pressure profile close to hydrostatic for most of the stratigraphic column; with a small gradient of overpressure (~ 9.5 - 10.5 lb/gal) in the deeper portion of the zone of interest. For the younger sediments (Early Silurian), the predicted pressure profile ranges from hydrostatic to a maximum of 11.0 lbs/gal with depth, ramping to a hydrostatic profile on the flanks of the structure and getting higher towards the North-Northwest. This study added important insight into pore pressure distribution within the Early Silurian and Late Ordovician sequences.The extracted pore pressure maps highlighted potential drilling hazards, and most importantly, areas with good gas production potential.