Overpressure Modelling in High Pressure, High Temperature HP/HT Reservoirs: A Case Study Using Wells and Seismic from Field in Niger Delta

Exploration onshore Niger Delta is moving into deeper HP/HT plays where pore pressure prediction is challenging due to secondary overpressure mechanisms and lack of data for calibration. Accurate pore pressure prediction is required to ensure safe and economic drilling campaign. This study aims at finding acceptable parameters and building models for predicting pore pressure where secondary overpressure mechanisms are observed in addition to undercompaction. Well logs and 3D seismic data were used to understand the evolution of pore pressure in the field. This was established from offset well data by making cross-plots of acoustic velocity versus density, temperature vs depth, geothermal gradient computation, computation of normal compaction trends, overburden gradients, shale pore pressure and fracture gradients. Then cube models were built for the field. Post-drilling pore pressure and fracture gradient estimation of eleven offset wells were done using Eaton's and Matthews & Kelly's methods respectively. Velocity analysis results from 3D seismic data were then used to predict 1D pore pressure models at six selected prospect locations. Analyses reveal depths shallower than 3800m TVD/MSL with geothermal gradient 3.0°C/100m and pressure gradient less than 1.50sg EMW are affected mainly by undercompaction and depths greater than 3800m TVD/MSL with geothermal gradient of 4.1°C/10m and pressure gradients reaching 1.82sg – 2.12sg EMW are affected by unloading. A general regression in pore pressure between 2000 – 3600m TVD/MSL interval were due to depletion by ongoing production from the developed reservoirs. The results of pore pressure modeling reveal a narrow drilling margin for the HP/HT prospect intervals.