Assessment of Pore Pressure Prediction Methods for Accuracy and Precision Using Geophysical Data: A Review of Balkassar Oil Field, Pakistan

Abstract

Pore pressure prediction is significantly important for successful drilling and completion of exploration/development wells. Unrestrained geopressure conditions can lead to well abandonment which costs billions of dollars’ loss to the oil and gas companies. The Potwar Basin, a significant hydrocarbon-producing basin of Pakistan, experiences severe drilling problems owing to abnormal pressures reported in the subsurface. Geopressure analysis is carried out in this study for a constrained pressure prediction model by integrating well and seismic data. The demarcation of structural geometry is critical in understanding abnormal pressures for which seismic interpretation is performed. Log-based pore pressure prediction using Eaton’s, Bower’s and Equivalent Depth method is then employed on the wells selected from the study area to identify the overpressure intervals and check which method provides the best of its accuracy. The predicted pressures are calibrated with the measured pressure of Balkassar OXY-01 well to validate the results. Seismic inversion followed by neural network analysis is then applied to determine the spatial and vertical pressure variations within the reservoir zone. A combination of linear regression method with Probabilistic Neural Network (PNN) is applied to predict 2D pressure sections. Based on the results of the log-based pore pressure prediction, Eaton’s and Equivalent Depth Method provided promising results having well-constrained predicted geopressure curves validated with the measured pressure. However, pressure predicted from Bower’s method showed significant variations. Therefore, predicted pressure curves from Eaton’s and Equivalent Depth Method were further used to compute the seismic-based pressure section. The results showed significant overpressured intervals within the post-Neogene sediments whereas, pre-Neogene carbonate rocks exhibit underpressured to hydrostatic conditions in the study area.Pressure estimation using Bower’s Method reached around 5000 psi in Balkassar OXY-01; 5500 psi in Balkassar OXY-02 and 5000 psi in Balkassar POL-01 in the murree formation, but with low precision. Eaton’s Method provided a more consistent result of 4200 psi in Balkassar OXY-01; 4700 psi in Balkassar OXY-02 and 4400 psi in Balkassar POL-01, while the Equivalent Depth Method estimated pressure at approximately 4100 psi Balkassar OXY-01; 4800 psi in Balkassar OXY-02 and 4500 psi in Balkassar POL-01.