Abstract:
Precise geopressure predictions in petroliferous sedimentary basins remains the most challenging job. Pore pressure prediction is significantly important for successful drilling and completion of exploration/development wells. Uncontrolled geopressure conditions can lead to well abandonment which costs billions of dollars to the petroleum industry. The Potwar Basin, a significant hydrocarbon producing basin of Pakistan, experiences severe drilling problems owing to abnormal pressures reported in the Murree Formation. Geopressure analysis is carried out in this study for a constrained pressure prediction model by integrating well and seismic data. The demarcation of structural geometries is critical in understanding abnormal pressures for which seismic interpretation is performed. Log-based pressure prediction using Eaton’s method is then employed on the wells selected from these structural compartments to identify the overpressure intervals. The predicted pressures are calibrated with the measured pressure/DST data to validate the results. Seismic inversion followed by neural network analysis is then applied to determine the spatial and vertical pressure variations within the Murree Formation. A combination of linear regression method with Deep Feedforward Neural Network (DFNN) is applied to predict 2D pressure sections. Based on the evidence of undercompaction, geohistory analysis is finally performed to determine overpressuring mechanism. Based on the variations of deformation mechanisms, eastern Potwar has been categorized into three different zones. Log-based pressure prediction revealed overpressuring in high porosity sands of Murree Formation. Due to excessive pressure at shallow depths exceeding 4000 psi, the Qazian field experiences critical drilling conditions. Pindori field exhibits highest pressures where Murree Formation encounters significant overburden stress at 2400 m and pore pressures exceeds 6000 psi. 2D pressure sections via DFNN showed an average correlation coefficient of 92% which
extended up to 97% in some fields. Geohistory analysis revealed that Murree Formation has undergone two subsidence phases during Neogene times due to high sedimentation influx. This resulted in a low porosity reduction rate due to the inability of fluid to escape out of the pores. The geopressure analysis revealed that compaction disequilibrium is the primary cause of overpressure conditions in response to rapid rate of sedimentation within Murree Formation. However, deformation mechanisms and the existence of various structural geometries also contributed to the generation of abnormal pressures.
