Modeling Pore Pressure, Fracture Pressure and Collapse Pressure Gradients in Offshore Panna, Western India: Implications for Drilling and Wellbore Stability

Abstract

Pore pressure modeling has proven and direct implications in oil and gas exploration and development. Abnormal pore pressure leads to drilling complexity and well control issues because of reduced mud window, contributing to major non-productive times and steep drilling cost. A comprehensive pore pressure–fracture pressure model plays a critical role in successful well drilling. This work caters to the pressure modeling of Panna area in Mumbai offshore basin, western India. Two offshore wells, drilled through 4 km of Tertiary sedimentary succession down to Cretaceous basaltic basement, were analyzed to interpret the vertical stress, pore pressure, fracture gradient and collapse pressure. Vertical stress profile was generated from density logs; pore pressure was estimated using Eaton’s method by employing resistivity and sonic logs. Calculated pore pressure was calibrated with various direct downhole measurements and various well events. Compaction disequilibrium was inferred as key mechanism for generating mild overpressure in Oligocene to early Miocene shales (14–15 MPa/km), while it increases sharply against early Eocene sediments, and hard overpressure with near-lithostatic gradient (22 MPa/km) was detected in the underlying Paleocene shales. The mid-Eocene Bassein formation, the primary hydrocarbon reservoir, reveals sub-hydrostatic condition (7.5 MPa/km) resulting from production-related depletion. Estimated fracture pressure was calibrated with available leak-off test data. Mohr–Coulomb rock failure criterion was employed to estimate collapse pressure and validated with the observations from caliper log to address the wellbore stability issues. This study provides insights on downhole pressure behavior across stratigraphy, to achieve optimum drilling mud designing as well as safe and successful operational planning.