Making sense of the L-shaped pore pressure ramps in Brunei: Compression-induced vertical fluid flow in the basin and its impact on the petroleum system

Abstract

Overpressure prediction for exploratory drilling has become robust in most basins with increasing well control, high-quality seismic datasets, and proactive real-time overpressure monitoring while drilling. However, accurate overpressure prediction remains challenging in offshore Northwest Borneo despite several decades of drilling experience. This paper focuses on two exploration wells drilled by Brunei Shell Petroleum 40 years apart that faced similar challenges with overpressure prediction and well control. An integrated lookback study is attempted using seismic and well-log data to explore the causes of the unsatisfactory Pore Pressure Prediction (PPP) outcome in pre-drill and real-time operation settings for these wells. Our study indicates that the misprediction of overpressures is due to real differences in shale pressure (basis of pre-drill work and monitoring) and sand pressure (source of drill kick and well control challenges) due to large-scale vertical leak or expulsion of deep-seated fluids into pre-compacted normally pressured overlying sediments in several regions through a mix of shear and tensile failure mechanisms. Such migrated fluids inflate the sand pressure in the normally compacted shallower sequences with the shale pressure remaining low. A predictive framework for upward fluid expulsion was attempted but found impracticable due to complex spatial and temporal variations in the horizontal stress field responsible for such leakage. As such, it is proposed that these migratory overpressures are essentially ‘unpredictable’ from conventional PPP workflows viewed in the broad bucket of compaction disequilibrium (undercompaction) and fluid expansion (unloading) mechanisms. Further study is recommended to understand if such migrated overpressures in the sand can produce a discernible and predictable geophysical or petrophysical signature in the abutting normally compacted shales. The study highlights the possibility of large lateral variability in the sand overpressure within the same stratigraphic unit in regions with complex tectonostratigraphic evolution like Northwest Borneo.