Abstract
Abnormal pore pressures, mostly overpressures, exist in many sedimentary formations. The overpressures deteriorate drilling safety and may cause borehole influxes, kicks, and even blowouts. To reduce these risks, origins and mechanisms of abnormal pore pressures are analyzed, including compaction disequilibrium, hydrocarbon generation, smectite diagenesis, formation uplift, and unloading. If formation overpressures were generated by compaction disequilibrium, it is often recognized by higher-than-expected porosities at a given depth and the porosities deviated from the normal porosity trend. Hydrocarbon generation is a process of the creation of mobile fluids from an original solid immobile kerogen, which greatly increases fluid volume or porosity and causes a great increase in pore pressure. Case studies show that hard overpressures created from hydrocarbon generation are consistent with the increase in vitrinite reflectance. In addition, a composite normal compaction trendline for pore pressure prediction is proposed to honor smectite and illite transformation and pore pressure generation. Abnormal pore pressures in several petroleum basins and shale oil and gas plays are examined to illustrate the mechanisms of abnormal pressure generations.
Published Version
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