A new method for predicting shale pore pressure: A case study of the Bodong Depression in the Bohai Bay Basin

Abstract

Objective China's oil and gas exploration is advancing towards the field of shale oil and gas. Overpressure is commonly developed in organic shale, and the lack of prediction methods for shale pore pressure restricts the research on shale oil and gas. Methods This article analyses the physical response characteristics of rock to hydrocarbon generation, pressurization and disequilibrium compaction. Based on density and sound velocity data, the differential responses of hydrocarbon generation pressurization and disequilibrium compaction pressurization were analysed. The hydrocarbon generation pressurization amount is calculated using sound velocity and density, and the disequilibrium compaction is calculated using the density data. Finally, the sound velocity rebound method was established. This article selects the Bodong Depression, which has superior hydrocarbon generation conditions, as a case study. First, comprehensive mudstone compaction curves, loading-unloading curves, and sound velocity-density diagrams are used to comprehensively identify the cause of overpressure. Then, the sound velocity rebound method is used to quantitatively calculate the pressure increase and formation pore pressure of a single well under compaction and hydrocarbon generation, and the results are compared with those of numerical simulations and conventional calculation methods. Results Taking the LD21-A well as an example, shale overpressure is caused by a combination of hydrocarbon generation and disequilibrium compaction. The pressure prediction results show that the overpressure of nonsource rock layers is contributed by the disequilibrium compaction. The overpressure of the source rock formation is contributed by hydrocarbon generation and disequilibrium compaction, with hydrocarbon generation pressurization mainly distributed between 5 MPa and 15 MPa (accounting for approximately 35% to 65%), and the measured pressure point hydrocarbon generation pressurization is 11.09 MPa (accounting for 45%). Conclusion This new method is vital of importance for the research of both shale and conventional oil and gas fields.

Keywords

• sound velocity rebound method
• overpressure generation by hydrocarbon generation
• disequilibrium compaction
• overpressure
• overpressure mechanism
• pressure prediction
• shale pore
• bodong depression