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A New Method for Predicting Pore Pressure in Shale Gas Reservoir

Abstract

Pore pressure in shale gas reservoirs may change dramatically within a short depth interval increasing from hydrostatic pressure at a shallow depth to being over-pressured deeper in the formation. Without accurate estimation of pore pressure, drilling through these over-pressured zones is very troublesome for drilling safety and formation evaluation. However, the conventional prediction techniques for pore pressure do not work well in shale gas formations, because the effect of hydrocarbon generation and the impact of gas and organic matter on acoustic velocity. Through a review of conventional methods, pitfalls related to several challenges in describing over-pressured shale gas formation are drew as following three sections:(1) most of prediction methods are established on the foundation of mechanical compaction theory, which can only estimate overpressures generated by disequilibrium compaction;(2) during industry practice, the model parameters can't be easily obtained from traditional measurements;(3) the effect of gas and organic matter on acoustic velocity are not included in these methods. Consequently, a new method based on acoustic velocity model was proposed for predicting pore pressure in shale gas reservoir. Combined with a large amount of statistical experimental data, the acoustic velocity model was built by fully considering the influence of porosity, organic content and effective stress on acoustic velocity. The effect of gas to overpressure's imprints on sonic well logs can be avoided via this new method, because it corrects compressional velocity by using an appreciated Vp-Vs relation. Effective stress (difference of overburden stress and pore pressure) can be calculated from the acoustic velocity model with a key input of corrected compressional velocity. Thus, pore pressure will be easily predicted when overburden stress is obtained from traditional integration of density log data. This new method is applicable for estimating overpressures generated by disequilibrium compaction, hydrocarbon generation and some other mechanisms in shale gas formations. Besides, performance results show that it produced highly consistent results with pore pressures measured from MDT(Modular Formation Dynamics Tester). Evidently, it solves the limitations of those techniques that are established on mechanical compaction theories, and is more successful in shale gas reservoirs compared to the conventional methods.