Excess pore pressure behavior and evolution in deep coalbed methane reservoirs

Abstract

Deep coalbed methane (DCBM), an unconventional gas reservoir, has undergone significant advancements in recent years, sparking a growing interest in assessing pore pressure dynamics within these reservoirs. While some production data analysis techniques have been adapted from conventional oil and gas wells, there remains a gap in the understanding of pore pressure generation and evolution, particularly in wells subjected to large-scale hydraulic fracturing. To address this gap, a novel technique called excess pore pressure analysis (EPPA) has been introduced to the coal seam gas industry for the first time to our knowledge, which employs dual-phase flow principles based on consolidation theory. This technique focuses on the generation and dissipation for excess pore-water pressure (EPWP) and excess pore-gas pressure (EPGP) in stimulated deep coal reservoirs. Equations have been developed respectively and numerical solutions have been provided using the finite element method (FEM). Application of this model to a representative field example reveals that excess pore pressure arises from rapid loading, with overburden weight transferred under undrained condition due to intense hydraulic fracturing, which significantly redistributes the weight-bearing role from the solid coal structure to the injected fluid and liberated gas within artificial pores over a brief timespan. Furthermore, field application indicates that the dissipation of EPWP and EPGP can be actually considered as the process of well production, where methane and water are extracted from deep coalbed methane wells, leading to consolidation for the artificial reservoirs. Moreover, history matching results demonstrate that the excess-pressure model established in this study provides a better explanation for the declining trends observed in both gas and water production curves, compared to conventional practices in coalbed methane reservoir engineering and petroleum engineering. This research not only enhances the understanding of DCBM reservoir behavior but also offers insights applicable to production analysis in other unconventional resources reliant on hydraulic fracturing.