An improved capillary pressure model for coal seam gas reservoirs

Abstract

Coal cleats are the main flow path for gas and water during coal seam gas (CSG) production, and the two-phase flow behaviour in coal cleats is sensitively affected by the capillary pressure. Cleat aperture is a key factor controlling the magnitude of capillary pressure. The deformation of coal cleats is significantly affected by the reservoir depressurization and desorption-induced matrix shrinkage, but there does not appear to be any existing capillary models explicitly considering this effect. In this work, an improved capillary pressure model is developed, the impact of the cleat aperture change on capillary pressure value and residual saturation is considered, which is a key contribution of this work compared to the existing models. The model is then extended to reservoir scale to understand the impact of cleat aperture change on capillary pressure and CSG production performance.The results show that the cleat aperture change can alter the capillary pressure and gas production rate considerably. Larger Young's modulus and Langmuir volumetric strain constant can lead to smaller capillary pressure and higher gas rate as the cleat aperture rise more. Higher Poisson's ratio delivers larger capillary pressure and smaller gas rate as it gives less cleat aperture increase. Influence of Langmuir pressure constant on capillary pressure and production rate transit varies along the production process due to its complex impact on the change of cleat aperture. For the two newly introduced parameters in improved model, the cleat geometry factor can reduce capillary pressure and raise the production rate, while a larger residual saturation fitting factor gives smaller capillary pressure and production rate. Sensitivity analyses are also conducted to evaluate the relative importance of various factors. The findings from this study can help better understanding the impact of cleat aperture change on capillary pressure during CSG production, and is expected to deliver more reliable production estimation.