Multiphysics of coal-gas interactions: the scientific foundation for CBM extraction

Coal permeability is perhaps the most important parameter for the implementation of primary methane recovery and CO-ECBM technology. It is well known that sorption-induced strain and thermal gradients have significant influence on permeability change. In this study, a general form of permeability model is developed, which includes the impact from the geomechanical process, coal swelling/shrinkage, gas pressure change, and thermal change. Then it is extended to apply on three different coal mediums: (1) Unfractured coal; (2) Fractured porous coal media; (3) Fractured coal with rigid matrixes. Based on this permeability model, a set of governing equations is built up, which fully couples the geomechanical deformation, gas and heat flow processes. This coupled model is used to evaluate the influence from different media types on permeability evaluation, gas injection and production performance, and the effective stress change around the wellbore. In addition, numerical simulations under both isothermal and non-isothermal conditions are conducted. Significant influence on the permeability evolution, injection and production performance was observed for both cases. Copyright 2010, Society of Petroleum Engineers