The coalbed methane (CBM) extraction usually begins with dewatering the coal seams to reduce the reservoir pressure. Then the gas desorbs from coal matrix into coal cleats and flows towards production wells. The CBM extraction requires a thorough understanding of the interactions among gas desorption, transport, capillary pressure and coal deformation. Although this issue has been investigated comprehensively in recent years, their combined impact is still poorly understood. There are two reasons for this: one is a lack of effective permeability models that take the effective stress changes into consideration, and the other is the mechanical influences are not rigorously coupled with the gas transport and water flow system. In this work, such permeability models are developed and implemented into a fully coupled finite element (FE) model of coal deformation, water flow and gas transport. The FE model represents important non-linear responses due to the effective stress effects that cannot be recovered where mechanical influences are not rigorously coupled with the water flow and gas transport system. The FE model is verified through the history matching of the gas and water production profiles in the Powder River Basin, and applied to forecast the gas production for a series of hypothetical production scenarios.
Coalbed methane (CBM) has become an important source of unconventional energy around the world. Forecast of gas production from coal seams relies on the understanding of the multiple physical and chemical interactions in the water-gas-coal system. The effective permeability is one of the most important parameters which determine the gas production. Coal seams are uniformly fractured media which are composed of the matrix and the cleat networks. The coal matrix is the main storage site for coalbed methane which is tightly adsorbed on the coal surface.
