Coalbed Methane Production: Why Coal Permeability Matters

Abstract

Abstract When coalbed methane is recovered, complex interactions of stress and chemistry have a strong influence on the properties of the coal. These include influences on gas desorption and flow, coal deformation, porosity change and permeability modification. Although coal-gas interactions have been comprehensively investigated, all of these prior studies usually assume that these interactions are under conditions of invariant total stress where effective stresses scale inversely with applied pore pressures. Here we develop a novel coal permeability model that defines the relations between directional permeabilities and strains, and implement a FE model that couples desorption of the coalbed methane and water flow within a deformable medium (coal) where the effects of deformation are rigorously accommodated. This relaxes the prior assumption that total stresses remain constant and allows exploration of the coalbed methane production under the full range of mechanical boundary conditions from invariant stress to restrained displacement. We apply this model to investigate the productivity of coalbed methane production under in situ conditions. The results show that increasing the absolute permeability could shorten the dewatering stage of the coalbed methane production, but the shape of the production curves does not change with the absolute permeability. For permeability anisotropy coal seams, the fluids mainly flow in face cleats direction and locating more wells in butt cleats direction is high efficient for coalbed methane production. The relative permeability dominates the fractional flow and thus controls the shape of the production curves. It determines when the commercial gas rate could be achieved and how much water should be coped with during the dewatering stage. Therefore both the absolute permeability and relative permeability are key factors for the dewatering stage of the coalbed methane production.