Relations between coal permeability and directional strains and their application to San Juan Basin

In this study a permeability model is developed to define the evolution of gas sorption-induced permeability anisotropy under the full spectrum of mechanical conditions spanning prescribed in-situ stresses through constrained displacement. In the model, gas sorption-induced coal directional permeabilities are linked into directional strains through an elastic modulus reduction ratio, R_m. This defines the ratio of coal mass elastic modulus to coal matrix modulus (0<R_m<1) and represents the partitioning of total strain for an equivalent porous coal medium between the fracture system and the matrix. Where bulk coal permeability is dominated by the cleat system, the portioned fracture strains may be used to define the evolution of the fracture permeability, and hence the response of the bulk aggregate. The coal modulus reduction ratio provides a straightforward index to link anisotropy in deformability characteristics to the evolution of directional permeabilities. The validity of the model is evaluated against results for special cases representing uniaxial swelling, constant volume reservoirs, and for the case of ten coalbed methane production wells in San Juan Basin.