Development of anisotropic permeability during coalbed methane production

Although coal-gas interactions have been comprehensively investigated, prior studies have focused on one or more component processes of effective stress in driving only isotropic changes in coal permeability. In our previous work (Wu et al., 2009) a general porosity and permeability model was developed to represent the behavior of both the primary medium (coal matrix) and the secondary medium (fractures) under conditions of variable stress. In this study the permeability model is extended 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. These models are implemented into a fully coupled model of coal deformation, gas flow and transport in the matrix system, and gas flow and transport in the fracture system. The model separately accommodates compressible gas flow and transport in the coal matrix and fracture systems and rigorously accommodates the role of mechanical deformations for a dual-porosity continuum. Since mechanical interactions and the role of sorption-induced strains are rigorously accommodated, these micro-mechanical models are capable of following the evolution of porosity and permeability in both the coal matrix and the fracture network. This 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 transport system. The permeability model for fractures is verified against the analytical solution for a constant volume reservoir and applied to successfully match a suite of field data from the San Juan Basin. The fully coupled model for coal deformation and gas flow has been applied to quantify the impacts of fracture spacing and in situ ground stresses on coal properties and active processes: those involving shrinkage, swelling and direct changes in effective stress. Model results demonstrate the complex interactions of fracture-matrix induced by CBM production.