Estimation of Pressure-Dependent Diffusive Permeability of Coal Using Methane Diffusion Coefficient: Laboratory Measurements and Modeling

Gas diffusion in coal is critical for the prediction of coalbed methane (CBM) production, especially for the late-stage CBM reservoir when both gas pressure and permeability are relative low. Using only Darcy permeability to evaluate the quality of gas transport might not be effective. Diffusive flow can be the dominant flow at low reservoir pressures. In this work, the methane diffusion coefficient was measured for pulverized San Juan sub-bituminous and Pittsburgh bituminous coal samples using the classic unipore model and particle method. The diffusion coefficient results showed a negative correlation with pressure, as reported previously. The significance of diffusion flow is strongly related to the rate of the methane ad-/desorption process, more severely in the low pressure range [<2 MPa (280 psi)]. The measured diffusion coefficient can be converted to the equivalent permeability. This equivalent permeability, termed diffusive permeability, can be considered as the contribution of diffusion flow, in terms of Darcy permeability, used to evaluate the total gas flow in the late production decline stage when matrix flow dominates. As expected, this diffusive permeability was found to be much lower than the fracture/cleat permeability. An increasing trend at low pressure due to pressure drop was obtained and coincides with sorbing gas permeability behavior when the pressure is extremely low.