New permeability model considering multiscale migration mechanism of deep coalbed methane and its application

Deep coalbed methane (CBM) flow exhibits various transportation mechanisms within the multiscale pore structures of reservoirs, including continuous flow, Knudsen diffusion, and surface diffusion. Current research predominantly emphasizes the effects of individual or partial flow mechanisms and single-factor influences on the multiscale migration of CBM. We proposed a new apparent permeability model that integrates multiple flow mechanisms to enhance our understanding of the factors governing CBM flow in complex fractured networks. This model accounted for stress sensitivity, adsorbed gas desorption, and matrix shrinkage. By assigning appropriate weights to different flow mechanisms, the model yielded a more accurate representation of the deep CBM apparent permeability, avoiding the overestimation resulting from the linear superposition of diverse migration mechanisms. Our findings indicated that the apparent permeability was positively correlated with compressibility and negatively correlated with the tortuosity and Poisson's ratio. In the presence of the adsorbed gas, the apparent permeability of organic matter showed heightened sensitivity to formation pressure, rock compressibility, and tortuosity. However, the impacts of these factors became less pronounced when the pressure differential was small. The proposed model was applied to the flow simulations for a multi-fractured horizontal well within a deep coal reservoir characterized by a complex fracture network. The simulation results agreed well with the production data. We found that continuous flow was the dominant contributor to the apparent permeability of organic and inorganic matter within the coal rock, followed by Knudsen diffusion and surface diffusion. This study provided insights into the evolution of apparent permeability of CBM during development and offered valuable guidance for the analysis of CBM production dynamics, productivity forecasting, and production system design.