Mass Transfer Characteristics of Coalbed Methane: Experimental and Numerical Simulation Insights

Summary During coalbed methane (CBM) extraction after hydraulic fracturing, gas production involves a multiscale mass transfer process in which CBM migrates from matrix micropores to conductive hydraulic fractures. To investigate CBM transport characteristics in coupled pore-fracture systems, we conducted series and parallel seepage experiments using combinations of coal samples with different pore-structure attributes and performed numerical simulations of matrix/fracture composite media using a representative elementary volume (REV)–scale lattice Boltzmann method (LBM). Furthermore, based on nuclear magnetic resonance (NMR) measurements on water-saturated coal columns under confining pressure, we developed permeability prediction models for different stimulated zones as well as for the overall production region. Results show that the flow rates in both series and parallel seepage pathways decay exponentially with confining pressure. Hydraulic fractures dominate the transport pathway, whereas the overall flow intensity is strongly limited by matrix mobility and supply capacity. Increasing the areal/volumetric proportion of the hydraulic fracture domain can improve the effective reservoir permeability. For deep, low-permeability CBM reservoirs, maintaining adequate fracture connectivity while enlarging the stimulated reservoir volume and increasing the fracture/matrix contact area to strengthen matrix supply and exchange can substantially enhance CBM production efficiency.