Modeling anisotropic permeability of coal and its effects on CO2 sequestration and enhanced coalbed methane recovery

Combined with experimental observations, the permeability model adopts a discontinuum medium approach, by which coal is treated as a discontinuum medium containing anisotropic matrixes and cleats. The permeability variations and anisotropic permeability ratios under isotropic net stresses were tested with relatively large coal samples to provide experimental information for model parameterization and validation. The model was further incorporated into a 3D coal reservoir model for reservoir simulations to investigate the impact of the anisotropic permeability of coal on coalbed methane (CBM) recovery. The simulations show good agreements with the experimental data, revealing that the developed model is superior for describing stress- and sorption-induced permeability variations in coals compared with commonly used permeability models that treat coal as isotropic media and use constant values for stress-dependent parameters. The results suggest that anisotropic permeability has significant effects on gas production and CO2 breakthrough time, implying that it is a critical parameter in determining well pattern and orientation of horizontal wells.