Modeling the effects of gas slippage, cleat network topology and scale dependence of gas transport in coal seam gas reservoirs

Gas flow in coal cleats plays a significant role for the permeability of coal seams. Investigations of gas transport in coal are carried out in this work using a numerical scheme with a coal sample scanned by the micro-computed tomography. We analyse a coal sample from the Bowen Basin (Australia) to characterize the natural discrete fracture network defined by connected cleats and matrix blocks. Due to the heterogeneity of coal micro-structures, the numerical simulator is combined with a grid-adaptive scheme. The accuracy of the simulator is validated for gas slip regime (0.01 < Kn < 1.0) in the micro-straight channel. Predictions of permeability and porosity are conducted through the method. The results show that the porosity and permeability of the digitalized coal model change along with the length and stabilizes at > 400 voxels and > 700 voxels, respectively. The Knudsen number (Kn) has an important effect on the gas slippage which can enhance the coal seam gas transport for narrow cleat networks. The increase of Kn ranging from 0.0005 to 0.01 increases the mass flux as well as the permeability of the coal for a given cleat aperture. The investigations of the gas transport in cleats prove the capability of the numerical simulator for modeling gas slip flow. It also provides a potential tool for solving gas transport in the complex geometry of a natural coal seam gas (CSG) reservoir.