How sorption-induced matrix deformation affects gas flow in coal seams: A new FE model

The influence of sorption-induced coal matrix deformation on the evolution of porosity and permeability of fractured coal seams is evaluated, together with its influence on gas recovery rates. The porosity-based model considers factors such as the volume occupied by the free-phase gas, the volume occupied by the adsorbed phase gas, the deformation-induced pore volume change, and the sorption-induced coal pore volume change. More importantly, these factors are quantified under in situ stress conditions. A cubic relation between coal porosity and permeability is introduced to relate the coal storage capability (changing porosity) to the coal transport property (changing permeability). A general porosity and permeability model is then implemented into a coupled gas flow and coal deformation finite element model. The new FE model was used to compare the performance of the new model with that of the Palmer–Mansoori model. It is found that the Palmer–Mansoori model may produce significant errors if loading conditions deviate from the assumptions of the uniaxial strain condition and infinite bulk modulus of the grains. The FE model was also applied to quantify the net change in permeability, the gas flow, and the resultant deformation in a coal seam. Model results demonstrate that the evolution of porosity and of permeability is controlled by the competing influences of effective stresses and sorption-based volume changes. The resulting sense of permeability change is controlled by the dominant mechanism.