The effect of stress, pressure and temperature on CBM migration with elastic–plastic deformation

Abstract

The laws relating to changes in permeability in coal reservoirs is central to the simultaneous exploitation of coal and gas. Under the condition of multi–field influences caused by stress, pressure, and temperature, elastic–plastic deformation in the matrix–fracture is complex, which has made it difficult to analyze the coalbed methane (CBM) migration law. The coal matrix is deformed by gas adsorption and thermal expansion. However, deformation in the coal matrix is not equal to fracture deformation, meanwhile, the coal mining process causes obvious plastic deformation. Therefore, in this work, the sensitivity coefficient (Sf) was used to quantify fracture deformation, and the sensitivity coefficient of fracture number to the damage value (ζ) was specified to quantify matrix deformation. Further, a coal reservoir permeability model under elastic–plastic deformation was established. The proposed permeability model was verified by laboratory test data under different boundary conditions. On this premise, the contribution of effective stress and adsorption to permeability was discussed, with the influencing factors of Sf being analyzed, and a sensitivity analysis of Sf and ζ was studied. The results revealed that the permeability model proposed in this work can predict the change law of permeability with elastic–plastic deformation effected by stress, pressure and temperature. The theoretical results from the proposed permeability model were helpful in understanding the law relating to changes in permeability in a complex geological environment during the simultaneous exploitation of coal and gas.