Numerical simulation of stress and strain due to gas sorption/desorption and their effects on in situ permeability of coalbeds

Most past studies on coal shrinkage/swelling due to gas adsorption/desorption were based on experiments under no constraint conditions. In this paper, the changes of stress and strain measured on one coal specimen under uniaxial compression in a vacuum and under axial constraint conditions during CO, adsorption are presented and numerically simulated. The simulation results show that a linear elastic deformation model, suitable to isotropic continuum media but widely assumed in analytical permeability models, cannot adequately simulate the deformation behaviour of coal mass even under uniaxial compression in a vacuum. The equivalent continuum medium (ECC) model considering the discontinuities of coal mass is successfully applied to simulate the deformation behaviour of the specimen for the uniaxial compression case and the axial constraint case before the occurrence of shear failure in the specimen. A detailed review of the analytical permeability models is presented and their limitations in application are discussed in this paper. The permeability, in situ stress, and production simulated with two representative analytical permeability models are compared with those calculated using the discontinuum medium coupled (DMC) permeability model and the coupled simulation. The results indicate that the DMC model provides better estimates of permeability and production than the analytical permeability models because it considers the influence of many factors such as the discontinuities and anisotropies that are ignored in analytical permeability models.