The Effect of Sorption-Induced Shrinkage on the Ground Surface Movement Above Gas-Producing Coalbeds

During production from gas reservoirs the formation pressure decreases, leading to increased levels of effective stress. As a result, the reservoir rock is compressed and deformed, leading to displacement of overlaying formations, which can propagate all the way to the ground surface. In unconventional reservoirs such as coalbeds, the production of gas is associated with desorption of gas from the coal matrix, which induces relatively large volumetric changes in addition to the pressure decrease. While reservoir models consider this phenomenon when investigating the variation of the fracture aperture and permeability, mechanical models usually either neglect or oversimplify its influence on the bulk volumetric response of the coal and consequently the displacement of the ground surface. In order to address this issue, a finite element model has been constructed, in which a horizontal poro-elastic hydrogeological profile is subjected to depressurisation in combination with a time-dependent Richard’s equation and linear elasticity. The effect of sorption-induced shrinkage is included in the model using the definition of and the concurrent effects of shrinkage on both permeability and bulk deformation. The results presented in this paper show that implementation of sorption-induced shrinkage can have a significant impact on the modelled geomechanical response of the coalbed and the estimated ground surface subsidence.