Cleat-scale modelling of the coal permeability evolution due to sorption-induced strain

Abstract

Coal permeability is known to be affected by the sorption-induced strain. Indeed, coal swells with gas sorption and shrinks with desorption, which is likely to modify the cleat aperture and thus the permeability. Coal permeability evolution is crucially important for either Coalbed Methane production (CBM) or Carbon dioxide Capture and Storage (CCS).A model is developed at the scale of the fractures to take into account the hydro-mechanical couplings observed experimentally. It is implemented in the finite element code Lagamine. This numerical model is developed at the scale of the fractures and the matrix blocks. Fractures are modelled with interface elements specially adapted to manage sorption/desorption by taking into account the Langmuir's isotherm (Langmuir, 1918). In the matrix blocks, the sorption strain is assumed proportional to the adsorbed gas content. Depending on the boundary conditions, sorption strain affects the stress state and thus the fracture aperture. Contrary to macroscale models (Bertrand et al. 2017), this model does not require the use of shape factors since the geometry can be explicitly represented. The implementation of the model is validated by comparison with the analytical solution on a simple geometry. The model is finally used to simulate gas injection on a representative elementary volume (REV) made up few matrix blocks. This modelling highlights the interests of a numerical approach to compute the permeability evolution compared to a porosity-based model.