Influence of gas adsorption induced non-uniform deformation on the evolution of coal permeability

When adsorbing gas is injected into coal, the gas fills in the fractures quickly and a pressure difference between matrix and fractures is created. Because of this difference, there is a pressure gradient within the matrix. The gradient evolves with time from the initial equilibrium (zero gradient) to the final equilibrium (zero gradient), so does the adsorption-swelling induced matrix deformation. Previous studies have not taken this effect into consideration. In this study, we hypothesize that the pressure gradient affects the expansion of the gas-invaded area/volume with the matrix and the propagation of the expansion front creates a non-uniform deformation within the matrix. Under this hypothesis, a relation between coal permeability and the expansion of the gas-invaded area with the matrix can be established. When the gas-invaded area is localized in the vicinity of the fracture wall, the expansion of the matrix within this area narrows the fracture opening. We define this as local swelling/shrinking. This local swelling/shrinking is controlled primarily by the coal internal structure. When the gas-invaded area is further spread over the matrix, the expansion of the whole matrix may narrow or widen the fracture opening depending on the external boundary conditions. This global swelling/shrinking is controlled primarily by the external boundary conditions. These conceptual understandings are defined through strain rate-based coal permeability models for both the matrix and the fractures. This strain rate based time-dependent permeability model was verified against experimental observations, that couples coal deformation, the gas flow in the matrix system and gas flow in the fracture system.