Dual poroelastic response of coal seam to CO₂ injection

Although the influence of gas sorption-induced coal deformation on porosity and permeability has been widely studied, these studies are all under the invariant total stress condition. According to the principle of effective stress, the induced coal deformation is determined by the change in effective stress, which can be replaced by the change in pore pressure, under the assumption of null change in total stress. This is why terms representing effective stress or total stress are absent in all of these existing permeability models. In our previous work (Zhang et al, 2008), this assumption was relaxed and a new porosity and permeability model was derived. The FE model was also applied to quantify the net change in permeability, the gas flow, and the resultant deformation in a coal seam. In this work, the general porosity and permeability model was modified to represent both the primary medium (coal matrix) and the secondary medium (fractures), and implemented into a fully coupled coal deformation, CO₂ flow and transport in the matrix system, and CO₂ flow and transport in the fracture system model. The novel dual-poroelastic model was applied to quantify the mechanical responses of coal seam to the CO₂ injection under in situ stress conditions. The simulation results illustrate how the CO₂ injectivity is controlled both by the competition between the effective stress and the gas transport induced volume change within the matrix system and by the dynamic interaction between the matrix system and the fracture system.