Numerical prediction of in situ horizontal stress evolution in coalbed methane reservoirs by considering both poroelastic and sorption induced strain effects

As an organic-rich dual-porosity medium, coal distinctively manifests unique deformation and permeability evolution behaviors due to the adsorption/desorption induced matrix swelling and/or shrinkage. Being different from the elastic strain, the sorption induced strain shows a non-linear behavior with continuous sorbing gas injections, and these two types of strains are competing process of the volumetric change for coal. With primary gas depletion, the horizontal stress decreases within coalbed methane (CBM) reservoirs due to the in situ boundary confinement, namely, uniaxial strain boundary condition. However, the coal solid skeleton deformative behavior with primary depletion is not specifically clear due to the difficulty both in field and lab measurement of horizontal stress. This paper, based on a gas-solid coupling model incorporated with swelling effect, focuses on the horizontal stress variation with respect to the primary depletion under uniaxial strain condition. It was found the simulated results of volumetric change under the hydrostatic condition well agree with experimental data. The increase of horizontal stress for methane is four times as for helium, with injected pore pressure at 7.6 MPa. Also, the conceptual dual-material model is proposed to demonstrate the competing process of compression and swelling, which leads to the bulk modulus change of coal. It was found the Biot's coefficient can be larger than unity and be a variable during the primary depletion due to the swelling induced equilibrium negative bulk modulus.