Modeling of anisotropic sorption-induced coal deformations

Gas sorption induced volumetric strain of coal is one of the primary geomechanics parameters in coalbed methane reservoirs. The apparent bulk volumetric strain model is proposed to define the coal volumetric deformations considering combined effects of gas sorption induced coal matrix swelling strain, mechanical compression induced strain, and cleat volume strain induced cleat compressibility. Based on transversely isotropic assumption, the proposed model can effectively describe the anisotropy characteristics strain evolution of coal by involving geomechanics parameters including two Young’s moduli and three Poisson’s ratios. The unconstrained coal deformation experiments were carried out. The experimental data agrees well with the evolution model under the specified hydrostatic boundary condition. The results also show that the volumetric strains are highly related to the adsorption capacities of gases. Furthermore, the positive matrix swelling strain induced by adsorption dominates during low pressure period and thereby the volumetric strain will continuously increase with the pressure increasing at this stage. As the pressure continuously increases, the volumetric strain will decrease under high pressure due to the dominant mechanical compression effects and even the volumetric strain may become negative value at highly enough pressure. Moreover, the sensitivity study shows that the anisotropy elastic properties mainly affect the volumetric response at high pressure stage.