Permeability evolution mechanism in deep coalbed methane extraction: Considering the competitive effects of adsorption-induced swelling, creep, and aperture compression

During gas extraction from deep coal, the rock endures high effective stress, with both the time-dependent deformation and anisotropic structure of the rock controlling the permeability evolution. To reveal this phenomenon, a numerical simulation framework of the finite volume method and transient embedded discrete fracture model is proposed to establish a new constitutive model that links poroelastoplastic deformation, adsorption-induced swelling, and aperture compression. From this model, anisotropic permeability tensors were derived to further achieve the simulation of coevolution. Meanwhile, our permeability model was verified against the measured permeability data, and the history match of the numerical model showed better results where the mismatch was less than 5 %. The results indicate that (1) the long-term permeability evolution clearly showed the competitive effects of multiple deformation mechanisms, which involves three stages: compaction-dominated decline, adsorption-dominated rebound, and creep-controlled loss. (2) The increased number of compressible cleats/fractures accelerated the initial permeability decline, while the increased desorption-induced strain promoted faster rebound and enhancement and higher viscosity coefficients enhanced the creep effect, which led to significant long-term permeability loss. (3) Massive hydraulic fracturing created a larger drainage area, accelerating methane desorption and causing sharp permeability rebound with reduced residual gas, which shows that the permeability remained higher than the initial values even after the extensive extraction via the fractured horizontal wells. The permeability evolution mechanisms displayed varying properties, such as coal rank and burial depth, and distinct characteristics. A precise understanding of multiple competitive stress effects is crucial for optimizing coalbed methane extraction techniques and improving recovery efficiency.