Gas sorption-induced coal swelling kinetics and its effects on coal permeability evolution: Model development and analysis

- A semi-analytical kinetic swelling model is proposed.
- A swelling kinetics-improved coal permeability model is developed.
- Swelling kinetics can mitigate the gas sorption-induced permeability change.
- Swelling kinetics can benefit ECBM production but harm primary CBM recovery.

Gas sorption-induced coal swelling plays an important role in coal permeability evolution. Experimental measurements have observed the kinetic feature of this swelling and this feature is normally referred to as 'swelling kinetics'. However, existing coal swelling models only consider the pressure-dependence of coal swelling, none coal swelling models have been developed to describe the swelling kinetic behavior. This paper proposes a semi-analytical swelling kinetic model based on the quasi-steady state diffusion model. The proposed model agrees reasonably with the literature swelling kinetic data. A swelling kinetics-improved coal permeability model is then developed by substituting the proposed swelling kinetic model into our previously developed coal permeability model. The swelling kinetics-improved permeability model also agrees well with the literature permeability data. Based on the swelling kinetics-improved permeability model, the effects of the swelling kinetics on coal permeability evolution are evaluated. The results show that the permeability variation also exhibits the kinetic feature due to the effects of the swelling kinetics. The swelling kinetics can mitigate the gas sorption-induced permeability change, both permeability increase induced by gas desorption and permeability decrease induced by gas adsorption, under uniaxial strain conditions. These effects may be favorable for CO2 injection-enhanced coalbed methane (ECBM) production but be detrimental to primary coalbed methane (CBM) recovery. Because of the significance of the swelling kinetics for coal permeability evolution, CBM production, and ECBM production, more efforts are needed to investigate the swelling kinetics of coal in the future.