Quantifying and modeling of coal permeability spatiotemporal response: Implications for gas recovery and CO₂ sequestration

Coal permeability features the long- term sustained evolution during methane recovery and CO₂ sequestration. Surprisingly, little is known about how permeability responds to the changing reservoir pressure/stress in spatiotemporal. Focusing on the actual dynamic deformation of coal, a novelty spatiotemporal-dependent permeability (STDP) model was proposed to depict the real propagation behavior of permeability in coals with time. A set of fully coupled governing equations was derived to determine the coal deformations, gas migration, and permeability behavior. Subsequently, model applicability was verified experimentally, the evolutionary patterns of real permeability is revealed theoretically, and the geological controllers were further analyzed numerically. The results showed that the differential pressure arising from asynchronous redistribution of gas pressure within the coal matrix and fracture system is the origin of the permeability spatiotemporal dependency. Geomechanical effects and sorption effects are intrinsic regimes of the permeability variability, with the former dominating early changes and the latter exhibiting a stronger control in the long-term evolution. Both mechanical and sorption properties are crucial to the coal spatiotemporal-dependent permeability evolution path. Moreover, gas diffusion kinetic property and extraction pressure are the main controllers of equilibrium time for whole coal-gas system. This study nicely fills the gap in the long-term spatiotemporal variability of coal permeability under pressure disturbance, and the findings are of practical significance for CBM recovery and CO₂ sequestration.