Why does coal permeability time dependency matter?

Permeability is a key property of coal for both natural gas extraction from and/or carbon dioxide sequestration in coal seams. It is normally defined as a function of effective stress. A fundamental assumption is that both gas pressure and its associated strains are independent of time. In these efforts, the gas diffusion process and its associated volumetric transformation between fractures and matrices were rarely considered. In this study, a special apparatus was setup to continuously monitor/calculate all essential variables under controlled gas injection conditions, including (1) coal permeability, (2) overall strain, (3) fracture strain, and (4) matrix strain. Two tests were conducted: (1) CO₂ injection test and helium test for reference. Both were conducted under the condition of constant effective stresses. In the CO₂ test, permeability experienced four distinct stages: (1) increase due to fracture pressure, (2) decrease due to local matrix swelling, (3) recovery due to matrix swelling expansion, and (4) stability due to equilibration between fractures and matrices over a period of 90 days. Based on these observations and analysis, we conclude that coal permeability evolves even under constant injection conditions. This suggests that coal permeability is time-dependent even under constant gas extraction/injection conditions and this dependency must be considered during natural gas production or gas sequestration in coal seams.