A study on the transient permeability behaviour of coal core under confinement

A high-volatile bituminous coal core was maintained at 40°C, for flooding separately with helium, methane, and carbon dioxide in steps, up to 6 MPa and under various confining pressures (8.2 MPa, 9.2 MPa, and 10.2 MPa). Time-variations and correlation among the flow parameters were analysed during the non-steady state flow. Changes in the effective stress and the specific pore volume were significant at higher confining pressure but less/negligible at lower confinement pressure. Permeability varied inversely with gas pressure within a pressure step but as pore pressure was decreased, the permeability was found to decrease. Permeability also varied with the volumetric flow rate, whereas negative trends were observed with effective stress and cleat compressibility. Effective stress dominated the gas permeability significantly and directly, but indirectly affected specific pore volume and cleat compressibility. In general, helium and CO2 permeability were observed to be the lowest and highest among the gases, respectively. Helium, methane, and CO2 permeability in coal are tested at different effective stresses. Changes in permeability and important parameters with time are investigated. [Received: September 19, 2023; Accepted: February 29, 2024]