Permeability of bituminous coal to CH4 and CO2 under fixed volume and stress boundary conditions: Effects of sorption

We report permeability data of a cylindrical sample of high volatile bituminous coal (25mm in diameter) with a clearly visible cleat system, to CH4 and CO2, performed under (near) fixed volume versus fixed stress conditions. To isolate the effect of sorption on permeability evolution, helium (non-sorbing gas) was used as a control fluid. All flow-through tests reported here were conducted under conditions of single-phase flow at 40℃, employing Terzaghi effective stresses of 14-35MPa. This experimental study aimed to gain a better understanding of the effects of fully coupled stress-strain-sorption-diffusion behaviour on permeability evolution in coal reservoirs during CO2-Enhanced Coalbed Methane (ECBM) recovery. Our results obtained permeability evolution versus changes in effective stress under both fixed volume and fixed stress conditions, showing an exponential correlation. Importantly, permeability (κ) obtained at similar effective stresses showed κ_helium > κ_(CH_4 ) >> κ_(CO_2 ), and κ measured at fixed volume is higher than fixed stress. This all demonstrates that permeability to CH4 and CO2, under in-situ boundary conditions, is strongly influenced by the coupled effects of a) self-stress generated by swelling, b) the change in effective stress coefficient upon sorption, c) sorption-induced closure of transport paths independently of poro-elastic effect, and d) heterogeneous gas penetration and equilibration, dependent on diffusion. Our results also show that the Walsh permeability model offers a promising basis for relating permeability evolution to in-situ stress evolution, using appropriate parameter values corrected for the effects of stress-strain-sorption-diffusion. These data are supplementary to: Liu, J., Spiers, C.J., 2022. Permeability of bituminous coal to CH4 and CO2 under fixed volume and stress boundary conditions: Effects of sorption. Submitted to Frontiers in Earth Sciences.