CO2-enhanced shale gas recovery (CO2-ESGR) has been demonstrated to be a promising method for increasing shale gas production and simultaneously achieving CO2 storage. During the CO2-ESGR process, the permeability of shale reservoirs is an important parameter significantly influencing the shale gas production and the CO2 sequestration efficiency. However, the quantitative relationship between CO2 and CH4 competitive adsorption induced differential deformation, and its impact on the permeability of shale is not yet fully understood. In this study, experimental investigations of pure gas (CH4 and CO2) and binary gas (CH4/CO2 = 3:1, 1:1, and 1:3) adsorption-induced volumetric deformation and the subsequent permeability changes on shale were conducted. The results show that under the constant effective stress conditions, adsorption-induced shale deformation increased with the increase of gas pressure for pure CO2, CH4, and their gas mixtures, while the permeability of shale shows a tendency of first decreasing rapidly, followed by a slow decrease, and then increasing slightly. In addition, binary CO2/CH4 gas adsorption-induced deformation increased with the increase of the mole fraction of CO2, leading to the decrease in the permeability of shale with the increase of the CO2 mole fraction, which means a significant permeability loss as the CO2 adsorption-induced swelling increases. In addition, with the increase of the CO2 mole fraction, the impact of the slippage effect on the permeability of shale decreased. The final slight rebound in the permeability of shale can be attributed to the dynamic variation of the Biot coefficient of effective stress in shale caused by adsorption. The results can provide theoretical support for optimization of the engineering parameters of CO2-ESGR.
Zhou et al. (Fri,) studied this question.