Experimental study on effect of multi-stage stress variations on permeability and pore structure of sandstone

The variations of reservoir stress caused by the multi-stage and discontinuous nature of the process in the CO₂ geological storage, leading to the change of permeability and pore structure of the rock, have a direct impact on CO₂ injection and storage. Through laboratory experiments, variations in the permeability of sandstone in the Liujiagou formation of the Ordos CCS (CO₂ capture and storage) demonstration project are analyzed under cyclic variations in injection pressure and confining pressure and multi-stage loading and unloading. In addition, the variations in the micro-pore structure are analyzed based on micro-pore structure tests. The main conclusions are as follows: (1) Both the confining pressure and the injection pressure have a significant effect on the permeability of the reservoir rock. And the influence degree can reach 30% and 80%, respectively. The relative permeability changes with pressure, and it between the loading and unloading stages is higher at low pressures than at high pressures. (2) Mathematical models of permeability as a function of confining pressure and injection pressure are constructed. The mathematical models representing different cyclical processes are quite different. (3) The effects of the interval between the experiments on permeability changes are different for different pressure modes. The rock permeability can recover better under the injection pressure variation than confining pressure variation. (4) The variations during multiple stress cycles have a significant effect on the micropore structure. The increase in micropores with mesopores at small widths and the decrease of macropores result in a decrease in the permeability of the rock samples.