Analysis of Stimulation Efficiency and CO2 Storage Capacity for CCUS-EGR in Tight Gas Reservoirs in Ordos Basin

Under the background of “Peak Carbon Dioxide Emissions” and “Carbon Neutrality”, CCUS-EGR (carbon capture, utilization and storage and enhanced gas recovery) technology has been brought to the forefront in the oil and gas industry. CCUS-EGR can not only sequestrates CO2 to reduce greenhouse gas emissions, but also significantly enhances reservoir recovery factor. Currently, China has initiated the first field trial of tight sandstone gas reservoir CCUS-EGR in the S pilot area in Ordos Basin, necessitating the evaluation of stimulation efficiency and CO2 storage capacity of CCUS-EGR in such reservoirs. Firstly, the physical properties of CH4 (methane) and CO2 were analyzed through PVT (pressure-volume-temperature) experiments. Subsequently, the stimulation mechanism of CCUS-EGR was explored based on physical simulation experiments of CO2 flooding in tight sandstones. Finally, by conducting numerical simulations for CCUS-EGR in the S pilot area of Sulige gas field, the stimulation efficiency and CO2 storage capacity of CCUS-EGR in tight gas reservoirs was comprehensively evaluated. The following conclusions were drawn: (1) The density of supercritical CO2 is close to that of a liquid, with a viscosity slightly higher than CH4, and its diffusion coefficient in CH4 is relatively low, which is advantageous for its efficient displacement of CH4 in the reservoir. (2) After depleting rock cores to abandonment pressures of 10, 7.5, and 5 MPa, CO2 was injected into rock cores. When the mole fraction of CO2 in produced gas reached 20%, the recovery factor increments were 27.3%, 20.3%, and 12.5%, respectively, and the corresponding CO2 sequestration volumes were 0.17, 0.11, and 0.06 PV; (3) The S pilot area is a lithological trapped tight gas reservoir with poor reservoir connectivity, and CO2 sweep efficiency in the pilot area is approximately 27.4%, resulting in an estimated recovery factor increment of only 5.1% based on numerical simulations, which is lower than the experimental value of 20.3%; (4) It is estimated that after 1000 years, structural sequestration and solubility sequestration will decrease to 2.19 × 105 t and 2.27 × 104 t respectively, while residual sequestration and mineralization sequestration will rise to 2.57 × 104 tand 7.49 × 104 t respectively in the S pilot area. This paper systematically investigates the mechanisms of enhanced gas recovery and storage capacity of CCUS-EGR in tight gas reservoirs. The research results provide reference for the future large-scale investment and implementation of CCUS-EGR projects in tight gas reservoirs in China.