Study on CO Injection in Tight Sandstone Gas Reservoirs to Enhance Recovery and Burial Mechanism.

To study the mechanism of CO injection in tight sandstone gas reservoirs for enhanced recovery and geological storage, we conducted experimental studies based on the reservoir conditions in West Sichuan. These studies focused on the properties of CO and CH, their adsorption and diffusion under high temperature and high pressure, the interactions of CO with tight sandstone and formation water, and the displacement efficiency of the CO injection in long core samples. The results show that under reservoir conditions, the adsorption capacity of CO in tight sandstone is nearly four times higher than that of CH, and its diffusion capacity is also four times higher. This enables CO to replace adsorbed CH through competitive adsorption. Due to the greater density of CO compared to CH, gravitational differentiation causes CO to sink below CH, pushing CH upward in the reservoir. This process facilitates the sequestration of CO and enhances the recovery of natural gas. Additionally, CO injection dissolves feldspar and calcite in dense sandstone minerals, generating kaolinite, iron dolomite, and chlorite. The dissolution effect exceeds precipitation, leading to an increase in permeability by 73.25 to 91.15% and an increase in porosity by 0.65 to 0.72%. This improves the reservoir's seepage characteristics, reduces the minimum flow pressure of liquids in the pores, decreases water-phase trapping damage, and ultimately increases gas recovery. This study provides technical guidance for the efficient development of tight sandstone gas reservoirs and geological storage of CO.