Experimental study of microscopic oil production and CO₂ storage in low-permeable reservoirs

Enhanced CO₂ sequestration (ECS) within low-permeable reservoirs during CO₂-enhanced oil recovery (CO₂-EOR) processes has gained significant interest, primarily driven by the need to mitigate the greenhouse effect caused by excessive CO₂ emissions. In this work, the in-situ nuclear magnetic resonance (NMR) is applied to investigate the oil production and CO₂ sequestration within the micropores of low-permeable reservoirs. Additionally, the impact of CO₂–water–oil–rock reactions on CO₂-EOR and CO₂ sequestration is studied by analysis of the changes in minerals, pore structures, and wettability of cores by scanning electron microscopy (SEM), X-ray diffraction (XRD), and contact angle measurements with the experiments of CO₂–water–oil–rock interaction in the high-temperature and high-pressure (HT-HP) reactor. The results reveal that the residual water saturation (S_wr), CO₂ injection pressure, and the interaction among CO₂, water, oil, and rock all exerted a considerable impact on oil recovery and CO₂ sequestration. Compared with the oil recovery and CO₂ sequestration of the two oil-saturated cores (Core No. 2 and Core No. 3) after CO₂ injection, the accumulated oil recoveries of the two cores with S_wr = 0.5 are enhanced by 1.8% and 4.2%, and the CO₂ sequestration ratios are increased by 3% and 10%, respectively. Compared with the CO₂–water–rock that occurred in oil-saturated cores, the CO₂–water–rock reaction for cores (S_wr = 0.5) is more intense, which leads to the formation of more hydrophilic rock on pore surfaces after the reaction, thereby reducing the adhesion work of CO₂ stripping oil. The oil and water mixtures in pores also inhibit CO₂ premature breakthrough from cores, therefore expanding the swept volume of CO₂ in cores. Otherwise, oil recovery and CO₂ sequestration in small pores of cores are significantly improved with the rise in CO₂ injection pressure due to the enhanced driving pressure degree and also the improved mutual solubility and mass transfer between CO₂ and oil.