The Analysis of CO₂ Enhanced Oil Recovery Based on CO₂-Water-Rock Coupling Experiments

Abstract The Ordos Basin is rich in unconventional oil and gas resources, but the reservoirs generally exhibit poor reservoir properties, making traditional extraction methods challenging in terms of improving recovery. CO 2 flooding for the development of unconventional reservoirs offers the potential not only for enhancing recovery rates but also for contributing to carbon sequestration, presenting broad application prospects. This paper focuses on the Yanchang Formation shale oil reservoirs. First, CO 2 -brine-rock coupling experiments were conducted to analyze the changes in the shale’s micro structure and mineral composition under different conditions. The effect of CO 2 injection on oil displacement efficiency under varying injection conditions was investigated. The nuclear magnetic resonance (NMR) T 2 spectra results under different coupling times indicate that during the first 0–7 days, the amplitude of the T 2 signal increased significantly as fractures and larger pores were filled with the fluid. Between 7 and 15 days, the amplitude change was less pronounced, suggesting that fluid migration between fractures and pores had largely been completed. From 15 to 30 days, capillary forces in the micro-pores that had been filled appeared to play a more significant role, resulting in a continued increase in the NMR signal. X-ray diffraction (XRD) results show that with increasing treatment time, the percentage content of albite gradually decreased, while the quartz content initially decreased and then increased. In contrast, the trends for potassium feldspar and chlorite were opposite. Core displacement experiments following coupling treatment demonstrated that as the coupling time increased, the CO 2 EOR efficiency also improved, with the final recovery rate reaching 29.52%. This demonstrates that the changes in the rock’s micro structure induced by CO 2 injection play a significant role in improving recovery rates, although the trend of this effect gradually stabilizes. These findings provide a theoretical basis for enhancing shale oil recovery.