Coupled Optimization of Enhanced Oil Recovery and CO ₂ Storage in Low-Permeable Reservoirs

Injecting CO2 into low-permeable reservoirs can not only improve oil recovery but also achieve CO2 storage, which has both economic and environmental benefits. The current study on coupled optimization of the CO2 enhanced oil recovery (EOR) and enhanced CO2 storage (ECS) primarily relies on numerical simulations. However, the optimization method considering the combined impact of multiple factors for CO2 EOR and ECS is insufficient. To improve the efficiency of oil recovery and CO2 storage during the CO2 flooding process, the Box-Behnken design (BBD) and response surface method (RSM), combined with simulations of CO2 flooding and storage, are conducted to reveal the coupled optimization effect of CO2-enhanced oil recovery (CO2-EOR) and enhanced CO2 storage (ECS) within reservoirs. The results indicate that the sensitivity ranking of the induced factors on CO2-EOR and ECS is the same as C (CO2 injection method) > A (CO2 injection rate) > B (CO2 injection stage) > D (reservoir permeability). From the RSM analysis results, the oil recovery is mainly influenced by the interaction effect of factors AC and BC. However, the interaction effect of AC and CD on the CO2 storage percentage in reservoirs is more significant. In the low water cut stage, the CO2-EOR effect does not significantly differ under different CO2 injection methods. However, as the water cut of reservoirs exceeds 50%, the difference in oil recovery becomes significant with the best displacement method of continuous CO2 injection. In the high water cut stage, severe water locking occurred, and the CO2 sweep volume in reservoirs was low by CO2 huff and puff (CO2 HnP) and CO2-water-alternating (CO2–WAG) injection, which inhibited the mass transfer between CO2 and crude oil. The change in permeability of reservoirs has little effect on CO2-EOR and ECS for low-permeability reservoirs. The optimized scenario for CO2-EOR and ECS is with the reservoir permeability of 10 mD, a water cut of 30%, CO2 HnP, and daily CO2 injection of 9770.60 m3. The optimal scenario significantly improves oil recovery, reaching 87.89% and CO2 storage percentage at 76.82%.