Experimental Study on the Optimization of CO2 Flooding Miscibility-Reducing Agents for Low Permeability Reservoirs

CO2 flooding is one of the paramount approaches to ameliorating the oil recovery rate of low-permeability reservoirs. Additionally, the physical and geological characteristics of low-permeability reservoir crude oil are less desirable owing to the influence of continental sedimentary structures. The minimum miscibility pressure (MMP) between CO2 and crude oil is generally higher than the formation fracture pressure, thereby rendering it difficult for CO2 to form miscibility with crude oil. Under such circumstances, a miscibility-reducing agent can be adopted to lower the miscibility pressure between CO2 and crude oil, thus achieving CO2 oil miscible phase flooding and elevating recovery efficiency. This paper first selected the most effective miscibility-reducing agent through molecular simulation experiments and core displacement experiments, and subsequently evaluated the injection effect of the miscibility-reducing agent on heightening the recovery rate of CO2 flooding in low-permeability reservoir cores. This research obtained the effect of the miscibility-reducing agents on the aggregation degree of CO2 molecules and asphaltene molecules during the oil gas mixing process from the radial distribution function and analyzed its function mechanism. On this basis, the core flooding experiment method was adopted to screen for the optimal miscibility-reducing agent with the best miscibility-reducing agent effect. Afterward, the impact of miscibility-reducing agent injection on the CO2 flooding development effect of the low-permeability core was evaluated systematically. As demonstrated by the molecular simulation results, selected miscibility-reducing agents displayed desirable electron transfer ability with CO2; Triisobutyl citrate can effectively heighten the aggregation degree of CO2 molecules and lessen the aggregation degree of asphaltene molecules, with the most conspicuous effect; The results of the core flooding experiment revealed that upon the injection of triisobutyl citrate, the MMP reduction between CO2 and crude oil exhibited the most remarkable trend, so it was selected as the best miscibility-reducing agent. The mechanism by which triisobutyl citrate can effectively reduce MMP mainly includes lowering crude oil viscosity and oil gas interfacial tension, and promoting the CO2 extraction ability. The indoor three-dimensional well network model CO2 flooding experiment results suggested that after injecting 0.07 PV of triisobutyl citrate preflush slug, the CO2 flooding recovery rate increased from 47.78% OOIP without the injection of miscibility-reducing agents to 59.86% OOIP. The selected triisobutyl citrate displayed a striking effect on augmenting the CO2 flooding recovery rate of low-permeability reservoirs. The research results provide a reference and inspiration for the high-quality and efficient development of CO2 flooding in low-permeability oil reservoirs.