A review of chemical-assisted minimum miscibility pressure reduction in CO2 injection for enhanced oil recovery

Abstract Miscible CO2 injection appears to be an important enhanced oil recovery technique for improving sweep efficiency and eliminating CO2-oil interfacial tension resulting in up to 10% higher oil recovery compared to immiscible flooding, in addition to the environmental benefits of reducing greenhouse gas emissions through carbon capturing utilising and storage (CCUS). Moreover, this technique could be similarly applicable to natural gas and nitrogen projects to increase oil recovery and to reduce the associated gas flaring. However, miscible displacement may not be achievable for all reservoirs, in particular, reservoirs with high temperature where high injection pressure would be needed to reach miscibility which likely exceeds the formation fracture pressure. Therefore, to further achieve reservoirs’ potential, there is a pressing need to explore a viable means to decrease the miscibility pressure, and thus expand the application envelop of miscible gas injection in reservoirs with high temperatures. In this work, we aim to provide insights into minimum miscibility pressure (MMP) reduction by adding chemicals into CO2 phase during injection. We achieved this objective by performing a comprehensive review on chemical-assisted MMP reduction using different chemical additives (e.g., alcohols, fatty acids, surfactants) and different experimental methodologies. Previous experimental studies have shown that a fraction of chemical additives can yield up to 22% of MMP reduction in CO2-oil system. Based on results analysis, surfactant based chemicals were found to be more efficient compared to alcohol based chemicals in reducing the interfacial tension in the CO2-oil system. Based on the current experimental results, adding chemicals to improve the miscibility and reduce the MMP in the CO2-oil system appears to be a promising technique to increase oil recovery while reducing operating cost. Selection of the effective chemical additives may help to expand the application of miscible gas injection to shallow and high temperature reservoirs. Furthermore, our review provides an overall framework to screen potential chemical additives and an injection strategy to be used for miscible displacement in CO2 and/or gas systems.