A Novel Approach to Enhanced Oil Recovery (EOR) in Fractured Reservoirs: A Mechanism Study on Micro-Model of a Viscoelastic Gemini Surfactant-Polymer Enhanced Foam (VEGS-PEF)

Summary CO2 foam technology has demonstrated its effectiveness as a tool in advancing Carbon Capture, Utilization and Storage (CCUS) strategies during oil production. Significant quantities of crude oil are found in fractured reservoirs, but conventional water and gas flooding effectively recover only the oil within the fractures, leaving oil in the matrix unrecovered. Although surfactants can reduce the interfacial tension (IFT) between oil and water and alter the wettability of reservoir rocks, the significant heterogeneity of reservoirs continues to pose challenges for enhanced oil recovery (EOR) technology. Injection methods involving foams, especially polymer-enhanced foam (PEF), are advancing and improving oil recovery efficiency. This paper elucidates the action mechanism of an innovative green viscoelastic Gemini surfactant-polymer-enhanced foam (VEGS-PEF) in fractured reservoirs. CO2 foam and CO2 PEF, prepared with nonionic surfactant, anionic surfactant, zwitterionic surfactant, and VEGS, were tested in flooding experiments on fractured micro-models, with observations also made on their foaming and stability in crude oil. High-definition images capturing fluid transport were recorded using a digital camera, and sweeping efficiencies at various stages were calculated. Results indicated that CO2 foam was more susceptible to sheet rupture compared to PEF. VEGS/SDS-PEF prepared with VEGS/SDS composite surfactants had a 103.7% longer half-life than VEGS-PEF. More importantly, heavy oil, which has a destructive effect on ordinary PEF, was able to extend the half-life of VEGS/SDS-PEF by 10.3%. Additionally, the increased apparent viscosity of VEGS/SDS-PEF facilitated higher injection pressures, enhancing fluid transfer from fractures to the matrix and improving the replacement of crude oil post-water injection. In conclusion, this study highlights the synergistic effects of VEGS, SDS, and polymer in enhancing the stability and apparent viscosity of CO2 foam, thereby aiding heavy oil recovery in fractured reservoirs. VEGS/SDS-PEF, compared to single-surfactant PEF, offers improved mobility control and oil carrying capacity, enhancing heavy oil recovery by 6.28%-10.39%. Injecting CO2 foam not only helps carbon utilization and CO2 geological sequestration but also offers dual benefits in carbon reduction and increased production.