Synergistic Effects between Supercritical CO2 and Diluted Microemulsion on Enhanced Oil Recovery in Shale Oil Reservoirs
Authors: Yuan Shuai , Wang Bin , Yang Maoqin , Zheng Leyi , Liu Hao , Li Yuan , Zhou Fujian , Liang Tianbo 
Published: November 2024

Summary
In the context of developing unconventional liquid-rich reservoirs, the application of supercritical carbon dioxide (sc-CO2) has shown many advantages, including enhanced oil recovery (EOR), reducing formation damage, reducing water usage, and promoting the formation of complex fracture networks. However, sc-CO2 faces certain limitations in shale oil reservoirs that hinder its widespread application, such as ultralow viscosity, asphaltene deposition, and high miscible pressure. The addition of chemical agents is expected to overcome some limitations of sc-CO2 and further improve the CO2-EOR performance. Diluted microemulsion (DME) shows great potential as a chemical additive in water-based fracturing fluids to improve oil recovery by wettability alteration during the shut-in period after hydraulic fracturing. It is essential to explore the synergistic mechanism of DME and sc-CO2 through laboratory experiments to understand the microscopic mechanism of oil mobilization in shale reservoirs and to guide field applications. In this study, three soaking sequences were designed and compared to explore the EOR mechanisms combining sc-CO2 with DME using crude oil and core samples from the Lucaogou shale formation. To distinguish the DME at different stages, the DME injection in the subsequent stage is referred to as post-DME (pDME). The soaking sequences consist of only sc-CO2 soaking, water–sc-CO2–pDME (W-C-D) soaking, and DME–sc-CO2–pDME (D-C-D) soaking. They correspond, respectively, to the CO2 fracturing process and the process of CO2-EOR technology after hydraulic fracturing with different water-based fracturing fluids. Low-field nuclear magnetic resonance (NMR) technology was used to quantify the oil distribution among different pores and to monitor changes in the fluid state during each soaking stage. Additionally, the component changes of the produced oil were characterized using gas chromatography (GC). The T2 spectra results indicate that sc-CO2 soaking yields the highest oil recovery in the first soaking stage compared with water soaking and DME soaking. DME soaking effectively mobilizes more oil in small pores than water soaking. Subsequent sc-CO2 soaking and pDME soaking exhibit better EOR performance in the W-C-D soaking sequence than in the D-C-D soaking sequence, primarily mobilizing the remaining oil in larger pores. The sequence of total oil recovery is D-C-D soaking &gt; only sc-CO2 soaking &gt; W-C-D soaking. While the total oil recovery from large pores is similar across different soaking sequences, the D-C-D sequence achieves the highest oil recovery in small pores. The GC results suggest that DME can enhance the recovery of heavy components (C17+) by reducing interfacial tension (IFT) and altering wettability, thereby providing a conducive environment for heavy component mobilization in the subsequent soaking period. DME enables balanced mobilization of both heavy and light components, while sc-CO2 enhances oil mobilization from the unswept area by the aqueous phase. Therefore, combining sc-CO2 and DME can result in a higher ultimate oil recovery factor in shale oil reservoirs. The findings of this study provide an in-depth understanding of the oil mobilization mechanism during the soaking period and inform the design of soaking sequences for field applications in shale oil reservoirs.