CO2-Surfactant Composite System Screening and Microscopic Enhanced Oil Recovery Mechanism Study

Carbon Capture, Utilization, and Storage (CCUS) involves separating and capturing CO2 from industrial emissions or the atmosphere, then transporting it via tankers, pipelines, or ships to sites for use or storage (Jiang et al. 2020, Chen et al. 2022, Yan and Zhang 2019). This technology reduces overall emission reduction costs and provides flexibility for achieving greenhouse gas reduction targets, making it a key technology for reaching carbon peak and carbon neutrality. As a CCUS technique, CO2-enhanced oil recovery (CO2-EOR) involves injecting CO2 into underground oil reservoirs to boost crude oil recovery while enabling large-scale geological storage, and has been widely applied internationally (Yan and Zhang 2019, Yang et al. 2024). With growing global energy demand and depleting traditional oil fields, improving oil recovery has become crucial for the oil and gas industry. CO2-EOR technology, which uses CO2 to reduce oil phase viscosity and enhance oil-gas interfacial activity, has been widely applied and studied abroad. The U.S., one of the earliest countries to initiate CO2-EOR projects, has relatively mature technology and has achieved significant recovery rate improvements (Hill et al. 2020, Webb et al. 2024). CO2-EOR can be divided into miscible and immiscible flooding. Miscible flooding forms a single mixed phase under reservoir conditions for efficient crude oil displacement, while immiscible flooding enhances recovery rates through mechanisms like viscosity reduction, oil expansion, and light component extraction. Emerging technologies like water-alternating-gas injection are also being explored to further improve CO2-EOR efficiency. China's research on CO2-EOR started later but has developed rapidly. Currently, pilot tests and industrial applications of CO2-EOR have been conducted in various oil fields, such as the Daqing and Shengli oil fields. Studies indicate that the minimum miscibility pressure (MMP) of CO2 with crude oil is a key factor affecting CO2-EOR performance (Adel et al. 2018, Chen et al. 2014). By adjusting formation pressure and optimizing CO2 injection parameters, the MMP can be lowered to enhance recovery efficiency. Researchers are also actively exploring suitable CO2-EOR solutions for different reservoir types, such as low-permeability and shale oil reservoirs (Luo 2024). Scholars have conducted early studies on improving recovery rates using CO2 and surfactants, achieving certain results. For example, adding surfactants can reduce interfacial tension between CO2 and crude oil, improving compatibility and oil displacement efficiency(Dowlati et al. 2023). Some studies also found that specific surfactants can synergistically act with CO2 to enhance its solubility in the oil phase, further boosting crude oil recovery. China has also made progress in this field. Researchers have investigated the compatibility of various surfactants with CO2, screened out suitable CO2-surfactant composite systems for different reservoir conditions, and verified their effectiveness in improving recovery rates through laboratory simulations and field tests.Common laboratory methods for measuring CO2-crude oil miscibility pressure include the capillary tube method, bubble characteristics analysis (BRA), and interfacial tension disappearance method (VIT)(Zhang et al. 2015, Zhang et al. 2020, Safaei et al. 2023). The capillary tube experiment, a widely recognized and accepted method for determining minimum miscibility pressure (MMP), involves conducting displacement simulations in a long, thin tube model. In China, the MMP assessment standard is based on injecting gas at 1.2 times the pore volume in the capillary model(Yu et al. 2024). When the recovery rate reaches 90% - 95%, the system is considered miscible. By plotting the relationship between recovery rate and displacement pressure for the immiscible and miscible phases, the curve's inflection point is identified as the MMP. If higher-pressure experiments show no significant increase in recovery rate compared to the MMP, the system is deemed miscible. To address the time-consuming and costly nature of the capillary tube experiments, some scholars have proposed the bubble characteristics analysis method, which determines MMP by measuring the rise time and velocity of bubbles in gas-liquid mixtures under different pressure conditions(Yuan et al. 2004, Song et al. 2024). Compared to the traditional capillary tube method for measuring MMP, the interfacial tension disappearance method offers the advantages of faster experimentation and lower costs. However, it is more subjective and requires highly precise instruments.In recent years, researchers have explored surfactants soluble in CO2 and studied their impact on CO2-crude oil miscibility pressure. This study selected four surfactants: NP9 (nonylphenol ethoxylate), DY604 (polyoxyethylene acetylene glycol), SPO5 (polyoxyethylene-polyoxypropylene copolymer), and TXIB (cyclohexyl-2-ethylhexanoate)(Gong et al. 2024, Kuang et al. 2021, Zhang, Gong, et al. 2024, Zhang et al. 2020). All these surfactants have unique molecular structures that enable interaction with CO2 and crude oil, reduce interfacial tension, and enhance oil displacement efficiency in reservoirs. DY604's acetylene group and polyoxyethylene chain form an amphiphilic structure. The acetylene group's high electron-density region can interact with CO2's electron-deficient carbon atoms via dipole-quadrupole interactions, significantly increasing CO2 solubility. The polyoxyethylene chain also enhances affinity with crude oil components through hydrogen bonding. NP9's nonylbenzene structure can efficiently embed into the asphaltene/resin layers of crude oil, disrupting heavy component aggregation through π-π stacking and enhancing CO2's asphaltene extraction ability. In SPO5, the polyoxypropylene (PO) segment becomes more hydrophobic at high temperatures, while the polyoxyethylene (EO) segment remains hydrophilic. This thermally sensitive balance maintains a low micelle aggregation number in reservoirs at 80–120°C, preventing high-temperature precipitation. TXIB, as an ester small molecule, can insert into the surfactant micelle's fence layer, expanding the micelle core volume and increasing the solubilization of light hydrocarbons (C6–C10). This study represents a significant exploration in this field(Sun et al. 2019). By selecting various gas-soluble surfactants, the research investigated their solubility in CO2 and their impact on CO2-crude oil miscibility pressure, further deepening the understanding of the mechanisms by which CO2 and surfactants enhance oil recovery(Liang et al. 2023). This study used microscopic visualization experiments to measure the miscibility pressure of CO2-surfactant composite systems with crude oil. Similar to the interfacial tension disappearance method but with differences, this approach allows intuitive observation of changes in oil-gas interface phenomena as pressure increases. Pressure control can be precise to three decimal places. When the pressure increases to a certain level and the oil-gas interface disappears, the pressure at this point is considered the minimum miscibility pressure of the CO2-surfactant composite system with crude oil. The study also investigated crude oil recovery under high-temperature and high-pressure conditions using the composite system, clarifying its mechanisms for improving recovery rates.