Abstract:

Miscibility between oil and supercritical carbon dioxide (scCO₂) phases has attracted significant attention in the field of oil recovery because it can be utilized in miscible gas displacement of oil, achieving nearly 100% recovery efficiency. The high recovery efficiency of miscible CO₂ flooding originates from the valuable heavy components of oil and CO₂ gas phase forming a homogenous phase with high mobility in the oil-scCO₂ miscible system. However, the high pressure required for oil-scCO₂ miscibility is a nontrivial obstacle for practical applications of scCO₂ flooding recovery. Therefore, it is important to develop assist-miscible agents to lower the necessary miscibility pressure. In oil and water systems, well-developed amphiphiles (such as surfactants) have shown great promise for reducing the interfacial tension and maintaining the stability of the emulsion system. Therefore, "oil-CO₂ amphiphiles" that can assist the miscibility between oil and scCO₂ have been proposed. Among potential oil-scCO₂ amphiphiles, a series of polyester-based oil-CO₂ amphiphiles with esters as the CO₂-philic groups and long carbon chains as the oil-philic groups were prepared. The polyester-based oil-CO₂ amphiphiles, acting as assist-miscible agents, showed great ability to lower the needed miscibility pressure. A visualized miscible method was used to examine the efficiency of the assist-miscible agents with white oil and kerosene as the oil phase. The height of the oil phase inside the chamber was measured through a glass window to monitor the miscibility with increasing CO₂ pressure. When the height of the oil reached the top of chamber, the oil filled the entire space, indicating miscibility. Using this method, the following conclusions could be drawn: First, amphiphiles with more ester groups exhibited stronger CO₂-philicity, providing stronger ability to dissolve carbon dioxide. Second, amphiphiles with hydrocarbon chain lengths of 16 carbons exhibited the optimal assist-miscible efficiency. Third, greater differences between the oil and scCO₂ phase showed more obvious differentiation among amphiphiles, showing the leveling and differentiating effect of oil. The temperature range of 50–80 ℃ did not influence the assist-miscible efficiency of the polyester-based amphiphiles. The best miscibility-assisting performance was obtained with CAA8-X, which contains eight ester groups and a palmitic acid chain. CAA8-X at a concentration of 1% (w, mass fraction) lowered the miscibility pressure in the white oil-scCO₂ system by 16.04%. Amphiphiles with polyether (PEO) groups also showed excellent assist-miscible efficiency. The findings presented herein extend the concept of "amphiphilicity" from oil-water phases to oil-scCO₂ phases and have the potential to guide future studies regarding scCO₂ flooding in actual CO₂ flooding oil recovery. Moreover, for other two-phase systems, according to the general amphipathic law and particular system parameters, it should be possible to design the optimal "amphiphiles".

Key words: Oil-CO₂ amphiphile, Polyester groups, Amphiphilicity, Assist-miscible effectiveness, Leveling and differentiating effect