Effect Evaluation and Mechanism Study of Polyether Surfactant-Assisted Supercritical Carbon Dioxide–Oil Miscibility Based on the Visible Vanish of Interface Measurement Method

Research on reducing the minimum miscible pressure (MMP) of CO₂ and oil is imperative for the advancement of CO₂ flooding, which can effectively enhance the recovery of low-permeability reservoirs and enable CO₂ geological storage. However, the conventional methods of determining the MMP (e.g., slim-tube experiments, supercritical extractor) are costly and time-consuming. In this paper, an experimental method for measuring the MMP, the visible vanish of interface measurement (VVIM) method, was proposed, and the effects of stirring rate, mass ratio, temperature, and oil phase properties on the scCO₂-oil miscible pressure were explored. At the same time, the assist-miscible effect of polyether surfactants, such as polyoxyethylene sorbate (Tween 80), C₈PO₁₅EO₁₀ (containing a polyether block), and phPO₁₅EO₁₀, were investigated. The experimental results show that the miscible pressure of scCO₂-oil is directly proportional to the stirring rate and inversely proportional to the temperature, and the mass ratio is the most difficult to achieve miscibility when the mass ratio is close to 1:1. Compared with conventional nonionic surfactants, short carbon chain-extended polyether has stronger amphiphilicity and a more suitable spatial matching effect, resulting in a better assist-miscible effect. The proposed materials and methods provide a reliable reference for the study of the structure-activity relationship of surfactants for supercritical carbon dioxide flooding.