Abstract:

Surfactants are widely applied for promoting miscibility and reducing interfacial tension between oil and water phases because of their remarkable amphiphilic morphology. Along with development and popularization of tertiary oil recovery techniques, surfactants play a significant role in crude oil exploitation. Among the various tertiary oil recovery techniques, supercritical CO₂-enhanced oil recovery is a promising method for improving oil recovery. However, the establishment of CO₂-enhanced oil recovery brought new requirements and challenges to traditional surfactant research and development, especially for molecular design. In this method, the reduction of the minimum miscibility pressure between supercritical CO₂ and crude oil is required to achieve miscible flooding—an important means to enhance oil recovery. Therefore, a novel miscible flooding agent that exhibits oil-water miscibility analogous to conventional surfactants is desirable for this method. Meanwhile, a conspicuous difference of polarity matching the high polarity of H₂O molecule against low polarity of alkane molecule, which is the essential feature of traditional surfactant, won't suit this case well due to a medium polarity of CO₂ molecule. According to previous work done in our laboratory, surfactants with multiple ester groups considerably reduce the minimum miscibility pressure between supercritical CO₂ and crude oil. Therefore, inspired by the "oil-water-amphiphilic molecules" design, herein, we replaced the hydrophilic moiety with multiple ester groups and designed a new type of "oil-CO₂ amphiphilic molecule" as a miscible flooding agent, which is composed of an alkane tail and multiple ester groups as the lipophilic and CO₂-philic groups, respectively. In the strategy based on the proposed agent, the number of ester groups and the length of the alkane tail are the main parameters. In addition, we optimized the molecular structure of the proposed agent, CAA8-X, which comprises cetyl and acetyl sucrose esters as the lipophilic and CO₂-philic groups, respectively. We verified that the as-synthesized agent can remarkably reduce the minimum miscibility pressure between supercritical CO₂ and various types of oil samples, including kerosene, white oil, and crude oil from the Changqing region. The crude oil-CO₂ minimum miscibility pressure reduction ratio was 20.5% as measured by the vanishing interfacial tension method and the slim tube test. In this study, we also established a method called the rising height method to measure the minimum miscibility pressure with significantly reduced time and equipment cost. Furthermore, to demonstrate the mechanism of this miscible flooding agent for CO₂-enhanced oil recovery, the affinity between the CO₂-philic group and molecular CO₂ was investigated via molecular dynamics simulation. The results indicated that the "oil-CO₂ amphiphilic molecule" can reduce oil-CO₂ interfacial tension because of lower affinity potential energy between the CO₂-philic group and molecular CO₂.

Key words: Alky sucrose ester, Supercritical-CO₂, Amphiphile, Molecular dynamics, Enhanced oil recovery, Miscible pressure