Supercritical carbon dioxide thickening for enhanced oil recovery: Methods, mechanisms, challenges, and outlook

Supercritical carbon dioxide (sc-CO₂) for enhanced oil recovery has gained increasing importance recently, driven by the growing interest in carbon capture, utilization, and storage, which plays a crucial role in mitigating the greenhouse effect. However, the lower viscosity of sc-CO₂ compared to crude oil has led to challenges such as viscous fingering and gas channeling along high-permeability zones, limiting its widespread application in oilfields. Since the 1990s, researchers have begun utilizing thickeners to increase sc-CO₂ viscosity, and significant progress in both theoretical understanding and technical implementation has been made in recent years. In this work, we review methods for thickening sc-CO₂ using different thickeners, including polymers, surfactants, small molecule compounds, and nanoparticles, outlining their thickening mechanisms and focusing on the differences in thickening processes and intra-molecular interactions. Additionally, we highlight advanced techniques like molecular dynamics simulations and quantum chemical calculations that are used to study these mechanisms. Based on our comprehensive review, we identify challenges in sc-CO₂ thickening and propose promising directions for future development, offering insights to guide the creation of novel technologies.