Research progress on and outlook of direct CO₂ thickeners for enhanced oil recovery

Supercritical CO₂, as an environmentally friendly and pollution-free fluid, has been applied in various EOR techniques such as CO₂ flooding. However, the low viscosity of the gas leads to issues such as early breakthrough, viscous fingering, and gravity override in practical applications. Although effective mobility-control methods, such as CO₂ WAG (water alternating gas)-, CO₂ foam-, and gel-based methods, have been developed to mitigate these phenomena, they do not fundamentally solve the problem of the high gas-oil mobility ratio, which leads to reduced gas sweep efficiency. Adding CO₂ direct thickeners to displacing fluid can increase its viscosity, achieve deeper mobility control, and thus improve the CO₂ flooding oil-recovery effect. Unlike other methods, direct thickeners can alter the physical and chemical properties of CO₂, making it a fundamentally effective means of achieving mobility control. This approach can be applied in various reservoir environments and formations, or it can assist other methods for more in-depth mobility control. This article reviews the development and application of CO₂ direct thickeners and introduces the thickening mechanisms and effects of different types of thickeners as well as their existing problems and future development directions.