A comprehensive review on CO2 thickeners for CO2 mobility control in enhanced oil recovery: Recent advances and future outlook

Carbon dioxide (CO2) has been utilized to recover the residual oil from the geological reservoirs through enhanced oil recovery (EOR) methods for over 50 years. Despite its long history of success as an EOR technique, CO2 flooding recovers only about 20–40% of the original oil in place (OOIP) from the geological reservoirs. The small amount of oil recovered by CO2 flooding is associated with the low viscosity of CO2 injected into the reservoir, resulting in CO2 viscous fingering, CO2 gravity override and unfavourable mobility. To address these problems, the CO2 viscosity needs to be enhanced considerably using CO2 thickeners or viscosifiers. Despite more than five decades of intensive research work in formulating and identifying effective CO2 thickeners such as polymers, surfactants, small molecules and nanoparticles; as yet none of these chemicals can be regarded as effective CO2 thickeners for EOR field applications. Thus, CO2 thickener is an interesting research topic for future studies to come up with effective and affordable CO2 thickeners for EOR field applications. This article presents the recent developments in CO2 thickening technologies in EOR. Furthermore, the CO2 thickening mechanisms, screening criteria, field scale applications, challenges and future research directions on CO2 thickeners are evaluated.