Fingering inhibition triggered by CO2 dissolution and viscosity reduction in water-alternating-CO2 injection

As a CO2 capture, utilization, and storage (CCUS) technology, water-alternating-CO2 (WAG) injection has demonstrated excellent results in enhancing oil recovery. Current research on WAG injection primarily focused on factors such as increasing injection pressure and optimizing the water-gas slug ratio (W:G) to enhance the driving force, reduce instability due to the significant viscosity difference between oil and CO2, thereby inhibiting fingering phenomenon and improving oil recovery. However, in immiscible flooding, CO2 dissolution reduces the viscosity of the oil, changing the instability of the interfaces and affecting oil recovery. We employed computational fluid dynamics to study the effect of CO2 dissolution and viscosity reduction on fingering patterns and its effect on enhanced oil recovery (EOR) under capillary numbers Ca = 0.12 x 10- 2-1.14 x 10-2 and W:G = 1:3-3:3. The results indicated that: (1) the dissolution of CO2 reduced oil viscosity, inhibiting the fingering phenomenon, promoting stable displacement and enhancing oil recovery. (2) The viscosity reduction effect of CO2 dissolution was more effective in viscous fingering. (3) Analysis of the EOR capacity after injecting a unit volume of displacement fluid confirmed that the optimal W:G remains 1:3. These findings highlight the importance of considering CO2 dissolution and its viscosity reduction effect to optimize WAG injection strategies for enhanced oil recovery.