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

Abstract

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 × 10⁻²–1.14 × 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.