Mechanisms and influencing factors of CCUS-EOR in heterogeneous reservoirs using in-situ emulsification-assisted WAG

Prolonged reservoir development commonly leaves a substantial amount of residual oil trapped in low-permeability zones, where conventional water-alternating-gas (WAG) flooding is often limited by poor sweep efficiency and gas channeling. In-situ emulsification-assisted WAG (ISE-WAG) has therefore emerged as a promising enhanced oil recovery (EOR) strategy for heterogeneous reservoirs with high permeability contrasts. However, compared with in-situ emulsification (ISE) applied during waterflooding, its role in WAG processes and its potential contribution to CO 2 storage remain insufficiently understood. In this study, the effects of an ISE system on oil recovery and CO 2 retention were investigated through core flooding experiments and numerical simulation. The core flooding experiments were conducted at 12 MPa and 60 °C, below the estimated minimum miscibility pressure of 19.8 MPa, indicating that the CO 2 -WAG process operated under immiscible conditions. The optimized formulation, consisting of 0.3 wt% dodecyl dimethyl betaine and 800 mg/L partially hydrolyzed polyacrylamide, exhibited ultra-low interfacial tension (IFT), favorable mobility control properties, and strong emulsion stability. Although the ISE-WAG stage provided only a limited direct contribution to oil recovery, it effectively reconfigured the subsequent displacement pattern and significantly enhanced oil mobilization during the following WAG cycles. An empirically calibrated engineering model was developed and history-matched, with a global simulation error of 2.23%. Simulation results further reveal that, under immiscible CO 2 -WAG conditions, the improvement in oil recovery and CO 2 retention is primarily attributed to ISE-induced flow resistance in high-permeability zones. ISE induced flow resistance suppressed rapid CO 2 recycling through preferential high-permeability pathways and prolonged the residence time of the free CO 2 -rich phase, thereby enhancing gas-phase retention and creating favorable conditions for subsequent residual trapping. Sensitivity analysis combined with Response Surface Methodology (RSM) indicates that polymer concentration is the dominant factor controlling oil recovery, whereas the liquid-gas slug ratio has the strongest influence on CO 2 storage. Moreover, a significant negative interaction exists between surfactant and polymer concentrations, indicating a coupled chemical-strength effect arising from the combined effects of interfacial-tension reduction, emulsification, and viscosity enhancement. These findings demonstrate that ISE-WAG is a viable CCUS-EOR strategy for heterogeneous reservoirs, particularly under immiscible CO 2 flooding conditions where sweep redistribution and flow resistance are critical to improving both oil recovery and CO 2 storage.