Mechanistic Investigation and Optimal WAG Ratio Design for CO2 Stable Flooding Through Multi‐scale Experiments

Carbon dioxide-enhanced oil recovery (CO2-EOR) is a widely adopted technique that improves oil production while enabling carbon sequestration. However, the high mobility and low viscosity of supercritical CO2 often cause early gas breakthrough and poor sweep efficiency, particularly in heterogeneous reservoirs. Water-alternating-gas (WAG) injection has been proposed as an effective mitigation strategy, yet its performance is highly sensitive to injection parameters, and a unified design framework for WAG ratio optimization remains lacking. To address this gap, this study conducted a series of long-core CO2 displacement experiments under high-pressure, high-temperature (HPHT) conditions using stratified artificial cores to simulate reservoir heterogeneity. Two WAG injection strategies—direct WAG and post-breakthrough WAG—were systematically evaluated through controlled injection sequences, monitoring of production performance, and analysis of effluent composition and pressure differentials. Experimental results demonstrated that direct WAG injection achieved higher oil recovery (81.88%) than post-breakthrough WAG (77.08%), attributed to delayed gas breakthrough, better CO2 utilization, and enhanced three-phase flow control. Produced fluid analysis revealed preferential extraction of light hydrocarbons (C1–C14) and selective retention of heavier components (C20+), driven by dissolution–fractionation mechanisms. Pressure differential analysis further showed that WAG injection introduced flow resistance and redistributed flow paths, particularly benefiting heterogeneous formations. This study provides a practical and scalable methodology for optimizing WAG injection ratios based on permeability contrast and flow stability, offering valuable technical insights for improving CO2-EOR performance in complex reservoir settings.