A quantitative evaluation method for gravitational override in thick carbonate reservoirs under water alternating gas flooding

• Quantitative evaluation index for gravity override in heterogeneous reservoirs and WAG numerical simulation. • Impact of heterogeneous scenarios on override degree and recovery efficiency. • Impact of different oil production rates on override degree and recovery efficiency. • Mutual offset of gravity and permeability breakthrough under specific conditions to enhance gas sweep. Carbon dioxide capture, EOR utilization, and storage (CCUS-EOR) reduce emissions while enhancing oil recovery, with Water-Alternating-Gas (WAG) flooding as a key technology. However, WAG performance is constrained by gas properties, notably gravitational override. Existing quantitative override indices mostly apply to homogeneous reservoirs and gas flooding, leaving challenges in evaluating override during WAG in highly heterogeneous carbonate rocks. This study develops a quantitative index to characterize override degree in heterogeneous reservoirs under WAG, for numerical simulation analysis. It uses the gas-unswept area proportion for homogeneous reservoirs and defines an override weight and a judgment coefficient for heterogeneous ones. Reservoir models with varied vertical heterogeneities and well-rates were built to analyze parameter impacts on override degree and the impact of override on recovery efficiency via this index. Results show the index accurately characterizes gas override in heterogeneous reservoirs during WAG simulations. Dominance of gravity or permeability contrast causes poor gas sweep and premature breakthrough, but these two factors can counterbalance under certain conditions, improving sweep efficiency and recovery. Findings aid understanding of gas sweep patterns in WAG of thick carbonate reservoirs, providing a theoretical basis for field adjustments.