Understanding and Managing Injectivity Behavior during CO2-WAG in Carbonate Reservoirs

Abstract The behavior of injectivity in CO2-WAG is contingent upon a combination of various phenomena. A downward trend is typically driven by factors such as relative permeability hysteresis, wettability alteration and low mobility of tertiary oil bank. Conversely, an upward trend is usually attributed to dominant effects such as a reduction in residual oil saturation, a decrease in water viscosity and rock dissolution. Notably, this latter phenomenon is more pronounced in carbonate reservoirs, as its reactivity to CO2-enriched water is higher than that in siliciclastic reservoirs, thereby contributing to enhanced injectivity by increasing the permeability in the vicinity of the injector well. In this study, the individual impacts of intrinsic reservoir parameters on the injectivity behavior are investigated through reactive transport simulation. The main objectives are to establish the scenarios most susceptible to a downward trend and then develop CO2-WAG operational strategies to minimize negative effects on oil production and CO2 storage capacity. The study revealed that inherent reservoir conditions that lead to a downward trend in injectivity include water-wetness wettability, high vertical heterogeneity, increased content of highly soluble minerals, such as calcite, in the reservoir mineralogy, and low reduction in the residual oil saturation (due to waterflood). This is primarily due to the predominance of relative permeability hysteresis over other phenomena that could potentially increase injectivity throughout the WAG cycles, particularly rock dissolution. Optimization led to a reduced WAG ratio, increased injection rate, longer WAG cycle periods, and greater CO2 content in the injected gas. Based on our extensive review of the literature, this study is the first comprehensive multi-scale examination of injectivity behavior incorporating the phenomenon of relative permeability hysteresis into reactive transport simulations when evaluating the effect of rock dissolution on the injectivity during CO2-WAG. This is crucial because these phenomena occur concurrently. Relative permeability hysteresis contributes to reducing injectivity, while rock dissolution tends to increase it. This study offers significant contributions into design and optimization of the CO2-WAG methods in carbonate reservoirs.