Water-Alternating CO2 Injection in Carbonate Reservoirs: A Review of Fluid-Rock Interaction Studies

Summary Water-alternating-CO2 (CO2-WAG) injection is a well-established method for enhanced oil recovery (EOR) and a promising option for geological carbon storage. The alternating injection of this gas with water also increases carbonate rock reactivity, which is higher than that in siliciclastic reservoirs, affecting the porosity and permeability near the well, thereby impacting the injectivity and well integrity. The composition of the produced water is also affected, increasing the potential for inorganic scaling. Moreover, reactivity also changes the pH of the produced water, thereby affecting material selection for producer wells. The characterization and modeling of such fluid-rock interaction effects are challenging but valuable for designing and optimizing the CO2-WAG process. To assess the current knowledge on this topic, we present a review encompassing laboratory- and field-scale studies of fluid-rock interactions resulting from CO2-WAG processes, particularly those pertaining to changes in the porosity, permeability, and produced water composition. Numerous studies within this scope have been published. This review summarizes the most pertinent findings and identifies opportunities for further research. In laboratory-scale studies, the main necessity is to expand the range of experimental conditions and parameters, either by conducting experiments with different mineralogies under representative reservoir conditions (e.g., pressure, temperature, and hydrodynamics) or by incorporating an oil phase, as such studies yield data essential for field-scale simulations, thereby enhancing their reliability. Addressing gaps in field-scale studies involves integrating the phenomenon of relative permeability hysteresis when assessing the impact of carbonate rock dissolution on the injectivity during CO2-WAG, as these phenomena are concurrent. Finally, we advocate for studies that establish an upscaling methodology for translating laboratory results into field-scale reactive transport simulations.