Ripening of Capillary-Trapped CO₂ Ganglia Surrounded by Oil and Water at the Pore Scale: Impact of Reservoir Pressure and Wettability

Mass transfer by Ostwald ripening can impact the life and volume of capillary-trapped CO2 in the subsurface. CO2 storage in depleted hydrocarbon reservoirs encounters various preferences for wetting of the porous rock, while different reservoir pressures impact the miscibility between CO2 and oil. The ripening behavior of CO2 ganglia under such conditions is hitherto unknown. Herein, we study the impact of reservoir pressure and wettability on the ripening of CO2 ganglia in the presence of oil (decane) and water at the pore scale, using a previously developed model that calculates the mass transfer based on chemical potential differences and the stationary three-phase fluid configurations with a multiphase level-set method. Through a comprehensive set of pore-scale simulations on 2D and 3D pore geometries, we show that ripening under immiscible conditions is faster than under near-miscible conditions, despite the fact that the permeability coefficients for CO2 in oil and water in the mass-transfer equation are higher for the near-miscible condition. The longer equilibration time with increased reservoir pressure occurs because lower CO2–liquid interfacial tensions and CO2–liquid contact angles closer to 90° lead to lower bubble capillary pressures, lower pressure differences between the bubbles, and lower gradients in bubble pressure with volume. Ripening is faster for strong wetting states where the CO2–liquid contact angles are far lower (or higher) than 90°. We find that reservoir pressure, wettability, and oil/water capillary pressure can alter the CO2 mass-transfer direction and hence the distribution of CO2 ganglia at thermodynamic equilibrium. Simulations on a residual three-phase configuration in sandstone show that ripening leads to the growth of larger CO2 ganglia, dissolution of small bubbles, and redistribution of trapped oil ganglia.