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

Abstract

Mass transfer by Ostwald ripening can impact the life and volume of capillary-trapped CO 2 in the subsurface. CO 2 storage in depleted hydrocarbon reservoirs encounters various preferences for wetting of the porous rock, while different reservoir pressures impact the miscibility between CO 2 and oil. The ripening behavior of CO 2 ganglia under such conditions is hitherto unknown. Herein, we study the impact of reservoir pressure and wettability on the ripening of CO 2 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 CO 2 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 CO 2 −liquid interfacial tensions and CO 2 −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 CO 2 −liquid contact angles are far lower (or higher) than 90°. We find that reservoir pressure, wettability, and oil/water capillary pressure can alter the CO 2 mass-transfer direction and hence the distribution of CO 2 ganglia at thermodynamic equilibrium. Simulations on a residual three-phase configuration in sandstone show that ripening leads to the growth of larger CO 2 ganglia, dissolution of small bubbles, and redistribution of trapped oil ganglia.