A continuum-scale representation of Ostwald ripening in heterogeneous porous media

Ostwald ripening is a pore-scale phenomenon that coarsens a dispersed phase until thermodynamic equilibrium. Based on our previous finding that multi-bubble equilibrium is possible and likely in complex porous media, we develop a new continuum-scale model for Ostwald ripening in heterogeneous porous media. In this model, porous media with two different capillary pressure curves are put into contact, allowing only diffusive flow through the aqueous phase to redistribute a trapped gas phase. Results show that Ostwald ripening can increase the gas saturation in one medium while decreasing the gas saturation in the other, even when the gas phase is trapped in pore spaces by capillary forces. We develop an analogous retardation factor to show that the characteristic time for Ostwald ripening is about 105 times slower than a single-phase diffusion problem due to the fact that separate-phase gas requires a much larger amount of mass transfer before equilibrium is established. An approximate solution has been developed to predict the saturation redistribution between the two media. The model has been validated by numerical simulation over a wide range of physical parameters. Millimetre to centimetre-scale systems come to equilibrium in years, ranging up to 10 000 years and longer for metre-scale systems. These findings are particularly relevant for geological CO2 storage, where residual trapping is an important mechanism for immobilizing CO2. Our work demonstrates that Ostwald ripening due to heterogeneity in porous media is slow and on a similar time scale compared to other processes that redistribute trapped CO2 such as convective mixing.