Gravity-Induced Bubble Ripening in Porous Media and Its Impact on Capillary Trapping Stability

Capillary or residual trapping is considered one of the safest geologic CO2 storage mechanisms due to its hydrodynamic stability. We present a first study of the impact of gravity on Ostwald ripening in porous media and show it may render capillary trapping thermodynamically unstable. Gravity induces a vertical chemical potential gradient that leads to the upwards diffusion of CO2. Thus, bubbles at shallower depths grow at the expense of bubbles in deeper strata, leading to the formation of an overriding gas cap. We first develop a pore-scale model for two bubbles trapped within adjacent pores and then upscale it to obtain a one-dimensional continuum model. We use the latter to predict the macroscopic evolution of a trapped bubble population. Factors controlling the ripening process are isolated to assist in selecting CO2 storage sites. Gravity-induced ripening may also play a role in geologic fluid emplacement and migration over millions of years.