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.