Long-term Ostwald ripening of gas bubble population in porous media: pore-geometry-driven anti-coarsening effect vs. gravity-driven coarsening effects

In this work, long term evolution of gas bubbles in porous media is studied with a simple pore-network modelling approach, based on analytical models and experimental results we previously published (Xu et al. PRL, 2017). The effect of gravity is emphasized and discussed. The evolution of gas bubbles without gravity matches our experiments well, which further confirms the anti-coarsening effect which is contrary to typical Ostwald ripening and leads to uniformly sized bubbles in a homogeneous medium. Ostwald ripening is shown to be the driving mechanism; however, the relationship between surface curvature and bubble size determined by the pore-throat geometric confinement reverses the ripening direction. Furthermore, the role of gravity is studied and we show that gravity can provide additional driving force that results in gas bubble shrinking at stratum bottom and expending around stratum roof, which lead to gas concentrating at the top. This effect can be considerable when the thickness of porous media is at or above the order of magnitude of 1 m. A field-scale model is then built and validated by this pore-network model, and long-term (decades to centuries) bubble evolution dynamics in porous media is predicted which explains some field CO₂ sequestration