H009-0009
A Comprehensive Investigation for Gravity-driven Ostwald Ripening in Porous Media

Monday, 7 December 2020
Poster
Yitian Feng, Zhejiang University, Hangzhou, China, Ke Xu, Massachusetts Institute of Technology, Cambridge, MA, United States and Yashar Mehmani, Stanford University, Energy Resources Engineering, Stanford, CA, United States
Abstract:
CO2 geological sequestration is promised to be a solution to greenhouse gas control. Among all trapping mechanics of CO2 in subsurface aquifers, capillary trapping, i.e., trapping CO2 as isolated bubbles by capillary pressure due to pore-throat structures, is considered to be efficient and hydrodynamically stable.

However, the thermodynamic stability of capillary trapping has been questioned by our recent work (Xu et al., PRL, 2017; Xu et al., GRL, 2019). We demonstrate that gravity induces the growth of bubbles at the top at expense of dissolution of bubbles at bottom, through upward molecular diffusion of gas component. In long term ( > thousands of years), this gravity-driven ripening may result in the formation of gas cap and pose the risk of leakage.

In this work, we improve and polish the gravity-induced bubble ripening model, and conduct comprehensive analysis accordingly. We show that the bubble ripening process highly depends on a modified Bond number, denoted by Bo*, as well as a critical gas saturation, denoted by Sc. Four regimes of bubble ripening are identified accordingly, with analytical solution achieved. We show that if the gas saturation S > Sc in a stratum of thickness h, the equilibrium time, teq, is proportional to h^2 when Bo*<<1, and is proportional to h when Bo*>>1; S <Sc, teq is proportional to h regardless of S, although the pre-factors are different for S>Sc and S<Sc.

We further establish numerical modelling workflow which enables the simulation of more complicated and heterogeneous situations. Specifically, we investigate the redistribution between two layers with pore size contrast, where completely different dynamics are found for small and large pore size contrasts. We also investigate the effect of a thin shale layer in slowing down the gravity-induced bubble ripening between two storage layers.

This work provides new physical perspective and mathematical tool in evaluating CO2 sequestration security.

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