Abstract
Exsolution and re-dissolution of CO2 gas within heterogeneous porous media are investigated using experimental data and mathematical modeling. In a set of bench-scale experiments, water saturated with CO2 under a given pressure is injected into a 2-D water-saturated porous media system, causing CO2 gas to exsolve and migrate upwards. A layer of fine sand mimicking a heterogeneity within a shallow aquifer is present in the tank to study accumulation and trapping of exsolved CO2. Then, clean water is injected into the system and the accumulated CO2 dissolves back into the flowing water. Simulated exsolution and dissolution mass transfer processes are studied using both nearequilibrium and kinetic approaches and compared to experimental data under conditions that do and do not include lateral background water flow. The mathematical model is based on the mixed hybrid finite element method that allows for accurate simulation of both advection- and diffusion- dominated processes.
Highlights
Geologic carbon sequestration has the potential to significantly reduce greenhouse gas emissions [1], and poses risks to groundwater resources including mobilization of contaminants in shallow aquifers due to leakage of CO2 from deep storage formations [2]
The extent and severity of the risks depend on complex multiphase flow and transport phenomena that govern the migration of CO2 through the shallow subsurface
A persistent issue with predicting these processes is the general difficulty of understanding CO2 phase change (a.k.a. inter-phase mass transfer) within macroscopic porous media systems, which is important in the case of CO2 due to its high solubility and potential mobility in the gas phase
Summary
Geologic carbon sequestration has the potential to significantly reduce greenhouse gas emissions [1], and poses risks to groundwater resources including mobilization of contaminants in shallow aquifers due to leakage of CO2 from deep storage formations [2]. If a leakage pathway is encountered, stored CO2 that is originally supercritical in a deep geologic storage formation may migrate upward due to buoyancy, dissolve into water, exsolve to form a separate gas phase, and eventually re-dissolve into clean water. These interrelated processes are collectively referred to as multiphase evolution, and have recently been studied in various continuum-scale systems. Numerical models have not yet been able to fully explain all of the observations from the experimental studies, of those that occur during 2D flow under the influence of background water flow
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