Fundamental study of multiphase CO₂ evolution and attenuation in shallow aquifers during possible leakage from deep geologic sequestration sites using multi-scale experimental testing and numerical modeling

Abstract

Geologic Carbon Sequestration has the potential to significantly reduce carbon dioxide (CO2) emissions, but the potential for leakage of CO2 from deep storage formations into the shallow subsurface presents a wide range of possible risks that must be understood and addressed. The type, extent and severity of the risks depend on the distribution and multi- phase behavior of the CO2. For instance, dissolved CO2 causes acidification of groundwater, which could lead to mobilization of other contaminants such as metals. On the other hand, when water containing dissolved CO2 migrates through shallow groundwater, gas phase CO2 may form (exsolve), expand, accumulate, flow, and/or re-dissolve into clean water. While exsolved gas causes its own risks such as eventual escape to the atmosphere, the presence of gas phase in shallow aquifers may also help to attenuate the transport of dissolved CO2 and other aqueous contaminants. The various processes that dissolved and gaseous CO2 concurrently undergo within porous media are complexly interrelated, and are collectively referred to in this dissertation as “multiphase CO2 evolution.” This research combined laboratory experimentation and numerical modeling, with the ultimate goal of contributing to scientific knowledge on the factors that control multiphase CO2 evolution within shallow aquifers. Specifically, the effects of geologic heterogeneity were investigated in several experimental test systems of various sizes and shapes that were packed with various types of porous media in heterogeneous configurations. The data from a set of one-dimensional (1-D) experiments were used to develop a new theory that quantitatively predicts the effects of geologic heterogeneity on CO2 evolution during 1-D flow. Then, two- dimensional (2-D) experimentation was used to develop and validate a general conceptual model for multiphase CO2 transport and attenuation in larger, more realistic systems. The 2-D data were also used to test the capability of a particular numerical model to capture the observed CO2 evolution behavior. The numerical model was then used to make predictions about the relative effects of different types of geologic heterogeneity on multiphase CO2 transport.