Spreading and dissolution of CO$_2$ in horizontal aquifers: theory and experiments

Injection of carbon dioxide into saline aquifers is widely regarded as a promising tool for reducing atmospheric CO$_2$ emissions. While an accurate assessment of the post-injection spreading and migration of the CO$_2$ is essential for estimates of storage security, many of the physical processes controlling CO$_2$ migration are poorly understood. CO$_2$ is buoyant relative to groundwater at reservoir conditions. This is undesirable because the presence of a pre-existing well or fracture, or the activation of a fault, could lead to leakage. It is well known, however, that the dissolution of CO$_2$ increases the density of the groundwater, resulting in convective currents that dramatically enhance CO$_2$ dissolution. Once dissolved, the CO$_2$ is considered to be securely stored within the subsurface. Recent numerical and experimental work has led to a greatly improved understanding of the resulting rate of CO$_2$ dissolution into groundwater. Here, we use analog experiments and simple theoretical models to study dissolution from a plume of CO$_2$ as it spreads upward against the caprock in an aquifer of finite thickness. We show that the interaction between spreading, dissolution, and the finite thickness of the aquifer has a strong influence on the ultimate distribution of the CO$_2$.