Controls on the dissolution of CO2 plumes in structural traps in deep saline aquifers

Abstract

Sequestration of carbon dioxide in deep saline aquifers has been proposed and investigated as a viable solution to help mitigate carbon emissions from fossil fuels. Much research has been directed at understanding the transitions of supercritical CO2 from being a mobile fluid phase to being trapped by capillarity or dissolved in groundwater; such transitions lead to a reduction in mobility of CO2 and hence in the risk of leakage to the surface. Following injection, buoyant plumes of CO2 migrate updip towards structural traps in the geological strata; however, some of this CO2 may be capillary trapped in pore spaces or dissolved in groundwater en route. Since CO2 saturated groundwater only has a small CO2 concentration, the dissolution of any large, structurally trapped plumes of CO2 may be controlled by the availability of unsaturated groundwater. In an aquifer of finite vertical extent, this may be rate limited by a combination of (i) the background hydrological flow coupled with (ii) the slow lateral exchange of relatively dense, CO2saturated groundwater with unsaturated groundwater. In an inclined aquifer, this may be controlled by the slow along‐aquifer component of gravity. Structurally trapped plumes of CO2 may therefore persist for many thousands of years, and, since they are potentially highly mobile, may represent an important contribution to the long term risks associated with CO2 sequestration at particular sites.