Interface pinning of CO₂ gravity currents

Abstract

Carbon capture and storage (CCS) is widely regarded as a promising tool for reducing global atmospheric carbon dioxide (CO₂) emissions, while allowing continued use of fossil fuels in the 21st century. In CCS, CO₂ is captured at point sources such as coal power plants and injected deep underground in geological formations like saline aquifers for long-term storage. Given the large scale of CCS required to significantly reduce anthropogenic CO₂ emissions into the atmosphere, it is critical to understand the migration of CO₂ after injection, so that we can design effective injection strategies to minimize the leakage risks of CO₂ . Recent studies have demonstrated that simple models that incorporate the essential physics involved in CO₂ storage are able to make significant contributions in addressing important questions such as storage capacity and leakage risks in large scale CO₂ sequestration projects. Here, we study the impact of capillarity on the migration of CO₂ plume through exchange flow experiments of immiscible fluids. We show that capillarity leads to the development of striking features not present in miscible exchange flows, including a vertical pinned interface and sharp corners. We show that interface pinning is caused by capillary pressure hysteresis, and the amount of pinning scales with the relative strength of capillarity relative to gravity, as measured by the inverse of the Bond number. We demonstrate that capillary pressure hysteresis in porous media is caused by the fundamental difference in pore-scale invasion patterns between drainage and imbibition. In addition, we propose a sharp interface gravity current model that incorporates capillary pressure hysteresis and quantitatively explains the experimental observations, including the x ~ t1/2 spreading behavior at intermediate times and the fact that capillarity stops the spreading of a finite release current. These results suggest that interface pinning has important implications in the migration of CO₂ plume in deep saline aquifers.