Abstract

Carbon dioxide (CO2) levels in the atmosphere are rising rapidly as a result of anthropogenic emissions. One proposal to reduce CO2 emissions and mitigate climate change is carbon capture and storage (CCS). The storage part of this involves large-scale injection of captured and compressed CO2 into deep porous rock formations such as saline aquifers. Many aspects of the resultant porous flows pose fundamental and interesting fluid-mechanical questions, and these will be the subject of the lecture. The physical ideas will be illustrated with movies of analogue laboratory experiments and numerical simulations.

The buoyant CO2 rises and spreads beneath the overlying impermeable “cap-rock” as a so-called gravity current, and the undesirable possibility of upward leakage through any fractures must be assessed. Fortunately, a number of long-term trapping mechanisms exist. One such, dissolution of CO2 into the underlying brine, produces a denser solution which, being denser, convects reassuringly downwards. A key question is then the rate of dissolution, which depends on study of vigorous convection in a porous medium. This has recently been shown to have a strikingly different form from the more familiar convection in a pure fluid. Given the dissolution flux, the evolution towards saturation in a confined aquifer, or the erosion of the spreading gravity currents in open aquifers, can be calculated.