Long-Term Dissolution Rate of Carbon Dioxide in Saline Aquifers

The aim of geological CO2 storage is the permanent removal of of the injected CO2 from the atmosphere. The buoyancy of the injected supercritical CO2 and the possibility of leakage along fractures faults and old wells may lead to leakage of CO2 back into the atmosphere over time. The brine density increases with increasing CO2 concentration, and therefore dissolved CO2 is unlikely to leak back into the atmosphere. The rate at which CO2 dissolves into the brine is a key constraint on the duration of possible leakage. Due to the low solubility of the CO2 in the brine a large volume brine is necessary to dissolve a given amount of CO2. Gravity driven flow induced by the increased density of CO2 saturated brine is necessary to contact this large volume of brine and therefore determines the long-term dissolution rate. We present high-order direct numerical simulations of the convective motion in the brine in homogeneous, horizontal, laterally-unbounded aquifers. At early time, before the plumes of saturated brine have reached the bottom, the overall dissolution rate is essentially constant due to rapid convective overturn. At late time the saturated brine forms a miscible gravity current propagating outward from the CO2 source. Simple models of constant density gravity currents predict a power-law decay of the overall dissolution rate. Direct numerical simulations show a similar power-law decay but slightly lower rates of decay. We attribute this to temporal variations of the average density of the gravity current.