Accelerating CO₂ Dissolution in Saline Aquifers for Geological Storage — Mechanistic and Sensitivity Studies

One of the important challenges in geological storage of CO₂ is predicting, monitoring, and managing the risk of leakage from natural and artificial pathways such as fractures, faults, and abandoned wells. The risk of leakage arises from the buoyancy of free-phase mobile CO₂ (gas or supercritical fluid). When CO₂ dissolves into formation brine, or is trapped as residual phase, buoyancy forces are negligible and the CO₂ may be retained with minimal risk of leakage. Solubility trapping may therefore enable more secure storage in aquifer systems than is possible in dry systems (e.g., depleted gas fields) with comparable geological seals. A crucial question for an aquifer system is, what is the rate of dissolution? In this paper, we address that question by presenting a method for accelerating CO₂ dissolution in saline aquifers by injecting brine on top of the injected CO₂. We investigate the effects of different aquifer properties and determine the rate of solubility trapping in an idealized aquifer geometry. The acceleration of dissolution by brine injection increases the rate of solubility trapping in saline aquifers and therefore increases the security of storage. We show that, without brine injection, only a small fraction (less than 8%) of the injected CO₂ would be trapped by dissolving in formation brine within 200 years. For the particular cases studied, however, more than 50% of the injected CO₂ dissolves in the aquifer as induced by brine injection. Since the energy cost for brine injection can be small (<20%) compared to the energy required for CO₂ compression for a 5-fold increase in dissolution, such reservoir engineering techniques might be viable and practical for accelerating dissolution of CO₂. The environmental benefit would be to decrease the risk of CO₂ leakage at reasonably low cost.