Numerical Simulation of CO2 Injection into a Carbonate Aquifer: Implications of Fluid–rock Reactions in the Aquifer and in an Overlying Clay Formation

Abstract

We use reactive transport simulations to evaluate the CO2 sequestration potential of a carbonate-hosted aquifer. Results show that in carbonate aquifers the dissolution of CO2 into the brine is the only effective chemical trapping mechanism. Its efficiency is controlled by the solubility of CO2 and by the mobility of the CO2 plume which affects its size and shape and thus the contact area between the plume and the brine. Porosity and permeability changes following CO2 injection are small and have little effect on the injectivity or the mobility of the plume. The model is extended to examine processes at the aquifer/seal interface and within the clay formation following the rise of the CO2 plume towards the seal. Reactive transport simulations include diffusion and electromigration into the clay and involve a new method for an explicit treatment of a diffuse layer on charged clay mineral surfaces. Simulations show that the diffuse layer exerts a strong impact on the breakthrough of the CO2-enriched, acidic plume into the clay as it influences all species as a function of their charge. The local pore water composition and mineral solubilities in the clay are affected, leading to patterns that cannot be predicted by conventional ion-exchange models.