Lattice-Boltzmann simulation of dissolution of carbonate rock during CO2-saturated brine injection

Geological carbon sequestration is one of the key effective technologies proposed to reduce the atmospheric CO2, which is essential to achieve the net zero-emission target. Injection of supercritical CO2 in saline aquifers can trigger geochemical reactions, which would lead to a change of pore morphology, porosity and permeability of the rock. Reservoir models - as the key tools to simulate and predict the behaviour of a saline aquifer - employ reaction rates that are obtained from the batch experiments. Here, there is a critical discrepancy in the physical scales of a grid cell in a reservoir model and the physical scale of a batch experiment. To address this shortcoming, pore-scale models are utilized to upscale the geochemical reactions in the CO2-brine-rock system. We have developed a pore-scale volumetric lattice-Boltzmann model coupled with the geochemical simulator, PhreeqcRM, to investigate the pore-scale reactive transport in carbonate rocks. The model was successfully validated against two sets of pore-scale experiments. Finally, the evolution of porosity and permeability with time was studied under various flow rates, pressures and temperatures of the injected CO2-dissolved brine. Results show that with increase of flow rate, dissolution occurs more homogeneously in the rock, while at low injection rates, dissolution is larger closer to the injection boundary. As a result, for a given porosity, the permeability increase at higher injection rates is much more compared to small injection rates. Upscaled reaction rates initially increase with time until they reach a plateau reaction rates but still are smaller than the batch reaction rates. Finally, the implication of the results for the time scale of dissolution to impede the caprock integrity was explored.