In-situ study of CO₂-saturated brine reactive transport in carbonates considering the efficiency of wormhole propagation

Deep limestone aquifers are potential CO₂ storage sites, but CO₂-saturated brine reacts with the carbonate rock, changing its transport and storage properties. This study provided a preliminary investigation of the optimal injection rate of CO₂-saturated brine in carbonate rocks. Indiana limestone cores were subjected to CO₂-saturated brine injection at varied rates using an HPHT coreflooding setup with X-ray CT monitoring. The samples were characterized pre- and post-treatment in terms of porosity and pore size distribution using a gas porosimeter and NMR T₂ measurements. Moreover, the reaction was evaluated by measuring the aqueous effluent calcium ions concentration as a function of throughput using ICP-OES analysis. A high-resolution micro-CT scan was used to capture the dissolution post-treatments and characterize the wormhole's size and patterns. Results showed that the wormholes broke through to the sample exit face after injecting 160, 48, and 36 pore volumes at 0.5, 1, and 2 cm³/min, respectively thereby revealing the importance of injection velocity. The ICP-OES analysis revealed that a larger dissolution rate was achieved at 2 cm³/min, which explained the fast wormhole propagation. An increase in rock porosity and the pore-size distributions was observed after coreflooding on all samples with minimum precipitation, as concluded from the NMR T₂ relaxation time. A universal optimum Damköhler number can be obtained that enables calculating the optimum injection rate of CO₂-saturated brine at different rock and fluid conditions. We speculated that the optimum Damköhler number could be different from the value of 0.29 proposed by Fredd and Fogler (1998). This study provides a preliminary understanding of the optimal CO₂-saturated brine injection velocity that has an application for CO₂ storage, water alternating gas (WAG) operations, and acid stimulation of carbonate formations.