Modification of Fracture Hydraulic Properties by CO2-Acidified Brine Flow

Geological storage of CO₂ is a promising option for carbon mitigation, but its application faces challenges such as the possibility of CO₂ leakage. Quantification of the leakage risks requires characterization of the porosity and permeability of the porous media, and prediction of dynamic evolution of these properties caused by rock-CO₂-brine interactions. Fractures in porous media are of special interest as they can be created, activated and propagated during CO₂ injection (e.g. In Salah CO₂ storage site). Moreover, they serve as preferential-flow pathways, and allow constant flushing of the solutes, which may lead to enhanced rock- CO₂-brine reactions.

In this study, we aim to quantify the changes in fracture permeability and flow velocity fields caused by exposing a fractured carbonate caprock sample to CO₂-acidified brine flow. Before and after a flow-through experiment, X-ray computed micro-tomography scans were taken. The images were segmented and processed to reconstruct the initial and final fracture geometries. 1-D empirical equation, 2-D steady state flow model, and 3-D computational fluid dynamics (CFD) simulation were applied on the reconstructed fracture geometries to estimate the intrinsic permeabilities prior to and after the experiment. The capabilities of the 1-D empirical equation, 2-D steady state flow model and 3-D CFD simulation in quantitatively capturing the permeability evolution are also examined. Furthermore, comparing the detailed pre- and post- fluid velocity field provides important insights on the direction of local geochemical reactions. The simulation results serve to deepen our understandings of the modification of fracture hydraulic properties by CO₂-acidified brine flow, and hence to improve our predictive capability on evolution of caprock integrity and leakage risks.