Experimental study of reactive flow in an Eau Claire fracture exposed to CO₂-rich brine

Fractures in caprocks represent potential integrity breaches and this integrity may be altered over time if the permeability evolves substantially due to reaction with flowing fluids. This is especially relevant in the context of CO₂ geological storage in which acid-promoted reactions are possible, leading to physical changes of fracture permeability and therefore leakage potential. In this study, we report an experimental study of reactive flow in a core sample from the Eau Claire formation. The core had been collected to characterize the primary sealing unit for the FutureGen 2.0 project, which is injecting CO₂ into the underlying Mt Simon sandstone. Synthetic brine was saturated with CO₂ at 11.0 MPa and flowed through the fracture at a confining pressure of 12.8 MPa for three weeks. Computed tomographic (CT) images from X-ray scanning before and after the experiment showed a small decrease in average aperture. The effective hydraulic aperture calculated from flow and pressure measurements decreased from 6 μm to 4 μm, and is consistent with the CT analyses. Composition maps of Ca abundance based on X-ray fluorescence indicate dissolution of Ca-bearing minerals close to the fracture surface. However, extensive mineralogical analyses from SEM-BSE, EDS and XRD showed low potential of reactivity and no evidence of any secondary precipitation. The decrease of the fracture permeability is consistent with pressure-enhanced dissolution of critical asperities, but the CT image resolution does not allow direct observation of this mechanism. This experimental study provides one piece of evidence supporting the suitability of the Eau Claire as a reliable caprock.