Safe Storage Under Strain: Geochemical and Geomechanical Alteration by Acidic Brine of Artificially Fractured and Strained Shale

The efficacy of geologic carbon sequestration is dependent on the sealing capacity of a caprock, often a low-permeability shale. During sequestration, injected CO₂ dissolves into brine, resulting in a low pH brine plume that expands through the reservoir and can eventually reach the caprock. The combination of injection pressure and dissolution of reactive minerals, such as carbonates, can lead to propagating fractures in the caprock, risking leakage. This work investigates coupled chemo-mechanic effects of acidic brine flow on a milled, artificially fractured shale under strain using Marcellus shale (40% carbonate content). Samples were held under strain in a shearing flow cell housed within an industrial CT scanner. Brine at pH 4 or at pH 9.5 (reservoir-simulated) was flowed through the fracture for 7 days. Geochemical change was analyzed by effluent elemental composition with ICP-OES and sample surface measurements with SEM-EDS. CT imagery was collected and used to evaluate fracture aperture change and strain release as well as use in flow modeling with 2D Navier-Stokes model. Ultimately, the acid-reacted shale exhibited faster reduction of strain than the reservoir-simulated shale through microfracturing and limited dissolution of reactive minerals.