Experimental Study of Porosity Changes in Shale Caprocks Exposed to Carbon Dioxide-Saturated Brine II: Insights from Aqueous Geochemistry.

Laboratory experiments evaluated two shale caprock formations, the Gothic Shale and Marine Tuscaloosa Formation, at conditions relevant to carbon dioxide (CO₂) sequestration. Both rocks were exposed to CO₂-saturated brines at 160°C and 15 MPa for ∼45 days. Baseline experiments for both rocks were pressurized with argon to 15 MPa for ∼35 days. Varying concentrations of iron, aqueous silica, sulfate, and initial pH decreases coincide with enhanced carbonate and silicate dissolution due to reaction between CO₂-saturated brine and shale. Saturation indices were calculated and activity diagrams were constructed to gain insights into sulfate, silicate, and carbonate mineral stabilities. Upon exposure to CO₂-saturated brines, the Marine Tuscaloosa Formation appeared to be more reactive than the Gothic Shale. Evolution of aqueous geochemistry in the experiments is consistent with mineral precipitation and dissolution reactions that affect porosity. This study highlights the importance of tracking fluid chemistry to clarify downhole physicochemical responses to CO₂ injection and subsequent changes in sealing capacity in CO₂ storage and utilization projects. [ABSTRACT FROM AUTHOR]