Supercritical carbon dioxide–brine–rock reactions in the Madison Limestone of Southwest Wyoming: An experimental investigation of a sulfur-rich natural carbon dioxide reservoir

The Madison Limestone on the Moxa Arch, southwest Wyoming, USA is a sulfur-rich natural CO2 reservoir. A series of hydrothermal experiments was performed to evaluate multi-phase (CO2–H2O)–brine–rock reactions and processes in this reservoir and to test the hypothesis that this reservoir is a natural analog for geologic carbon–sulfur co-sequestration. Idealized Madison Limestone (dolomite–calcite–anhydrite–pyrite) and Na–Cl–SO42 − brine (I = 0.5 molal) reacted at 110 °C and 25 MPa for approximately 81 days (1940 h). Supercritical CO2 was then injected and the experiment continued for approximately 46 days (1100 h). A parallel experiment was performed without supercritical CO2 to provide a basis of understanding for the interaction of supercritical CO2 with the brine–rock system. Two additional experiments were conducted in the same manner, but without anhydrite in the starting mineral assemblage, to examine supercritical CO2–sulfur reactivity. ion of supercritical CO2 decreases pH by 2.5 to 3.3 units, increases Eh by 0.19 to 0.23 V, and drives reaction pathways along the pyrite–anhydrite saturation boundary of an Eh–pH diagram. The dolomite–calcite–anhydrite mineral assemblage and reaction textures that are produced are consistent with those observed in the natural CO2 reservoir. The mineral assemblage does not change following emplacement of supercritical CO2; instead, minerals dissolve, mobilize and re-precipitate. Anhydrite precipitates in the dolomite–calcite–pyrite experiment following injection of supercritical CO2 and provides a mineral trap for sulfur. Anhydrite precipitation decreases SO42 − activity, ultimately leading to mineralization of CO2. Experimental results support the hypothesis that the Madison Limestone on the Moxa Arch is a natural analog for geologic carbon-sulfur co-sequestration.