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 degrees 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. Injection 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. (C) 2012 Elsevier B.V. All rights reserved.