Supercritical carbon dioxide and sulfur in the Madison Limestone: A natural analog in southwest Wyoming for geologic carbon–sulfur co-sequestration

The Madison Limestone on the Moxa Arch, southwest Wyoming, USA contains large volumes (65–95%) of supercritical CO2 that it has stored naturally for 50 million years. This reservoir also contains supercritical H2S, aqueous sulfur complexes (SO42− and HS−), and sulfur-bearing minerals (anhydrite and pyrite). Although SO2 is not present, these sulfur-bearing phases are known products of SO2 disproportionation in other water–rock systems. The natural co-occurrence of SO42−, S2−, supercritical CO2 and brine affords the opportunity to evaluate the fate of a carbon–sulfur co-sequestration scenario.Mineralogic data was obtained from drill core and aqueous geochemical data from wells outside and within the current supercritical CO2–sulfur–brine–rock system. In addition to dolomite, calcite, and accessory sulfur-bearing minerals, the Madison Limestone contains accessory quartz and the aluminum-bearing minerals feldspar, illite, and analcime. Dawsonite (NaAlCO3(OH)2), predicted as an important carbon sink in sequestration modeling studies, is not present. After confirming equilibrium conditions for the Madison Limestone system, reaction path models were constructed with initial conditions based on data from outside the reservoir. Addition of supercritical CO2 to the Madison Limestone was simulated and the results compared to data from inside the reservoir. The model accurately predicts the observed mineralogy and captures the fundamental changes expected in a Madison Limestone-brine system into which CO2 is added. pH decreases from 5.7 to 4.5 at 90 °C and to 4.0 at 110 °C, as expected from dissolution of supercritical CO2, creation of carbonic acid, and buffering by the carbonate rock. The calculated redox potential increases by 0.1 V at 90 °C and 0.15 V at 110 °C due to equilibrium among CO2, anhydrite, and pyrite. Final calculated Eh and pH match conditions for the co-existing sulfur phases present in produced waters and core from within the reservoir. Total dissolved solids increase with reaction progress, mostly due to dissolution of calcite with an accompanying increase in dissolved bicarbonate. The Madison Limestone is a natural example of the thermodynamic end point that similar fluid–rock systems will develop following emplacement of a supercritical CO2–sulfur mixture and is a natural analog for geologic carbon–sulfur co-sequestration.

► The Madison Ls is a CO2 reservoir containing S minerals, aqueous complexes, and gas.
► Equilibrium model accurately predicts changes expected due to CO2 emplacement.
► Equilibrium model also predicts coexisting S- and Al-bearing phases.
► The Madison Ls is an analog for geologic carbon–sulfur co-sequestration.