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 50millionyears. This reservoir also contains supercritical H₂S, aqueous sulfur complexes (SO₄ ²⁻ 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 SO₄ ²⁻, 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 0.1V at 90°C and 0.15V 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.