GEOLOGIC CARBON-SULFUR SEQUESTRATION (CO-SEQUESTRATION): EXPERIMENTAL INVESTIGATION OF SULFUR IN A NATURAL ANALOGUE, MADISON LIMESTONE OF THE MOXA ARCH IN SW WYOMING

Emissions from coal-fired power plants contain sulfur in addition to CO₂, therefore it is important to understand the reactions that occur when SO₂ is co-injected with CO₂ (i.e. geologic co-sequestration). For 50 million years, the Mississippian Madison Limestone of the Moxa Arch has naturally contained a mix of CO₂ (66%-95%), H₂S (5%) and variable amounts of CH₄, N₂, and He. SO₂ is not present in the gas, however if SO₂ is co-injected with CO₂ into a saline aquifer, the S⁴⁺ disproportionates into S⁶⁺and S^2-, both of which occur in the Madison Limestone in minerals and solution.

We performed laboratory experiments to understand potential brine-rock-carbon-sulfur reactions that take place during co-injection of supercritical CO₂ and SO₂ into a carbonate reservoir. Initial experiments (250 bars, 110°C) evaluated supercritical CO₂-brine-rock and brine-rock reactions in a Na-Ca-Cl brine (I=0.52 M, 80 mM SO₄^2-) and two different synthetic rock types: 83% Do, 10% Cc, 6% Anh, 1% Py and 86% Do, 13% Cc, 1% Py. The synthetic rock is composed of 75% chips for textural analysis, 0.5-3.0 mm in size, and 25% powder (<45 um) to enhance reaction rates. Fluid samples are collected in a time series throughout the experiment. On reaching steady state (~1080 hrs) supercritical CO₂ is injected into the ongoing reaction.

After injection, the dissolved CO₂ concentration in the brine of the Do-Cc-Anh-Py experiment increased from 1mM to 1.27 M (5.1% dissolved), and bench pH decreased from 7.4 to 6.4. In the Do-Cc-Py experiment, dissolved CO₂ increased from 0.78 mM to 1.22 M (5.1% dissolved) and bench pH decreased from 8.8 to 6.6. CO₂ injection mobilized Ba, Fe, Mn, Ni and Zn in both experiments. XRD and SEM data suggest calcite dissolved from the powders and re-precipitated as newly formed calcite. A decrease in Ca and SO₄^2- concentrations in both experiments suggests anhydrite precipitation, notably as seen in XRD in the Do-Cc-Py experiment. Calcite and anhydrite mineralization in the experiments is consistent with petrologic observations of the Madison Limestone. The injection of CO₂ drives the SO₄^2- concentrations down as the system is saturated with supercritical CO₂. Future experiments will incorporate SO₂ into supercritical CO₂-brine-rock experiments to evaluate carbon-sulfur co-sequestration.