EXPERIMENTAL ASSESSMENT OF CARBON-SULFUR CO-SEQUESTRATION IN A SILICICLASTIC BRINE AQUIFER

Co-sequestering SO₂ with CO₂ in saline aquifers could reduce parasitic energy costs associated with gas separation at coal-fired power plants. This practice would negate the need for subsidiary waste management and offer a financial incentive both when retrofitting existing and when building next generation power stations. In co-sequestration scenarios, characterizing each components' respective influence, as well as how this influence changes in the presence of impurities, is a prerequisite for implementation. Toward this aim, we performed two hydrothermal experiments using rocking autoclaves to simulate in-situ conditions (110°C, 250 bar), injection, and mixed fluid-rock interaction, and compared the results to a numerically modeled brine-rock system. The reservoir replicated the Weber Sandstone in southwest Wyoming, an anhydrite and dolomite cemented, pyrite-bearing, arkosic sandstone housing a Na-SO₄ brine (ionic strength=0.37 m, pH=8.00). Synthetic brine and rock were combined in a 27:1 ratio and reacted for 2,000 hours before injecting 0.33 mol CO₂ and 0.36 mol CO₂ + 0.11 mmol SO₂ into the two systems, respectively. Experiments reacted with injected fluids for an additional 500 hours. Aqueous samples were obtained at logarithmic time intervals and analyzed for major, minor, and trace elements, and CO₂ and reduced sulfur in brine and gas. Unreacted and reacted solids were compared using XRD and SEM.

Previous modeling hypothesizes that co-injected SO₂ will disproportionate into H₂S and H₂SO₄, decreasing pH in formation waters at least one pH unit more than acidification by CO₂ alone. In this study, SO₂ entirely dissolved into brine within 24 hours of injection, and in-situ pH decreased from 7.11 to 3.86 during this time. In-situ pH closely tracked the pure CO₂-brine-rock system, never differing by more than 0.06 pH units post-injection. Brine acidification dissolved solids, affecting anhydrite and dolomite most drastically. Aqueous concentrations of Fe, Mg, and Mn increased considerably after injection, but without an appreciable difference between experiments. The comparatively similar results in this study indicate that co-injected SO₂ does not generate extreme pH. Thus, co-sequestration is a potentially viable strategy for advancing carbon capture and storage closer to commercial reality.