57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Geologic sequestration of carbon dioxide generated by coal-fired power plants is a critical component of Carbon Capture and Storage (Pacala and Socolow, 2004). In addition to carbon dioxide, coal combustion generates SO_x, NO_x, and other constituents.  Geologic sequestration of carbon dioxide that contains these constituents is known as co-sequestration.  The geochemical effects of co-injected carbon dioxide, SO_x, and NO_x on a geologic storage reservoir and its caprock are largely unknown.  The essential problem is that sulfur dioxide, the most abundant constituent in SO_x, is very reactive in water-rock systems.  We are undertaking a series of hydrothermal experiments to evaluate carbon dioxide-sulfur dioxide-brine-rock reactions in saline reservoirs.  Our results will help us to understand the fundamental processes that can occur in these fluid rock systems.  In doing so, we can help to determine the viability of co-sequestering sulfur with carbon and bring the management and storage of carbon emissions closer to a practical reality. 

Our experiments emulate actual carbonate and siliclastic formations, the Madison Limestone and Weber Sandstone, respectively.  Both formations are viable sequestration targets in Wyoming (Surdam and Zhao, 2007).  In southwest Wyoming these formations also house natural accumulations of carbon dioxide.  These two "natural analogs" have stored supercritical carbon dioxide for geologic time scales.  The Weber Sandstone is an anhydrite and dolomite cemented, pyrite-bearing, arkosic sandstone housing a Na-SO₄ brine (ionic strength = 0.4 molal, pH = 8.0).  The Madison Limestone is a dolostone that also contains calcite, anhydrite, and accessory pyrite and silicate minerals.  It houses a Na-Cl-SO₄^2- brine (ionic strength = 0.5 molal).  Hydrothermal experiments emulate both formations by reacting rock and brine at in-situ conditions (110°C and 25 MPa) for approximately 2000 hours.  Supercritical carbon dioxide containing 500 ppmv sulfur dioxide is then injected and the experiments continued for an additional 500 to 1100 hours.  Two types of parallel experiments are performed to provide a basis of understanding for the interaction of sulfur dioxide with supercritical carbon dioxide, brine, and rock:  1) experiments that react brine with rock in the absence of carbon dioxide; and 2) experiments that react brine and supercritical carbon dioxide with rock.  We are also exploring new directions in co-sequestration in our laboratory.  To investigate the fate of co-injected sulfur dioxide, a brine-rock experiment containing sulfate-free brine, dolomite, and calcite was injected with supercritical carbon dioxide containing 1800 ppmv sulfur dioxide.  To examine the potential of co-sequestering carbon dioxide and NO_x, we are conducting another suite of hydrothermal experiments.  These experiments emulate injection of supercritical carbon dioxide containing 2000 ppmv NO into the Weber Sandstone.

Synthesis and interpretation of the Madison Limestone experiments is complete and represents the culmination of one MS student's graduate research.  This student defended his thesis in December.  The Weber Sandstone experiments are also complete.  The results are currently being interpreted and represent a second MS student's graduate research.  This student plans to defend her thesis in July.  Stipends for these students have been funded via University graduate assistantships whereas laboratory expenses and student travel expenses are being funded with this PRF grant.  The following highlights represent findings from the new experiments as well as findings from analysis of data from experiments completed in the previous review cycle.

1)  Calcite in the supercritical carbon dioxide-sulfur dioxide-brine-rock experiments displays extensive dissolution textures relative to calcite in the carbon dioxide-brine-rock experiments.  The in-situ pH values of all of these experiments is virtually identical, as presented in last year's annual report.  Therefore, these textural relationships suggest that increased acidity produced by co-injecting sulfur dioxide and carbon dioxide is buffered by the dissolution of calcite.  

2)  In the experiment that explored the fate of co-injected sulfur dioxide in a carbonate reservoir, supercritical carbon dioxide and sulfur dioxide was injected three times in the span of eight days.  Both brine and the supercritical fluid in the experiment were sampled approximately 24 hours after each injection.  Sulfate concentrations in the brine increased accordingly.  No sulfur was detected in the supercritical fluid.  These results suggest that sulfur dioxide readily partitions out of supercritical carbon dioxide and into the brine.  In conjunction with our other results (the previous highlight as well as the findings discussed in last year's report), our experiments suggest that co-sequestration of carbon dioxide and sulfur dioxide is a viable option for Carbon Capture and Storage.

3)  Little quantitative geochemical work has been performed to evaluate the effects of co-sequestering carbon dioxide and NO_x (Wilke et al., 2012 and references therein).  In the experiments that explored injection of supercritical carbon dioxide and NO into Weber Sandstone, pH values closely track values observed in experiments injected with supercritical carbon dioxide as well as supercritical carbon dioxide and sulfur dioxide.  pH values among all of these experiments are within 0.1 pH units.  These preliminary results suggest that co-sequestration of carbon dioxide and NO_x is also a viable option for Carbon Capture and Storage.

References

Surdam, R.C., and Zhao, Z., 2007.  The Rock Springs Uplift: An outstanding geological CO₂ sequestration site in southwest Wyoming.  Wyoming Geological Survey, WSGS-2007-CGRD-02.

Wilke, F.D.H., Vkgquez, M., Wiersberg, T., Naumann, R., Erzinger, J., 2012. On the interaction of pure and impure supercritical CO₂ with rock forming minerals in saline aquifers: An experimental geochemical approach.  Applied Geochemistry, v., 27, p. 1615–1622.