Susan D. Hovorka

Lecture SH1: Put it back: Geologic sequestration for greenhouse gas emissions reductions

Oxidation (by burning) of carbon stored in fossil fuels – coal, oil, gas – has transferred more than 250 billion tons of carbon from into the atmosphere since 1800. Changes in atmospheric concentration of CO2 show that the rates of emission exceed the rates of uptake, resulting in increase of atmospheric CO2 from less than 290 ppm in 1880 to 385 ppm present day. Fossil fuel reserves and rapidly growing global energy demand suggest that continuation of the current ”business as usual” evolution will result in rapidly increasing concentration of CO2 in the atmosphere. A variety of risks ranging from increased average temperature to decreased pH of ocean surface water are predicted to result from this changing atmospheric concentration.

A spectrum of alternatives to the ”business as usual” scenario have been proposed. The Gulf Coast Carbon Center (GCCC) is focused on research relevant to the option known as carbon capture and geologic sequestration. This process involves capturing the CO2 at point sources before it enters the atmosphere, compressing it, and injecting the CO2 to depths below and isolated from fresh water. This creates a closed cycle where carbon is extracted as fossil fuel, the energy is extracted by conventional or novel oxidation processes, and the resulting CO2 is injected into deep subsurface settings where it will be stored for geological time periods.

GCCC research shows that it is feasible to inject the relevant volumes and reasonable to expect them to be retained. Conventional oil and gas tools are used in novel ways to document the processes of retention. Seals should perform much as they do for oil and gas to limit the rise of CO2 under injection pressure and buoyancy. Much of the CO2 will be immobilized within intergranular pores by capillary processes by the same physics that limit extraction of oil and gas. The potential for CO2 escape is also limited by the solubility of CO2 in large volumes of subsurface brine. Assessed risks are moderate and manageable.

Lecture SH2: Risks and benefits of geologic sequestration of carbon dioxide – how do the pieces fit together?

Geologic sequestration of CO2 as part of the carbon capture and storage (CCS) process has proven potential to provide the needed reduction in atmospheric emissions. Before this process is accepted as a dominant part of the solution, it is important to consider the possibility that hazards resulting from this action will create unacceptable damages. Capture of CO2 from point sources would also create a significant revenue stream as CO2 could be sold for enhanced oil recovery (CO2-EOR). The benefit of CO2 EOR to the economy would be significant, but it is important to examine how CO2 EOR fits into the carbon balance and the long term solution.

Techniques to assure that a sequestration site will retain stored CO2 for geologically significant time periods include site characterization, modeling, and monitoring. Risks from failure of a sequestration site to perform properly include

• leakage to protected groundwater, impact on ecosystems and
• return to the atmosphere, and damage to other subsurface resources (gas reservoirs in particular).

Assessment shows that:

• maximum damage is limited and moderate and
• risks can be reduced by implementing a permitting process that favors high quality sites with adequate monitoring.

Ambrose, W. A., Lakshminarasimhan, Srivatsan, Nuñez López, Vanessa, Hovorka, S. D., and Duncan, I. J., 2007, Geologic factors controlling CO2 storage capacity and permanence: case studies based on experience with heterogeneity in oil and gas reservoirs applied to CO2 storage: Environmental Geology: Springer-Verlag, DOI 10.1007/x00254-007-0940-2, 16 p.

Hovorka, S. D., Holt, R. M., and Powers, D. W., 2007, Depth indicators in Permian Basin evaporites, in Schreiber, B. C., Lugli, S., and Babel, M., Evaporites through space and time: The Geological Society of London, Special Publication 285, p. 335–364.

Hovorka, S. D., Doughty, Christine, Benson, S. M., Freifeld, B. M., Sakurai, Shinichi, Daley, T. M., Kharaka, Y. K., Holtz, M. H., Trautz, R. C., Nance, H. S., Myer, L. R., and Knauss, K. G., 2006, Measuring permanence of CO2 storage in saline formations: the Frio experiment: Environmental Geosciences, v. 13, no. 2, p. 105–121.