Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Second Annual Conference on Carbon Sequestration, Hilton Alexandria Mark Center, Alexandria, Virginia, May 5-8, 2003
of CO2 Sequestered
as a Residual Phase
Mark H. Holtz
Geologic sequestration of CO2 in brine-saturated formations has been proposed as a possible method to reduce emissions of this greenhouse gas to the atmosphere. To optimize this method the largest possible volume of CO2 should be sequester over geologic time. This optimization goal can best be archived by sequestering CO2 as a residual phase under the most advantageous Geological conditions. Geological conditions that impact the volume of CO2 stored as a residual phase include petrophysics, burial effects, temperature and pressure gradients, and CO2 pressure-volume-temperature character. Analyzing and integrating all of these elements results in an optimal CO2 sequestration depth for a given geologic subprovience.
The integrated sequestration optimization model was constructed from petrophysical, geological, and CO2 characteristics. Sequestering CO2 as a residual nonwetting phase is the key to obtaining it's residency in rock over geologic time. Thus residual saturation and porosity were pivotal modeling characteristics. Sediment burial depth effects porosity, temperature, and pressure and thus they were integrated together. Finally, CO2 density as a function of temperature and pressure was accounted for, resulting in a model that combines all the salient properties that effects the amount of CO2 that can reside in buried rock.
A sequestration optimization curve for the Frio Formation, Upper Texas Gulf coast indicates that the largest volume of CO2 could be trapped as a residual phase at about 11,000 feet. The sequestration optimization curve of depth verses CO2 density is a concave down parabolic shape with a broad maximum indicating the optimum sequestration depth. Additionally, this depth decreases the risk of surface leakage and increases the pressure differential between hydrostatic and lithostatic, both characteristics with sequestration benefits.