Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
West Texas Geological Society Luncheon Meeting, March 8, 2005
Optimizing Permanent CO2 Sequestration in Brine-Bearing Aquifers: Example from the Frio Formation, Gulf of Mexico
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 sequestered over geologic time. Sequestration over geologic time can be thought of as permanent for the purposes of relieving climate-changing increases in atmospheric CO2 concentration. The least risky way to achieve permanent sequestration is to store the CO2 as a residual phase within a brine aquifer. This optimization goal can best be achieved by sequestering CO2 as a residual phase under the most advantageous geologic conditions. Geologic 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 parameters result in an optimal CO2 sequestration depth for a given geologic subprovince.
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 its residency in rock over geologic time. Thus residual saturation and porosity were pivotal modeling characteristics. Sediment burial depth affects porosity, temperature, and pressure; thus depth is a key input variable that integrates the other parameters. Finally, CO2 density as a function of temperature and pressure was accounted for, resulting in a model that combines all the salient properties that affect the amount of CO2 that can reside within buried rock.
A model for predicting residual nonwetting-phase saturation and a sequestration optimization curve (SOC) was developed. Results indicate that a sandstone porosity of 0.23 is optimal for CO2 sequestration. The SOC for the Frio Formation, Upper Texas Gulf Coast, indicates that the largest volume of CO2 could be trapped as a residual phase at about 10,000 to 11,000 ft. The SOC of depth versus CO2 residual phase bulk volume is a concave-down parabolic shape with a broad maximum indicating the optimal sequestration depth. Additionally, this depth decreases the risk of surface leakage and increases the pressure differential between hydrostatic and lithostatic, both characteristics having sequestration benefits.