From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Bureau Seminar, February 10, 2006

Optimizing Permanent CO2 Sequestration in Brine Aquifers: Example from the Upper Frio, Gulf of Mexico

Mark Holtz

Abstract:

Geologic sequestration of CO2 in brine-saturated formations has been proposed as a possible method for reducing emissions of greenhouse gas to the atmosphere. To optimize this method, the largest possible volume of CO2 should be sequestered over geologic time, which, for the purposes of relieving climate-changing increases in atmospheric CO2 concentration, can be thought of as permanent. The least risky way to achieve permanent sequestration is to store CO2 as a residual phase within a brine aquifer. This optimization goal can best be achieved by sequestering the gas 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 using petrophysical, geological, and CO2 characteristics because sequestering CO2 as a residual nonwetting phase is key to obtaining its residency in rock over geologic time. Sediment burial depth affects porosity, temperature, and pressure; thus, depth is a key input variable integrating the other parameters. Finally, CO2 density as a function of temperature and pressure was accounted for, resulting in a model that combines all 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 optimal sequestration depth. Additionally, greater depth decreases the risk of surface leakage and increases the pressure differential between hydrostatic and lithostatic, both characteristics having sequestration benefits.