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Holtz, M. H.,
Optimizing Permanent CO2 Sequestration in Brine Aquifers: Example from the Upper Frio, Gulf of Mexico
Mark H. Holtz1
1Praxair, Inc., Worldwide Headquarters, Danbury, Connecticut, U.S.A.; Present address: Praxair, Inc., International Business Development, EOR, Austin, Texas, U.S.A.
This research is funded by the U.S. Department of Energy National Energy Technology Laboratory under contract DE-AC26-98FT40417 to the Bureau of Economic Geology and by the GEO-SEQ project contract DE-AC03-76SF00098. The preparation and presentation of this chapter were funded by the John A. and Katherine G. Jackson School of Geosciences under matching funds to the Gulf Coast Carbon Center (GCCC). We thank our GCCC contributing members BP, Praxair, Kinder Morgan, and ChevronTexaco for their support. I also thank Larry W. Lake and Steven L. Bryant for their guidance in the study of this subject and Sue Hovorka for continued encouragement and Libby Stern for helpful guidance in writing. Susann Doenges reviewed the chapter. Patricia Alfano prepared the figures under the supervision of Joel L. Lardon, Media Information Technologies Manager. Publication was authorized by the director, Bureau of Economic Geology, The University of Texas at Austin.
Geological 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 the effectiveness of this method, the largest possible volume of CO2 should be sequestered over geological time. Sequestration over geological 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. 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 parameters result in an optimal CO2 sequestration depth for a given geological subprovince.
The integrated sequestration optimization model was constructed using petrophysical, geological, and CO2 characteristics. Sequestering CO2 as a residual nonwetting phase is one way to ensure its residency in rock over geological 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 3048–3657 m (10,000–12,000 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, greater depth decreases the risk of surface leakage and increases the pressure differential between hydrostatic and lithostatic so that higher injection pressures and, thus, higher injection rates can be obtained.
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