Geochemistry of CO2 sequestration in the Jurassic Navajo Sandstone, Colorado Plateau, Utah

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Abstract
The Jurassic Navajo Sandstone on the Colorado Plateau of Utah may be considered for sequestration of CO2, because it is thick, widely distributed, has a high porosity and permeability, is typically horizontal to gently folded, is favorably located with respect to seal strata, and underlies many large point sources of CO2. However, faulting common on the Colorado Plateau may provide pathways for leakage of the CO2 similar to present-day geysers and CO2-charged springs. Natural groundwater present in the Navajo Sandstone includes a range of low-salinity, moderate-salinity with high bicarbonate, and high-salinity waters. Higher salinity waters may have moved from deeper strata under artesian pressures or originated from solution of evaporite in pre-Jurassic rocks. These characteristics make the Navajo Sandstone an excellent analog for examining the geochemistry of CO2 injection into deep saline aquifers. The storativity of CO2 in solution is a function of the solubility of CO2 in these waters, which is dependent on salinity, temperature, and pressure. Geochemical modeling shows that the coolest, least saline water can contain the most CO2 in solution. Dissolving CO2 in the water lowers the pH, so that no minerals precipitate, and the Navajo Sandstone contains only small amounts of mineral that may consume the H+. The reaction of the acidic water produced by dissolving CO2 with K-feldspar and minor clays and calcite in the sandstone throughout 500 yr consumes little H+ and produces only small amounts of product minerals. The Navajo Sandstone likely would not store significant CO2 as mineral precipitate, and thus, stored volumes of CO2 would be limited by its solubility in the in-situ water and storage as free CO2 in pore space.

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