SUITABILITY OF THE NAVAJO SANDSTONE FOR CO2 SEQUESTRATION IN CENTRAL UTAH

Growing concern over possible global climate change has spurred research toward finding ways to reduce greenhouse gas emissions. One of the best options volumetrically is to collect CO2 from point sources and inject it into deep, saline aquifers. This study has examined the deep, saline aquifers underling the Drunkards Wash, Helper and Buzzard Bench gas fields, located in central Utah, to determine if this is a suitable location for the sequestration of CO2 gas. Salt water (as part of the production of coal bed methane) currently is being injected into the Navajo Sandstone and other formations at depths ranging from 4,558 to 8,218 feet. By studying the geochemistry, it may be possible to determine if the salt water is migrating from the deep formations into shallow aquifers along faults. Water samples have been analyzed for the stable isotope ratios of deuterium/hydrogen (D/H), oxygen-18/oxygen-16 (18O/16O) and carbon-13/carbon-12 (13C/12C). Samples were also analyzed for major ions and trace metals.

Based on the geochemical results, no mixing appears to be taking place between the deep, salt-water disposal formations and the overlying, fresh-water aquifers. A graph of delta-D/H versus delta-18O/16O values shows that the salt- and fresh-water samples plot in separate portions of the graph. The salt water has high delta-13C/12C values (isotopically heavy), indicating that the carbon source likely is calcite. Water from the shallow aquifers is isotopically light, thus the source most likely is atmospheric and/or soil CO2 gas. The major ion and trace metal geochemistry of the injected salt water and the shallow groundwater are drastically different. The chemistry of the shallow groundwater samples is nearly uniform, and the injected salt water samples have much higher concentrations for nearly every constituent analyzed, indicating that mixing does not appear to be occurring on either a local or a regional scale.

A geologic analysis of the area will be done using geologic and topographic maps and aerial photos. These will be examined with an eye toward locating faults that may exist in the subsurface but do not displace rocks at the surface, as is common in areas where sandstones, which are brittle and fracture, are overlain by shales, which are ductile and fold rather than fracture, as is the case in the study area.