BASELINE MEASUREMENTS OF SOIL GAS EXCHANGE WITH THE ATMOSPHERE AT THE TEAPOT DOME SEQUESTRATION TEST SITE, WYOMING

Baseline measurements of soil gas exchange with the atmosphere are critical in the assessment of future gas microseepage potential and quantification of microseepage rates at CO₂ sequestration sites. Triplicate measurements of CH₄ and CO₂ atmospheric exchange, and soil gas concentrations were determined at 40 locations at the Teapot Dome oil field, Wyoming (National Geologic Carbon Storage Test Site; NGCSTS). Measurements were made in January, 2004 to allow the determination of gas exchange and soil gas concentrations when soil biological activity was at a minimum. Methane is important because of its mobility and inertness in the subsurface, relative to CO₂. Methane was previously demonstrated to be of potential concern at the CO₂-EOR project at Rangely, Colorado, where significant localized CH₄ microseepage was demonstrated.

The average CO₂ flux into the atmosphere at Teapot Dome was 227 mg m^-2day^-1, and for CH₄ was 0.137 mg m^-2day^-1. Both positive (upward) and negative (downward) fluxes for CH₄ were observed. Downward CH₄ fluxes and sub-atmospheric CH₄ concentrations were observed in soil gas due to methanotrophic oxidation below the frost line, with the system in very close balance. Concentrations of CO₂ in soil gas averaged 618, 645, and 1010 ppmv at 30-, 60-, and 100-cm depth, respectively, contrasting with an average atmospheric concentration of 379 ppmv during the measurement period at Teapot. Stable carbon isotopic ratios indicated that soil gas CO₂ was dominated by biological processes, even in the winter. Methane concentrations averaged 0.94, 0.92, and 0.92 ppmv, respectively, at 30-, 60-, and 100-cm. Ten-meter deep holes have been drilled at selected locations and equipped for nested gas sampling at five different depths.