Is It Possible to Detect Gas Seepage from CO2-EOR Projects? Experience at Rangely, Colorado, with CO2-EOR, and Teapot Dome, Wyoming, at Baseline Condition

Ronald W. Klusman, Dept. of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401, phone: 303-273-3617, fax: 303273-3629, [email protected]

Carbon dioxide sequestration in spent oil and gas fields as part of a CO2-EOR program will likely be an early option that has economic advantages. A concern is the potential for gas microseepage under the overpressured conditions necessary for operation, particularly for CH4. A comparison was made between the overpressured Rangely, Colorado CO2-EOR operation and the underpressured Teapot Dome oil field in Wyoming.

Fluxes of CO2 and CH4 into the atmosphere under winter conditions of low soil biological activity were determined at both the Rangely and Teapot Dome fields. Shallow soil gas concentrations and stable carbon isotopes on soil gas CO2 were also determined. Shallow soil gas composition exhibited large differences for CH4 at Rangely, reflecting high rates of microseepage at a few locations. Stable carbon isotope measurements aided in the recognition of anomalous areas at both Rangely and Teapot Dome.

Ten-meter deep holes were augured for nested soil gas sampling at selected locations of interest for more thorough characterization, including areas of gas microseepage and background. In anomalous locations, a substantial proportion of deep-sourced CH4 was bacterially oxidized in the unsaturated zone, producing isotopically distinctive, and radiocarbon-depleted CO2. Carbon isotopic composition of surface materials, such as calcite veins, caliche, vegetation, soil organic and inorganic matter are essential in the characterization of processes operating in the near-surface, that reflect seepage.