Application of Soil Gas Composition, Carbon Isotopes, and Gas Fluxes to the Atmosphere in Order to Detect Low Rates of Leakage from CO2 Sequestration Projects

Carbon dioxide sequestration in spent oil and gas fields will likely be a significant early option that has economic advantages. A concern is the potential for gas microseepage under the overpressured conditions necessary for operation, particularly for indigenous CH4. A comparison was made between the overpressured Rangely, Colorado CO2 operation and the underpressured Teapot Dome oil field in Wyoming. Fluxes of CO2 into the atmosphere under winter conditions of low soil biological activity were similar, whereas fluxes of CH4 were different by a factor of 100, respectively, for Rangely and Teapot Dome. Shallow soil gas composition was similar for CO2, but also exhibited large differences for CH4, reflecting high rates of microseepage at a few locations related to faulting. Near-surface CH4 will be potential trouble in oil and gas field sequestration operations. Stable carbon isotope and carbon-14 measurements aided in the recognition of anomalous areas, and in identifying gas sources. Ten-meter deep holes were augured for nested soil gas sampling at selected locations of interest for more thorough characterization. In anomalous locations, a substantial proportion of deep-sourced CH4 was microbially oxidized in the unsaturated zone, producing isotopically distinctive and radiocarbon-depleted CO2. Carbon isotopic composition of surface materials, such as calcite, caliche, vegetation, soil organic and inorganic matter are essential in the characterization of processes operating in the near-surface.