Rate measurements and detection of gas microseepage to the atmosphere from an enhanced oil recovery/sequestration project,Rangely, Colorado,USA

One of the proposals for large-scale sequestration of fossil fuel-derived CO₂ is deep geologic disposal in depleted oil/gas reservoirs or deep aquifers. Previously published scenarios for this inadequately proven technology have either ignored or dismissed the possibility of vertical migration of gases caused by overpressure. Overpressuring of a reservoir or aquifer will be necessary in order to have acceptable rates for dispersal of injected CO₂. This research describes methodology and the results of measurement of microseepage of CO₂ and CH₄ at a large-scale CO₂-enhanced oil recovery (EOR) operation at Rangely, Colorado, USA. Shallow and deep soil gas concentrations, and direct transport of CO₂ and CH₄ into the atmosphere were measured. The interpretation of the measurements was complemented by both stable and radiogenic isotopic measurements of C. The results have demonstrated an estimated microseepage to the atmosphere of approximately 400 metric tonnes of CH₄/a from the 78 km² area of the Rangely field. Preliminary estimates of deep-sourced CO₂ losses are <3800 tonnes/a, based on stable isotope measurements of soil gases. Several holes up to 10 m deep were drilled on, and off the field for nested gas sampling of composition and stable C isotopic ratios for CO₂ and CH₄. Carbon-14 measurements on CO₂ from these holes indicate that deep-sourced CO₂ microseepage losses were approximately 170 tonnes/a.