Computer modeling of methanotrophic oxidation of hydrocarbons in the unsaturated zone from an enhanced oil recovery/sequestration project, Rangely, Colorado, USA

Measurements of CH4 and CO2 concentrations in soil gas, and as exchange rates (fluxes) with the atmosphere were made over the Rangely oil field, Colorado, USA. The Rangely field has been operated as a large CO2 enhanced oil recovery (EOR) project since 1986, and may serve as a prototype for CO2 sequestration in depleted oil/gas fields. The first of this pair of papers reported on the details of the methodology and results of field measurements of these gases, including light alkanes. The interpretation was enhanced by the extensive use of 13C and some 14C measurements. The fluxes of both CH4 and CO2 to the atmosphere were estimated. This research has demonstrated and quantified the methanotrophic oxidation of microseeping CH4 in the unsaturated zone. Strong spatial variations were found and studied further by nested soil gas sampling in holes up to 9 m deep. Areas of microseepage were found, as well as areas where atmospheric CH4 was the only source for methanotrophic oxidation. Computer modeling was used to determine rate constants for the oxidation process. Strong seasonal differences exist in the rates of CH4 oxidation because of the semiarid and severe climate of Rangely. Rate constants of <1 to >1000 day−1 were determined. During the summer, the highest rates were near the surface, but shifted downward to about 2 m depth in the winter. Methanotrophic oxidation of CH4 in the unsaturated zone will be the most important component of amelioration of CH4 microseepage induced by overpressuring of the reservoir undergoing CO2 injection. These types of measurements and interpretation will be necessary in order to fully understand the environmental effects of CO2 sequestration in oil/gas fields.