IDENTIFICATION OF SURFACE MICROSEEPAGE AT THE RANGELY CO2 EOR PROJECT, COLORADO

Surface flux to the atmosphere and soil gas measurements of methane and carbon dioxide are being made at the carbon dioxide-enhanced oil recovery project at Rangely, Colorado. This large-scale EOR project can be considered analogous to a large-scale prototype sequestration project that has been operating since 1986. Both winter and summer measurements have been made in order to understand noise caused by root respiration and microbiological oxidation of soil organic matter. Several deep holes have been drilled to obtain a profile of soil gas composition with depth. Deep-sourced carbon dioxide flux has been measured as in the range of 170-3800 metric tonnes per year over the 78 square km area of the field. This range was constrained by del C-13 ratios at the upper end of the range, and C-14 measurements at the lower end. Deep-source methane flux has been measured as 400 metric tones per year over the field. This was constrained by seasonal measurements over the field and a nearby control area. A deep thermogenic source for methane was supported by del C-13 measurements. The microseepage over the field appears to be patchy, or as plumes several tens of meters in diameter. Geochemical modeling of the soil gas concentration and fluxes suggests transport of methane due to buoyancy below the water table, and by diffusion above the water table. Methanotrophic oxidation of most of the microseeping methane occurs in shallow soils, particularly in the summer. Carbon dioxide production in soils, and transport to the surface is more difficult to model. Aqueous geochemical modeling suggests approximately 90% of the injected carbon dioxide has dissolved in the formation waters, with 10% by dissolution of carbonate minerals in the reservoir. A small amount of dawsonite is possibly forming.