Surface geochemical measurements applied to monitoring, verification, and accounting of leakage from sequestration projects

Abstract

A detailed study of [Formula: see text] and [Formula: see text] surface fluxes, shallow soil gas composition, 10-m hole soil gas composition was made at Rangely, Colorado, Teapot Dome, Wyoming, USA, and the results of geochemical verification measurements at Weyburn, Saskatchewan, Canada. Summer and winter soil gas and gas flux measurements were made at Rangely, and winter only at Teapot Dome. The objectives were to determine if leakage of [Formula: see text] and/or [Formula: see text] could be detected in the overpressured Rangely and the underpressured Teapot Dome systems. At Weyburn, the objective was the determination of the presence or absence of an alleged leak. Seasonal surface [Formula: see text] fluxes at Rangely were similar at on-field locations and an off-field control area. Methane fluxes were much higher at on-field locations than in the control area, suggesting a reservoir source. Seasonal differences in [Formula: see text] fluxes suggested methanotrophic oxidation was occurring in the soils. Shallow soil gas concentration measurements of [Formula: see text] were similar at a 100-cm depth, on-field and in the control area. Methane soil gas concentrations were higher on-field than in the control area; summer and winter. These data were used to select locations with and without evidence for seepage allowing nested sampling up to a 10-m depth. More complex chemical and isotopic measurements were made in 10-m holes at Rangely and Teapot Dome. Methanotrophy was operational at the active Rangely and the passive Teapot Dome systems. A small methane leakage rate of 400–700 tonnes [Formula: see text] and [Formula: see text] of [Formula: see text] tonnes [Formula: see text] were determined at Rangely, and rates near zero at Teapot Dome. Inert gases, carbon-containing gases, as well as isotopic ratios supported the presence of pathways at geochemically anomalous locations at Teapot Dome. Measurements of soil gas and inert gas isotopes in shallow groundwater by other authors at Weyburn, Saskatchewan, found no evidence of leakage.