Soil gas investigation of an alleged gas migration issue on a residential farm located above the Weyburn-Midale CO2 enhanced oil recovery project

Abstract

Stable (13C) and radiogenic (14C) carbon isotope tracers are recognized approaches to identify gas sources using soil gas monitoring techniques; however, definitive characterization from a subsurface injection/storage reservoir can be complicated by processes that produce and consume CO2. Few studies assess the combined use of compositional (fixed gases, hydrocarbons, and sulphurous compounds) and isotopic (13C, 18O and 14C) indicators from soil gases as tools to evaluate CO2 storage reservoir leaks. Compositional and isotopic indicators from soil gases were compared at an investigation site (IS) alleged to be impacted by CO2 leaking from an underlying injection reservoir with a baseline control site (BCS) outside of the injection area. Soil gas δ13CCO2 values collected from the IS (−23.4‰to −22.6‰) were comparable with the range observed at the BCS (−22.8‰ and −23.3‰), falling within the range anticipated for both biogenic and injected CO2. 14C values were also comparable between IS (98.0 pMC to 107.0 pMC) and BCS (102.8 pMC to 105.3 pMC) sites, representing a modern carbon source that was distinct from the CO2 injection gas (0.34 pMC). The results provide several lines of evidence that CO2 fluxes in soil gases at the IS were related to naturally occurring processes and were not caused by a gas migration issue from a geological CO2 storage reservoir.