Bulk and isotopic characterization of biogenic CO2 sources and variability in the Weyburn injection area

Abstract

To help evaluate surface monitoring tools for Weyburn, it is important to establish ranges of natural variation, and signal to noise ratio (SNR) of MMV tools in their intended setting. This study took place at three sites, two of which were in the injection field. For six months, we measured parameters at various temporal scales from half-hourly (CO2 surface flux and meteorology), to monthly (soil gas CO2 and δ13CO2), to bi-monthly (soil gas CO142), to compare SNRs of promising MMV techniques for Weyburn. Our summary of findings is as follows:1.All observed data fell within the range of values considered normal for Weyburn and for proximal control sites such as the Minard farm.2.High temporal variation in CO2 surface fluxes were observed. Lower atmospheric CO2 concentrations were also highly variant, and coupled with abiotic factors. A modelling strategy was able to reduce observed variability by 80–95%. When used together, soil CO2 surface flux + modelling methods can produce high SNRs for leak detection.3.Temporal variability in soil profile CO2 concentration was controlled by soil gas diffusivity (soil wetting/drying) and not biological production. Despite various sources of noise, we conclude that soil gas bulk CO2 investigations can still be useful for MMV.4.There were many possible influences on δ13CO2, including biological variation, normal steady and non-steady state physical transport (several ‰), spatial differences (0–3‰), and temporal fluctuations (0–3‰). The effects of these influences are cumulative. Relative to this background variation, the Cenovus-source δ13CO2 is not highly differentiated, and δ13CO2 is not a robust tracer.5.High precision radiocarbon soil profile data indicates that CO2 produced within the soil profile is modern and its average age is less than decades old. This age is consistent with other studies, and recent Kerr investigations (Trium, 2011). There was a tendency towards older CO142 production with increasing depth. There is a marked differentiation in CO142 signature from deep gases, and low variation. Radiocarbon is a very promising tracer for Weyburn with high SNR.