Physicochemical and microbiological effects of geological biomethane storage in deep aquifers: introduction of O2 as a cocontaminant

Biomethane is considered one of the most promising energy vectors to substitute fossil fuels during the global energy transition. Its production is steadily increasing, and high storage volumes are needed to cover seasonal needs. Existing underground gas storage (UGS) aquifers, which have been used for natural gas storage, are excellent candidates. Underground aquifers are known for being anoxic systems. However, dioxygen (O2) can be injected as an impurity with biomethane into these anoxic environments. O2 limitations in the underground vary worldwide; however projects are conducted to optimize these limitations. It has been shown that O2 presence can affect the aquifer's ecosystems and induce mineral reactions. Thus, a multidisciplinary study was conducted in which the in situ conditions were simulated in a high-pressure reactor. Water containing autochthonous microorganisms and reservoir rock were used as the aqueous and solid phases, respectively. Initially, the gas phase was composed of methane, 1% CO2, benzene and toluene under 60 bar and 36 °C conditions. Sulfate was depleted from the aqueous phase due to sulfate-reducing microorganismes. After 50 days, 100 ppm O2 was injected into the gas phase. Sulfate reducers were inactivated; however, other taxonomic groups became dominant, such as members of the class Acidobacteriae and the families Desulfitobacteriaceae and Kineosporiaceae. Hydrocarbon biodegradation was demonstrated by a benzene decrease in the aqueous phase, which was barely affected by O2 injection. However microbial analyses suggested a shift in the ecosystem to adapt to this new ‘low aerobic’ conditions. The findings of this study can help for better understanding of any other process including O2 as an impurity in UGS such as CCS and CAES.