Biological, geological and chemical effects of oxygen injection in underground gas storage aquifers in the setting of biomethane deployment.

The last few years have seen the proliferation of anaerobic digestion plants to produce biomethane. Oxygen (O 2 ) traces added to biogas during the desulfurization process are co-injected in the gas network and can be stored in Underground Gas Storage (UGS). However, there are no data available for the undesirable effects of O 2 on these anoxic environments, especially on deep aquifers. In addition to mineral alteration, O 2 can have an impact on the anaerobic autochthonous microbial life. In our study, the storage conditions of an UGS aquifer were reproduced in a high-pressure reactor and bio-geo-chemical interactions between the aqueous, gas and solid phases were studied. Sulfate was depleted from the liquid phase for three consecutive times during the first 130 days of incubation reproducing the storage conditions (36 °C, 60 bar, methane with 1% CO 2 ). Sulfate-reducers, such as Desulfovibrionaceae, were identified from the high-pressure system. Simulations with PHREEQC were used to determine the thermodynamic equilibrium to confirm any gas consumption. CO 2 quantities decreased in the gas phase, suggesting its use as carbon source by microbial life. Benzene and toluene, hydrocarbons found in traces and known to be biodegradable in storages, were monitored and a decrease of toluene was revealed and associated to the Peptococcaceae family. Afterwards, O 2 was added as 1% of the gas phase, corresponding to the maximum quantity found in biomethane after desulfurization process. Re-oxidation of sulfide to sulfate was observed along with the end of sulfate reducing activity and toluene biodegradation and the disappearance of most of the community. H 2 surprisingly appeared and accumulated as soon as hydrogenotrophic sulfate-reducers decreased. H 2 would be produced via the necromass fermentation accomplished by microorganisms able to resist the oxic conditions of 4.42·10 -4  mol.Kgw -1 of O 2 . The solid phase composed essentially of quartz, presented no remarkable changes.

Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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