Biogeochemical characterization of four depleted gas reservoirs for conversion into underground hydrogen storage

Abstract

Depleted gas reservoirs are a valuable option for underground hydrogen storage (UHS). However, different classes of microorganisms, which are capable of using free H2 as a reducing agent for their metabolism, inhabit deep underground formations and can potentially affect the storage. This study integrates metagenomics based on Illumina‐NGS sequencing of bacterial and archaeal 16S rRNA and dsrB and mcrA functional genes to unveil the composition and the variability of indigenous microbial populations of four Italian depleted reservoirs. The obtained mcrA sequences allow us to implement the existing taxonomic database for mcrA gene sequences with newly classified sequences obtained from the Italian gas reservoirs. Moreover, the KEGG and COG predictive functional annotation was used to highlight the metabolic pathways potentially associated with hydrogenotrophic metabolisms. The analyses revealed the specificity of each reservoir microbial community, and taxonomic and functional data highlighted the presence of an enriched number of taxa, whose activity depends on both reservoir hydrochemical composition and nutrient availability, of potential relevance in the context of UHS. This study is the very first to address the profiling of the microbial population and allowed us to perform a preliminary assessment of UHS feasibility in Italy.

Investigating the activity of indigenous microbial communities from Italian depleted gas reservoirs and their possible impact on underground hydrogen storage

“…Regarding pH, batch cultures obtained from R1 and R3 stabilized their pH between 7.5 and 8 by day 23, whilst cultures obtained from R2 and R4 reached pH between 6.5 and 7. pH values measured during batch cultivation were slightly higher or within the ranges previously reported ( Bassani et al, 2023 ). Nonetheless increase in average pH is likely to be attributed to interactions with the provided media, which could have partially altered the pH.…”

“…Nonetheless increase in average pH is likely to be attributed to interactions with the provided media, which could have partially altered the pH. This is also sustained by the fact that in batch culture performed without external nutrient provision pH values remained stable along the experiment (Data not shown) and close to those previously reported by chemical characterization Bassani et al (2023) .…”

“…The inocula used during the present study was obtained by sampling formation waters in anaerobic conditions from four NG reservoirs which have been previously characterized and named as R1, R2, R3 and R4, as previously described ( Bassani et al, 2023 ). Upon receipt, water samples were stored overnight at 4°C under N 2 atmosphere to ensure anaerobic conditions.…”

“…■ Metagenomics analysis ( Bassani et al, 2023 ) and low-pressure batch tests with formation water informed initial microbial compositions and active species.…”

“…As of 2023, 15 UGS facilities – from the conversion of depleted NG reservoirs - with a total capacity of 19 billion cubic meters are present in Italy ( Ministero dell’Ambiente e della Sicurezza Energetica (MASE), 2023 ). 1 We have previously characterized formation water samples collected from four depleted NG reservoirs located in different parts of Italy, named R1, R2, R3 and R4, by hydro-chemical and metagenomic analyses ( Bassani et al, 2023 ). 16S amplicon sequencing provided the profile of indigenous microbial populations highlighting the presence of microorganisms (i.e., HM, SRB and AB), which activity might pose a potential issue during future UHS operations.…”