Systematic Mapping of the Potential Microbiological Risk Associated with Underground Hydrogen Storage in Depleted Gas Reservoirs on the Italian Territory

Underground Hydrogen Storage (UHS) is currently regarded as a promising solution for large-scale storage of hydrogen (H2), a low-carbon energy carrier, expected to play a key role in the energy transition and in the near-future renewable energy systems. Despite its potential and the success of preliminary feasibility studies, the actual application of UHS technology still needs research and pilot-scale demonstrations. Microbiological aspects are a key feature for the feasibility assessment of UHS. As the main reducing agent of chemolithoautotrophic metabolisms, H2 could trigger the activation of different microorganisms, particularly methanogenic archaea and sulfate-reducing bacteria (SRB), which in turn could pose issues for UHS. The goal of the present work was to carry out a systematic study aimed at investigating, characterizing and mapping the potential microbiological risk of depleted reservoirs located on the Italian territory and currently converted into Underground Gas Storages (UGS). Preserved samples of formation fluids were collected from nine different UGS spread across Italy. The microbiological analyses involved a comprehensive metabarcoding characterization based on bacterial and archaeal 16S rRNA gene, functional gene sequencing based on dissimilatory sulfite reductase (dsrAB) and methyl coenzyme M reductase (mcrA) genes for sulfate reduction and methanogenesis, respectively, and predictive gene functional annotation based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Clusters of Orthologous Genes (COG) categories. Molecular analyses were cross-referenced with a comprehensive hydrochemical characterization to infer the potential activation of metabolic activity with respect to nutrient availability. Overall, the results of the characterization of the Italian reservoirs showed that formation waters typically have moderate salinity, low phosphate and sulfate levels, and variable carbon content. Only in two of the nine reservoirs investigated, the detection of sulfate and carbon sources, together with the dominance of microorganisms putatively able to reduce sulfur compounds, may sustain the activation of this microbial cluster. These results are the first indispensable step towards a comprehensive evaluation of the microbiological risk associated with UHS in Italian depleted reservoirs and lay the basis for pilot-scale activity studies.