The relevance of microbial processes in geo-energy applications

The subsurface is a vast reservoir which we exploit in various ways. We extract energy in the form of oil/gas or heat from it. We use it for the storage of energy, e.g., in shallow geothermal applications or for the underground storage of natural gas. A lot of recent research has studied the potential for storing hydrogen (H2) in the subsurface. We also use the subsurface to dispose of energy-related waste, e.g., radioactive materials, carbon dioxide (CO2), and acid gas. For a long time, the subsurface was considered sterile below a few metres, probably stemming from work carried out in the 1950s which suggested that bacteria in Pacific sediments most likely disappeared somewhere just below 8m (Morita and ZoBell, 1955). This observation seems to have been extrapolated to the subsurface in general, but over time, as methods developed and microbiologists probed harder, this view changed, and it is now recognised that microbial communities exist at depths where the subsurface is exploited for most types of geo-energy. This raises questions about what sort of microbial community exists, how active it is, what limits and drives that activity and how this might impact geo-energy operations.