Predicting microbial activity potential in salt caverns based on brine chaotropicity analysis.

Salt caverns are promising sites for hydrogen (H₂) storage, but microbial activity in these high-salinity environments poses risks, including H₂ consumption and subsequent toxic hydrogen-sulfide (H₂S) production by sulphate-reducing bacteria. While salinity influences microbial diversity, the role of chaotropicity, defined as a membrane-disrupting effect of salts like magnesium chloride (MgCl2) and calcium chloride (CaCl2), remains unexplored. We introduce a novel method using oscillatory rheology to measure solute-induced changes in agar gel point temperature, enabling prediction of chaotropicity and subsequent microbial activity. We assessed individual salts, salt mixtures, literature data, and original brine samples from four salt caverns. Our results show that chaotropic conditions arise when the ionic strength (I) of solution exceeds 3 mol/L with 55% MgCl₂, or 6 mol/L with 40% MgCl₂. One tested cavern exhibited chaotropic properties, suggesting reduced microbial risk. Microbial analysis and growth tests confirmed missing microbial activity and minimal cell numbers in the chaotropic cavern, in contrast to the more kosmotropic caverns. Therefore, we propose a strategy to mitigate microbial threats by adjusting salt cavern brine composition to induce chaotropicity as one additional factor to limit activity, which offers a new framework for microbial risk management.