Potential risk of H2S generation and release in salt cavern gas storage

Abstract

The storage of natural gas in salt caverns can entail the risk of H2S generation, which in turn leads to gas pollution. H2S is generated by bacterial sulfate reduction. The bacteria use aqueous sulfate(aq) as an electron acceptor to oxidize the dissolved hydrocarbons and generate sulfide. Anhydrite is available in the rock salt surrounding the cavern and acts as a sulfate(aq) source. The stored natural gas, with its main component, methane, is in solubility equilibrium with the brine and is additionally delivered by diffusion into the brine. The generated H2S reaches the stored gas by outgassing from the brine. In this study, these processes are simulated by one- and three-dimensional hydrogeochemical diffusive mass transport models, which are based on equilibrium reactions for gas-water-rock interactions and kinetic reactions for sulfate reduction. Modelling results show that the greatest amount of H2S is generated in the brine. The amount of generated H2S(g) is mainly controlled by the amount of available sulfate(aq) as well as the rate of diffusion, which is coupled with the maximum operating live time of salt caverns. Additionally, the amount of generated and released H2S(g) is sensitive to the chosen kinetic rate constant.To ensure constant gas quality over time, the gas and the brine must be analyzed continuously and technical methods must be applied when the H2S(g) concentration increases. According to the modelling results, H2S(g) generation is inhibited by addition of dissolved ferrous iron to the brine. Dissolved ferrous iron reacts with sulfide-sulfur to form mackinawite (FeS(s)) so that aqueous sulfide is no longer available for H2S(g) generation. Another method is the addition of NaOH to increase the pH of the brine. Then, higher fractions of generated sulfide-sulfur are transformed to free S2−(aq) instead of H2S(g) and H2S(aq).