H2S Generation and Release in Salt Cavern Gas Storage
Salt caverns are used for the underground gas storage to balance the fluctuations in the supply and demand of natural gas throughout the year. However, the storage of gas in the underground entails risks, e.g. the generation of hydrogen sulfide (H2S(g)) during methane storage in salt caverns. H2S is toxic, leads to gas souring and corrosion of the storage facilities. Therefore, technical regulations determine that the H2S(g) concentration in stored gas is limited to 5 mg/m3.
The H2S is generated by bacterial sulfate reduction (BSR) in the brine and the upper meters of the sump of a salt cavern. Generally, BSR occurs in aqueous anoxic environments. Sulfate-S is used by the bacteria as an electron acceptor to oxidize organic compounds and sulfide is generated. In salt caverns, the sulfate source is provided by anhydrite layers within the salt rock. The stored methane dissolves in the brine until saturation is established and serves continuously as reductant for BSR. Consumption of anhydrite and aqueous methane is accompanied by calcite formation. The H2S(g) generated in brine and sump contaminates the stored gas by outgassing. These processes are quantitatively retraced by a three-dimensional hydrogeochemical mass-transport model based on chemical equilibrium thermodynamics using the software PHAST. Reaction kinetics of methane oxidation by sulfate are integrated into the model. The modeling approach simulates a semi-generic salt cavern with data from several caverns. Despite the semi-generic nature of the model, the modeling results give basic and quantitative insights into the mechanisms of H2S(g) generation in salt caverns induced by BSR.
By varying the input parameters, the factors controlling H2S(g) generation are identified and explain why H2S(g) is just generated in some salt caverns. An important factor is the availability of anhydrite as sulfate source. Whereas the occurrence of Fe-bearing minerals like goethite inhibit the release of BSR-generated H2S into the the stored gas. The sulfur (S-II/-I) reacts with Fe+II, and mackinawite (FeS) or pyrite (FeS2) are formed. To identify early H2S generation and protect the stored gas from souring, a monitoring system should be installed in the brine of the salt cavern. If the aqueous H2S concentration increases, the addition of dissolved ferrous iron into the brine and the sump is a potential method to reduce H2S release.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017