Microbial hydrogen consumption leads to a significant pH increase under high-saline-conditions: implications for hydrogen storage in salt caverns

Salt caverns have been successfully used for natural gas storage globally since the 1940s and are now under consideration for hydrogen (H2) storage, which is needed in large quantities to decarbonize the economy to finally reach a net zero by 2050. Salt caverns are not sterile and H2 is a ubiquitous electron donor for microorganisms. This could entail that the injected H2 will be microbially consumed, leading to a volumetric loss and potential production of toxic H2S. However, the extent and rates of this microbial H2 consumption under high-saline cavern conditions are not yet understood. To investigate microbial consumption rates, we cultured the halophilic sulphate-reducing bacteria Desulfohalobium retbaense and the halophilic methanogen Methanocalculus halotolerans under different H2 partial pressures. Both strains consumed H2, but consumption rates slowed down significantly over time. The activity loss correlated with a significant pH increase (up to pH 9) in the media due to intense proton- and bicarbonate consumption. In the case of sulphate reduction, this pH increase led to dissolution of all produced H2S in the liquid phase. We compared these observations to a brine retrieved from a salt cavern located in Northern Germany, which was then incubated with 100% H2 over several months. We again observed a H2 loss (up to 12%) with a concurrent increase in pH of up to 8.5 especially when additional nutrients were added to the brine. Our results clearly show that sulphate-reducing microbes present in salt caverns consume H2, which will be accompanied by a significant pH increase, resulting in reduced activity over time. This potentially self-limiting process of pH increase during sulphate-reduction will be advantageous for H2 storage in low-buffering environments like salt caverns.publishedVersio

Microbial life in salt caverns and their influence on H2 storage – Current knowledge and open questions

Hydrogen will be one of the key components for renewable energy storage in the future energy systems as it can be stored in significant volumes to overcome daily to seasonal energy fluctuations. Subsurface storage in salt caverns will be a first step. These caverns are created by solution mining in underground salt formations. Despite the high salinity in this environment, salt caverns harbor microbial life. These microorganisms can not only survive in these caverns by using unique adaptation mechanisms, but they actually cause several risks to hydrogen storage. Different metabolisms can use hydrogen as electron donor, leading to hydrogen loss and in the worst case also to H2S formation. The knowledge on salt cavern microbiology and subsequent possible effects of hydrogen is still in its infancy and only a limited number of salt caverns have been investigated so far. This review summarizes the current knowledge and key questions about halophilic (salt-loving) microbes, their adaptation strategies, their origin and potential consequences of their metabolisms. It also discusses the major factors influencing microbial activities and potential risks. This review emphasizes that more research and field trials with extensive microbial monitoring are needed before hydrogen storage in a biologically active system can be safely achieved at a global scale.publishedVersio

Insights into the effects of anthropogenic activities on oil reservoir microbiome and metabolic potential

Microbial communities have long been observed in oil reservoirs, where the subsurface conditions are major drivers shaping their structure and functions. Furthermore, anthropogenic activities such as water flooding during oil production can affect microbial activities and community compositions in oil reservoirs through the injection of recycled produced water, often associated with biocides. However, it is still unclear to what extent the introduced chemicals and microbes influence the metabolic potential of the subsurface microbiome. Here we investigated an onshore oilfield in Germany (Field A) that undergoes secondary oil production along with biocide treatment to prevent souring and microbially induced corrosion (MIC). With the integrated approach of 16 S rRNA gene amplicon and shotgun metagenomic sequencing of water-oil samples from 4 production wells and 1 injection well, we found differences in microbial community structure and metabolic functions. In the injection water samples, amplicon sequence variants (ASVs) belonging to families such as Halanaerobiaceae, Ectothiorhodospiraceae, Hydrogenophilaceae, Halobacteroidaceae, Desulfohalobiaceae, and Methanosarcinaceae were dominant, while in the production water samples, ASVs of families such as Thermotogaceae, Nitrospiraceae, Petrotogaceae, Syntrophaceae, Methanobacteriaceae, and Thermoprotei were also dominant. The metagenomic analysis of the injection water sample revealed the presence of C1-metabolism, namely, genes involved in formaldehyde oxidation. Our analysis revealed that the microbial community structure of the production water samples diverged slightly from that of injection water samples. Additionally, a metabolic potential for oxidizing the applied biocide clearly occurred in the injection water samples indicating an adaptation and buildup of degradation capacity or resistance against the added biocide.publishedVersio

Microbial induced wettability alteration with implications for Underground Hydrogen Storage

Abstract Characterization of the microbial activity impacts on transport and storage of hydrogen is a crucial aspect of successful Underground Hydrogen Storage (UHS). Microbes can use hydrogen for their metabolism, which can then lead to formation of biofilms. Biofilms can potentially alter the wettability of the system and, consequently, impact the flow dynamics and trapping mechanisms in the reservoir. In this study, we investigate the impact of microbial activity on wettability of the hydrogen/brine/rock system, using the captive-bubble cell experimental approach. Apparent contact angles are measured for bubbles of pure hydrogen in contact with a solid surface inside a cell filled with living brine which contains sulphate reducing microbes. To investigate the impact of surface roughness, two different solid samples are used: a “rough” Bentheimer Sandstone sample and a “smooth” pure Quartz sample. It is found that, in systems where buoyancy and interfacial forces are the main acting forces, the impact of biofilm formation on the apparent contact angle highly depends on the surface roughness. For the “rough” Bentheimer sandstone, the apparent contact angle was unchanged by biofilm formation, while for the smooth pure Quartz sample the apparent contact angle decreased significantly, making the system more water-wet. This decrease in apparent contact angle is in contrast with an earlier study present in the literature where a significant increase in contact angle due to microbial activity was reported. The wettability of the biofilm is mainly determined by the consistency of the Extracellular Polymeric Substances (EPS) which depends on the growth conditions in the system. Therefore, to determine the impact of microbial activity on the wettability during UHS will require accurate replication of the reservoir conditions including surface roughness, chemical composition of the brine, the microbial community, as well as temperature, pressure and pH-value conditions

Pore-scale study of microbial hydrogen consumption and wettability alteration during underground hydrogen storage

Hydrogen can be a renewable energy carrier and is suggested to store renewable energy and mitigate carbon dioxide emissions. Subsurface storage of hydrogen in salt caverns, deep saline formations, and depleted oil/gas reservoirs would help to overcome imbalances between supply and demand of renewable energy. Hydrogen, however, is one of the most important electron donors for many subsurface microbial processes, including methanogenesis, sulfate reduction, and acetogenesis. These processes cause hydrogen loss and changes of reservoir properties during geological hydrogen storage operations. Here, we report the results of a typical halophilic sulfate-reducing bacterium growing in a microfluidic pore network saturated with hydrogen gas at 35 bar and 37°C. Test duration is 9 days. We observed a significant loss of H2 from microbial consumption after 2 days following injection into a microfluidic device. The consumption rate decreased over time as the microbial activity declined in the pore network. The consumption rate is influenced profoundly by the surface area of H2 bubbles and microbial activity. Microbial growth in the silicon pore network was observed to change the surface wettability from a water-wet to a neutral-wet state. Due to the coupling effect of H2 consumption by microbes and wettability alteration, the number of disconnected H2 bubbles in the pore network increased sharply over time. These results may have significant implications for hydrogen recovery and gas injectivity. First, pore-scale experimental results reveal the impacts of subsurface microbial growth on H2 in storage, which are useful to estimate rapidly the risk of microbial growth during subsurface H2 storage. Second, microvisual experiments provide critical observations of bubble-liquid interfacial area and reaction rate that are essential to the modeling that is needed to make long-term predictions. Third, results help us to improve the selection criteria for future storage sites.publishedVersio

The light in the dark: in-situ biorefinement of crude oil to hydrogen using typical oil reservoir <i>Thermotoga</i> strains

H2 is a CO2 free energy carrier that can be produced biologically through dark fermentation using specific bacteria. In general, biological production of H2 needs a carbon source and is more efficient at higher temperatures. Mature petroleum reservoirs have the required high temperatures for H2 production, and they contain a significant amount of organic matter in form of residual hydrocarbons. In this work, we evaluated whether indigenous microorganisms isolated from hydrocarbon reservoirs are able to biorefine hydrocarbons to H2. We observed that two Thermotoga strains, Pseudothermotoga hypogea DSM-11164 and Pseudothermotoga elfii DSM-9442, are able to convert hydrocarbons to H2. DSM 9442 produced 0.47 and 1.02 mmol H2 per liter of growth medium from 20 mL/L of n-hexadecane or a crude oil, respectively. DSM 11164 only produced H2 from n-hexadecane (0.94 mmol/L). Addition of 25 mg/L Tween 80, to reduce phase separation, together with 1 g/L glucose increased H2 production from hydrocarbons up to 12-fold. Via an energy analysis we show that bioconversion of crude oil into H2 can be more efficient than conversion of crude oil to gasoline. Therefore, we suggest dark fermentation as a promising alternative to biorefine crude oil and unlock the energy trapped in hydrocarbon reservoirs after abandonment.The light in the dark: In-situ biorefinement of crude oil to hydrogen using typical oil reservoir Thermotoga strainspublishedVersio