Abstract

Hydrogen, as an energy carrier, serves as a keystone for achieving both energy sustainability and stability. However, safe and effective geological underground hydrogen storage (UHS) is required for large-scale utilization of this technology. Our study presents a novel look at the effects of geochemistry on UHS. In this manuscript, we undertake a numerical investigation of hydrogen sulfide (H2S) generation within the context of hydrogen storage systems, elucidating its potential consequences on hydrogen recovery performance. For that, a detailed study is performed using a compositional numerical reservoir simulator, where the geochemical reactions are modeled using the Wolery geochemical database. Our results indicated the possibility of pyrite reduction, raising concerns about the formation of hydrogen sulfide in this sandstone saline aquifer.

Introduction

Hydrogen is emerging as a key element in the shift towards sustainable energy because of its adaptability and reduced carbon emissions throughout its lifecycle [1]. With an energy density that surpasses that of natural gas by weight, hydrogen acts as a strong candidate for being an alternative energy source and carrier. Developing effective storage methods is crucial in transitioning toward clean energy and net-zero emissions. Storing hydrogen in underground formations, such as depleted reservoirs or saline aquifers, is an effective strategy for long-term storage, ensuring that the energy supply remains consistent, particularly when the demand is high [2,3]. There are ongoing research efforts focused on understanding the detailed processes involved in storing hydrogen in subsurface formation. Primarily, the potential of geochemical reactions emerges as a critical research domain, which demands in-depth studies and evaluation of any potential risks, such as the alteration of the reservoir's properties over time [4],[5], and the formation of undesired gases like hydrogen sulfide (H2S) which can adversely affect the infrastructure and degraded the purity of hydrogen.

Herein, we shed light on the possibility of H2S generation, and we assess its influence on storage's feasibility and safety. The change in the stored gas composition was a common observation during the storage of town gas in Europe, which is the most relevant experience to the future vision of underground hydrogen storage in saline aquifers or depleted gas reservoirs [6]. A complete assessment of the potential reactions of hydrogen with the existing gases, brine, and minerals is required to avoid the reduction of hydrogen purity and the consumption of hydrogen through the geochemical reaction. The formation of hydrogen sulfide (souring), in particular, is a common problem that had accrued previously in the oil and gas industry, causing corrosion to the infrastructure and toxication to the existing gas, which requires further gas treatment to overcome this issue [7,8]. In the previous underground gas storage experience, the generation of H2S was documented but not thoroughly explained or investigated [9]. The town gas storage at Beynes in France, for instance, encountered some composition changes, including the generation of hydrogen sulfide. Bourgeois et al. [10,11] presented the hypothesis that the generation of hydrogen sulfide that was observed in Chemery, France, is more likely attributable to abiotic processes rather than the biotic reactions of sulfate-reducing microorganisms. Their theory is supported by empirical evidence suggesting that certain geological and chemical conditions, such as the range of temperatures and pressures associated with the saline aquifer and the alkalinity, can lead to the generation of hydrogen sulfide in the absence of microbial activity. The bacterial count tests conducted on the formation water extracted from these reservoirs yielded negative or inconclusive results. Therefore, the absence of bacteria in the formation's brine suggests that biotic processes are not responsible for H2S generation. On the other hand, the presence of impurities in the formation gas, especially CO2, is believed to be the contributing factor driving abiotic reactions. Furthermore, the presence of pyrite in their qualitative scanning samples is believed to be the source of H2S generation [11].

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