Integrated Modelling of Bio-Geochemical Aspects in Underground Hydrogen Storage: Implications for Reservoir Selection and Performance

Hydrogen offers a potential replacement for conventional fossil fuels as a sustainable energy vector. Despite this promise, its large-scale storage is one of the main bottlenecks. Utilizing depleted gas reservoirs for hydrogen storage could present a viable solution. However, introduction of hydrogen into the subsurface may induce microbial and geochemical reactions, resulting in possible hydrogen loss. Therefore, understanding the microbial and geochemical risks associated with underground hydrogen storage is essential for appropriate reservoir selection. To explore the bio-geochemical behaviour of subsurface hydrogen storage, we developed a coupled numerical model using PHREEQC. This model includes both geochemical and microbial reactions, with the former assumed to be at equilibrium and the latter governed by kinetics. The model incorporates three metabolic pathways: Methanogenesis, Acetogenesis, and Sulphate Reduction modelled by the Dual-Monod approach. Inputs such as reservoir mineralogy and brine composition determine the reservoir type for geochemical reactions, while kinetic drives microbial reactions. This adaptable model enables batch simulations across various reservoir types, contributing to a comprehensive understanding of hydrogen storage dynamics in subsurface environments. This understanding may then be applied to specific reservoir systems. The preliminary findings reveal a significant interplay between microbial and geochemical reactions, underscoring the substantial impact of reservoir choice - specifically mineralogy and initial brine composition - on microbial reactions. Storage performance and hydrogen loss are particularly sensitive to these reservoir characteristics. Developing on these initial insights, a comprehensive case study was undertaken, assessing hydrogen storage performance in some depleted/operating gas reservoirs in the North Sea with specified formation mineralogy and brine compositions. Observations indicate that reservoir type substantially drives hydrogen storage performance, with variations tied to the presence of calcite, dolomite, quartz, and anhydrite, and to the initial brine composition, as well as to the activity of microbial life (kinetic). This illustrates the need for a rigorous reservoir selection process to ensure optimal storage efficacy and purity of recovered hydrogen. This study offers novel predictive insights into the microbial and geochemical dynamics within any given reservoir during underground hydrogen storage projects, thereby facilitating screening processes.