Reservoir Engineering Aspects of Geologic Hydrogen Storage

Abstract Safe and effective large-scale storage of hydrogen (H2) is one of the biggest challenges of the global energy transition. The only way to realize this is storage in geological formations. The aim of this study is to address and discuss the reservoir engineering (RE) aspects of geological H2 storage (GHS). The study is based on two sources: first, a comprehensive literature review, and second, experimental and numerical work performed by our institute. The current state of the art regarding the principles of reservoir engineering on the application of GHS is reviewed and summarized. Atypical properties of H2, with its lower density, viscosity and compressibility factor higher than one, increase uncertainties in the definition of capacity, injectivity, and confinement. In addition, the abiotic and biotic reactivity of H2 should be considered in the associated changes in petrophysical properties and molecular mass transfer in subsurface storage formations. Therefore, both geochemistry and reservoir microbiology are inseparable components of reservoir engineering of GHS. The sealing of H2 storage in a porous reservoir with caprock is due to the interplay between potentially higher capillary threshold pressure but higher diffusivity of H2, while the technically impermeable assumption of most deep salt formations can be considered as valid for H2 storage in caverns. Such changes can also affect the injectivity of H2 through plugging or dissolution. Well integrity is of particular concern when abandoned-old gas wells are reused. Especially at higher temperatures, hydrogen can behave more actively to support metal oxidation processes at the casing-cement contact and microbiological activity can promote these reactions. In addition, the permeability of the hardened cement samples to H2 is highly dependent on the effective pressure. An overview of the reservoir engineering aspects of GHS is compiled from recent publications. We integrate key findings with our experimental results to provide essential guidance for front-end engineering and challenges to be addressed in future work. Monitoring of the reservoir pressure, as an indicator of microbial activity, is of great importance. Therefore, measures to control microbial activity have to be drawn, taking into account the site-specific characteristics.