Effects of Underburden Fault Properties, Operational Properties, and Stresses on Induced Basement Fault Seismicity During Hydrogen Storage in Depleted Gas Fields

ABSTRACT: The injection, storage, and withdrawal of hydrogen into depleted gas reservoirs can be a critical carbon-neutral technology in satisfying seasonal energy demand. In this paper, fault slip and induced seismicity are numerically investigated along a basement fault in response to gaseous hydrogen's injection, storage, and withdrawal in an overlying sandstone reservoir.. The model contains a chalk overburden, shale caprock, sandstone reservoir, and shale underburden, with an anticlinal structural trap and material properties representative of a generic North Sea candidate field. Various operational properties (i.e., injection rate, withdrawal rate), fault properties (i.e., friction coefficient, fault permeability) and stresses (isotropic versus anisotropic horizontal stresses) are parametrically varied to study their influence on basement fault slip, where the fault is hydraulically isolated from the storage reservoir. Simulations are conducted using a three-dimensional, fully-coupled, hydromechanical, Finite Element Method (FEM) code. It was found that a low injection rate should be used to minimize induced seismicity along basement faults. Fields with basement faults with high friction coefficients and low fault permeabilities are the preferable low seismic-risk storage candidates. Anisotropic horizontal stresses may reduce underburden seismic risk, where the horizontal stress in the plane parallel to the dip direction of the fault is the largest of the horizontal stresses. 1. INTRODUCTION Gaseous hydrogen storage in depleted natural gas fields can be a crucial technology for satisfying future energy demand in a low-carbon energy mix. However, as of 2023, a fluid with over 62% hydrogen concentration had never been injected into a subsurface porous medium, and specifically, a fluid in excess of 10% hydrogen concentration had never been injected into a depleted gas field (Burtonshaw et al., 2024). RAG Austria's hydrogen storage demonstration facility (the Underground Sun Storage) has become the world's first hydrogen storage project that stores pure hydrogen in a depleted gas reservoir (Andiappan et al., 2023). In the summer, 4.2 gigawatt-hours (GWh) of energy is stored in the form of green hydrogen in a depleted gas reservoir in Gampern, Upper Austria (Salmachi et al., 2024). The hydrogen is re-produced during the winter months as an energy source and for material usage. Due to the fact that hydrogen storage in depleted gas fields is still at an embryonic stage, numerical simulations of the injection, storage, and production of gaseous hydrogen is critical in understanding the potential geomechanical ramifications (i.e., caprock and reservoir integrity, surface subsidence, and induced seismicity). The problem of hydrogen storage is fundamentally different than other geo-energy storages for two main reasons: (1) hydrogen has an ultra-low density (∼10 kg/m3) and very-low viscosity (∼10 μPa s (Burtonshaw et al., 2024; Heinemann et al., 2021) and (2) the hydrogen must be withdrawn on short time-scales of months which induces cyclical stresses and fatigue (Kumar et al., 2023) in the caprock, reservoir, and faults, whereas in CCS and wastewater disposal, the carbon dioxide and water never return to the surface, and the reservoir pore pressure monotonously increases.