Modelling coupled flow with key biogeochemical reactions in subsurface hydrogen systems

Underground hydrogen storage (UHS) is a promising option for large-scale energy storage, and its success depends on how geochemical and microbial processes interact with reservoir heterogeneity. In this study, we applied UGFACT (Underground Gas Flow simulations with coupled bio-geochemical reACTions), an open-source reactive transport framework that couples MRST with PHREEQC, to investigate the impact of heterogeneity on microbial hydrogen consumption during a single storage cycle in a Berea Sandstone–like system. The microbial reactions considered were methanogenesis (MET), acetogenesis (ACE) and sulphate reduction (SRB), with kinetic parameters and pH dependencies taken from the literature. Results show that heterogeneity disperses the H 2 plume, enlarges the gas–brine–rock contact area, and thereby increases microbial consumption. Cumulative H 2 losses rose from 6.0% to 6.67% in areal models and from 6.48% to 7.07% in vertical models, with the vertical case exhibiting stronger losses due to gravity-driven override exposing a greater mixing zone to hydrogen. Also, even trace amounts of anhydrite were sufficient to sustain microbial sulphate reduction, leading to a local spike of H 2 S concentrations. These findings build on earlier project results showing that dolomite-rich formations impose greater risks of microbial consumption during UHS than sandstone or calcite formations. This is because dolomite supplies bicarbonate that stimulates MET and ACE, which drive hydrogen loss and buffering pH in a way that favours microbial activity. Overall, this work demonstrates that UGFACT provides an effective tool for rapidly screening candidate reservoirs by integrating mineralogy, brine chemistry and heterogeneity. The framework highlights critical risks associated with hydrogen loss and H 2 S generation, thereby supporting safer and more reliable deployment of subsurface hydrogen storage. • Reservoir heterogeneity increases microbial hydrogen consumption. • Heterogeneity disperses H 2 plumes and enlarges mixing zones. • Vertical models show higher H 2 loss due to gravity override. • Trace anhydrite can trigger H 2 S generation via sulphate reduction. • UGFACT enables rapid screening of microbial risks in UHS systems.