Investigation of hydrogen-rock-brine geochemical reactions in depleted gas reservoir: A study from an Italian case.

Underground hydrogen storage (UHS) in depleted gas reservoirs is increasingly considered as a viable option for large-scale and seasonal energy storage. While such reservoirs benefit from existing infrastructure and extensive subsurface characterisation, uncertainties remain regarding hydrogen–rock–brine interactions and their potential impact on hydrogen retention and reservoir integrity. In particular, the presence of reactive mineral phases may lead to abiotic hydrogen consumption and spatially variable geochemical behaviour that is difficult to assess at the reservoir scale.

This study investigates hydrogen–rock–brine geochemical interactions in a mixed-mineralogy clastic reservoir from an onshore depleted gas field in the Adriatic Basin, Italy. The reservoir is a Pliocene turbiditic sandstone with good petrophysical properties and a heterogeneous mineralogical composition, including a substantial carbonate fraction. Static geochemical modelling was performed using PHREEQC to evaluate mineral stability and potential hydrogen consumption under representative reservoir conditions.

The modelling results indicate that silicate minerals are largely stable in the presence of hydrogen, whereas carbonate minerals (calcite and dolomite) dissolve under equilibrium conditions, suggesting that carbonate-rich intervals may represent zones of enhanced reactivity. To explore how these mineral-scale results may translate to the reservoir scale, reactive facies were defined based on total carbonate content and implemented within a 3D geostatistical reservoir model. This approach allows the spatial distribution of relative geochemical reactivity to be assessed across the field.

The results provide a first-order assessment of potential abiotic geochemical hydrogen loss and its spatial variability. The study highlights the importance of mineralogical heterogeneity when evaluating depleted gas reservoirs for hydrogen storage and demonstrates how geochemical modelling results can be incorporated into reservoir-scale frameworks.