Understanding Subsurface Deformation Induced by Hydrogen Storage Operations: A Static Geological Model of the Rivara Area (Central Po Plain, Italy)

Underground hydrogen storage is emerging as a key component of future energy systems, but its deployment in deep geological formations requires a thorough understanding of subsurface structural complexity and fault-controlled reservoir behavior. In tectonically deformed settings, pressure changes associated with cyclic fluid injection and withdrawal may interact with pre-existing structures, potentially affecting reservoir-seal system integrity and fault stability. This study presents first-step results focused on the construction of a static geological model within an integrated geological/fluid-flow/geomechanics modelling workflow developed to assess deep saline aquifers as potential candidates for underground hydrogen storage. The Rivara site was selected as a pilot area based on geological criteria and the availability of extensive datasets, and it is used as a reference case to test the workflow for similar geological settings in Italy and across Europe.

The Rivara area is located in the transition zone between the outer sector of the Northern Apennines (Italy), formed during the Miocene–Lower Pliocene, and the Po Valley. The Rivara Reservoir was previously studied by ERS (Erg Rivara Storage) for a CH₄ storage project, later abandoned. In this work, documentation published by ERS (seismic interpretation, structural maps, petrophysical analyses, etc.) was integrated with public geological databases (ViDEPI) and geological literature. The investigated system consists of a condensed carbonate reservoir (Calcari Grigi Group) overlain by low-permeability marly units (Basinal Formation) acting as a regional seal, within a compressional tectonic framework. Seismic interpretations indicate a structural decoupling between shallow stratigraphic units and deeper levels hosting the reservoir, suggesting that reservoir-scale deformation is governed by a distinct fault system.

Based on available data, we carried out a full modelling workflow. Geological surfaces (associated to the seal and reservoir) were refined through contouring and gridding procedures constrained by explicitly modelled fault traces, preserving fault-related discontinuities. Major compressional structures, including a basal thrust, associated splay and back-thrust elements, and minor faults, were reconstructed in three dimensions and integrated into a comprehensive structural framework. The resulting structural model defines distinct compartments and captures fault offsets and geometries throughout the reservoir volume. The framework was discretized into a three-dimensional geocellular grid through depositional space calculations and structural refinement, and populated with petrophysical properties including interval velocity distributions, porosity, and permeability. A first-pass fracture intensity model was also developed to generate a dual-porosity geological model accounting for tectonic structures and the associated increase in porosity and permeability in their proximity. Finally, formation water salinity was characterized through detailed petrophysical analysis to assess interactions between waters of varying salinity and hydrogen molecules, supporting the evaluation of reservoir suitability for underground hydrogen storage. Overall, the resulting three-dimensional geological model provides a robust basis for investigating subsurface response to hydrogen storage operations in deep saline aquifers.