VESTA-CH - Very High Temperature Heat Aquifer Storage

High Temperature Storage systems (HTS) aim at balancing the mismatch between energy production and demand by injecting surplus or residual heat from industrial processes, waste-to-energy plants, or renewable energy sources into deep geological reservoirs for seasonal storage and subsequent recovery when required. As such, they constitute a key enabling technology for increasing the flexibility of renewable energy systems, improving energy security, and reducing greenhouse gas emissions. The VESTA-CH project addresses one of the main challenges limiting the large-scale deployment of High-Temperature Thermal Energy Storage (HTS) systems: the long-term preservation of storage performance under repeated high-temperature operation. Over successive injection and production cycles, coupled thermo-hydro-chemical (THC) processes may alter reservoir properties through permeability changes, mineral dissolution and precipitation, progressively reducing injectivity, productivity, thermal recovery efficiency, and ultimately operational safety. The project therefore aims to develop scientific and engineering methodologies that enable the definition of safe operating envelopes and support the design of more efficient, reliable, and durable HTS systems. To achieve this objective, VESTA-CH combines advanced numerical modelling with laboratory experiments and field observations to investigate the evolution of HTS under different operating conditions. The project develops fully coupled THC simulations to quantify the interaction between fluid flow, heat transport, and geochemical reactions over repeated storage cycles. Particular emphasis is placed on identifying the physical and chemical mechanisms responsible for performance degradation, evaluating the sensitivity of system behaviour to key geological and design and operational parameters, and assessing innovative engineering concepts, including distributed fibre-optic monitoring, horizontal multilateral wells, and advanced well architectures designed to improve heat exchange. Rather than defining site-specific operating limits, which inherently depend on local hydrogeological and thermal conditions, the modelling framework is used to establish generic relationships between reservoir performance and the governing geological, design, and operational parameters. This approach enables the identification of the factors primarily controlling thermal efficiency and provides transferable design principles that can be readily adapted to future HTS projects by incorporating site-specific geology, well architecture, and completion design. VESTA-CH benefits from datasets acquired in the context of the Forsthaus Geospeicher High-Temperature Aquifer Thermal Energy Storage (HT-ATES) project. This project aimed to store excess heat generated by the local utility company Energie Wasser Bern (ewb) within sandstone layers in the Lower Freshwater Molasse (Untere Süsswassermolasse; USM) located at depths ranging from 240 to 500 meters. Following extensive field-testing and stimulation campaigns, it became evident that the permeability of the sandstone formations was too low to allow significant thermal energy storage. As a result, only partial datasets, mainly related to the characterization of the hydrogeological system, were obtained. This limited the achievement of several of the original objectives of the VESTA-CH project, including: (1) the optimization of the injection scheme (pressure and temperature) for the Bern Forsthaus pilot site, (2) the design and validation of the monitoring system, and (3) the assessment of geochemical processes. In the absence of key datasets, this report provides generic recommendations based on the available data and numerical analyses. In particular, the outcomes of VESTA-CH are (1) an advanced, fully coupled numerical framework for simulating TH(M)C processes in both wellbores and reservoirs; (2) generic engineering recommendations for the design of future HTS systems; (3) an integrated monitoring and surveillance concept combining surface instrumentation, downhole sensors, and distributed fibre-optic technologies; (4) generic guidelines for defining safe operational envelopes, including injection and production rates and pressure limits; and (5) a comprehensive assessment of the geochemical risks affecting long-term HTS performance and reliability. By extending the operational lifetime and reliability of HTS installations, VESTA-CH will contribute to reducing the levelised cost of stored thermal energy and facilitate the broader deployment of seasonal heat storage technologies. The project therefore supports the objectives of the Swiss Energy Strategy by providing the scientific and engineering basis required for safe, efficient, and economically viable underground thermal energy storage systems capable of integrating increasing shares of renewable energy into the future energy system.