Technical Committee Underground Storage Technology

SUBI - Safety of underground storage facilities under cyclical load: functionality and integrity of storage facilities and wells

The energy transition and the fluctuations in energy supply associated with the provision of renewable energies require underground gas storage (UGS) facilities to be designed for a higher frequency and, in some cases, amplitude of storage operations. The influence of these cyclic loads on the geological and technical components and thus on the functionality and safety of UGS was therefore the focus of the project. To ensure the long-term safety of the protected assets, the underlying processes and mechanisms were investigated across scales, both qualitatively and quantitatively. The focus was on a holistic consideration of operation-related property changes of reservoirs and overburden as well as their connection to technical facilities (e.g. cementation of boreholes) of both pore and cavern reservoirs. Tightly integrated experimental and numerical investigations form the basis for model predictions, which were validated with field observations.

The aim of the project was to investigate the complex geomechanical processes that influence the reliability of underground storage facilities on a cross-scale and interdisciplinary basis. The focus was on the influence of cyclic stresses on technical installations and gas storage facilities in the regional stress field.

The following hypothesis was tested: Increased frequency and amplitude in storage operation (e.g., as a result of the energy transition) has an impact on the integrity of underground gas storage facilities.

Summary Results: Due to the cross-scale interdisciplinary approach, solutions for a safer and more sustainable operation of the storage facilities even under higher-cycle loading were developed and tested. The main result is that both pore and cavern storage facilities can be qualified and upgraded for higher-frequency operation. Boundary conditions to be observed and verification concepts were developed in order to further increase the safety of the storage facilities and to maintain the storage functionality.

Both salt and mudstone can be used as a safe overburden for higher-frequency loads if the frequency-dependent geomechanical properties and rock mechanical boundary conditions are sufficiently taken into account. For this purpose, suitable complex material laws have to be used, the anisotropic stress field has to be considered and corresponding high-resolution numerical models have to be applied. Corresponding models and "workflows" are presented in the report.

Satellite-based methods can be used to monitor pore and cavern reservoirs. Using suitable models, the filling levels can be derived from the observed deformation.

The boreholes are identified as the most critical element of the reservoirs under high cyclic loading, as long as the rock mechanics boundary conditions, e.g. for the overburden, are met. Experiments and models have shown how chemical alteration and cyclic loading affect tightness. It is proposed to increase the impermeability of cemented casings by roughening the surfaces of the casings by sandblasting and removing layers of corrosion and scale. In addition, suitable cement mixtures should be used and the tightness should be monitored. The increasing age of our storage infrastructure must be taken into account. Biological processes in the reservoirs must be taken into account during high-cycle operation in order to prevent any degradation of the well integrity due to
avoid microbially induced corrosion.

The results were presented to interested specialists from operators, authorities and scientific institutions as an online event in a stakeholder workshop and discussed intensively.