Effects of Fracture Network Distribution on Lined Rock Caverns Subject to High Internal Gas Pressure

We develop a two-dimensional (2D) numerical model to analyse the impact of fracture networks on the behaviour of pressurised lined rock caverns (LRCs). We use the discrete fracture network (DFN) approach to represent the fracture system in rock. The LRC consisting of an inner steel lining and an outer reinforced concrete is situated within the rock mass characterised by spatially distributed fractures. An elasto-brittle constitutive relationship is adopted to characterise the deformation/failure of intact rocks, while the Mazars damage model is used to simulate concrete cracking. The 2D model, representing the horizontal cross-section of an LRC with its surrounding rock mass, is subject to a prescribed in-situ stress condition. We explore various fracture network scenarios and analyse the damage evolution in rock/concrete and tangential strain in the concrete/steel linings. Damage within the rock mass mainly evolves in the form of wing cracks emanating from the tips of pre-existing fractures, while damage in concrete is primarily induced by tensile cracking under cavern pressurisation. Damage emerges in the lining sections where pre-existing fractures are located in the tensile region around the cavern and either intersect with the cavern wall or reach the cavern wall by promoting wing crack propagation. 1 INTRODUCTION Climate change is significantly impacting on our society, now more than ever. To meet the net zero carbon emissions target by 2050, many industrial processes need to be transformed to use fossil-free energy carriers. Hydrogen as a promising alternative to fossil fuels can serve as the energy carrier for many renewables such as wind and solar energy with significant intermittency and strong dependency on weather conditions. Converting off-peak surplus renewable electricity via electrolysis into hydrogen that can be stored in the subsurface for dispatchable, optimal, and sustainable use has been recognised as an integral part of future renewable energy development (Tarkowski, 2019; Krevor et al., 2023; Schultz et al., 2023a,b). Different concepts have been proposed for underground hydrogen storage, including depleted oil/gas reservoirs, saline aquifers, salt caverns, and unlined/lined rock caverns (Tarkowski, 2019). The first two options are plagued by the associated risks of induced seismicity as well as the uncertainties related to hydrogen loss via leakage or microbial consumption (Heinemann et al., 2021; Krevor et al., 2023). The third option relies on the presence of salt deposits which are often limited in many countries. Unlined rock caverns require significant burial depths to ensure hydrodynamic sealing (Lindblom, 1985) and are therefore expensive to construct and maintain. In contrast, lined rock caverns (LRCs) that can sit at relatively shallow depths with great geographical flexibility are much more cost-effective (Johansson et al., 2018). Such an LRC concept for underground storage of natural gas has been developed since 1985 with a pilot plant constructed at Grägesberg, Sweden in late 1980s to experimentally test this concept (Johanasson, 2003). Then, an LRC demonstration plant was constructed between 1998 and 2002 at Skallen, Sweden and further commissioned to operate until now (Johanasson, 2003; Glamheden and Curtis, 2006). The pilot and demonstration projects provide valuable datasets for evaluating the feasibility and effectiveness of the LRC approach. In 2016, the Hydrogen Break-through Ironmaking Technology (HYBRIT) project was launched for developing fossil-free steelmaking technology, where LRC-based hydrogen storage plays a critical role in ensuring continuous steel production (Johansson et al., 2018). A pilot LRC facility, as a key part of the HYBRIT initiative, has been recently built and put into operation at Luleå, Sweden from summer 2022 (Bäcklin, 2022; Vattenfall, 2022).