A Comprehensive Review of Experimental Techniques for Determining Crack Propagation in Caprock during Underground Hydrogen Storage

This study comprehensively reviews the state-of-the-art laboratory-scale fracture mechanics testing methods to assess their suitability for investigating stress-induced critical cracks and geochemically induced subcritical cracks in caprock during underground hydrogen storage. Subcritical crack propagation is primarily examined using empirical techniques such as double torsion and constant stress-rate methods. Both methods determine stress intensity factors and crack velocities without requiring crack length measurements. Comparatively, the double torsion method provides advantages such as simple sample preparation and pre-cracking process, continuous data acquisition, and fracture toughness measurements, which makes it more reliable for caprockrelated studies. The International Society for Rock Mechanics recommends four standard methods for critical crack propagation to determine fracture toughness values. Chevron-notched specimens, including the Chevron Bend specimen, Short Rod specimen, and Cracked Chevron Notched Brazilian Disk specimen, exhibit higher uncertainty in fracture toughness data due to specimen size effects, additional fixture requirements, and undesirable crack formations. In contrast, the Semi-Circular Bend specimen method is frequently employed due to its smaller specimen size, simplified testing, and well-balanced dynamic forces. Despite these advancements, studies on multiple cracking behaviour in caprock under subsurface hydrogen storage conditions remain limited. The conventional methods discussed in this review are primarily designed to function at ambient conditions, making it challenging to replicate subsurface geochemical interactions. Future studies should focus more on developing new laboratory techniques and enhancing existing specimen configurations by incorporating specialised apparatus such as high-pressure cells and reaction chambers to implement typical subsurface conditions observed during underground hydrogen storage. Additionally, more parametric studies on caprock samples are recommended to generate a comprehensive dataset on subcritical and critical crack propagation and validate the reliability of these testing methods for underground hydrogen storage applications.