Compressibility of unconventional gas shale formations: implications for hydrogen geo-storage

Underground hydrogen storage (UHS) offers an effective solution for large-scale and safe hydrogen storage to deploy H₂ as a clean energy carrier that accelerates energy transition and decarbonization. Geological H₂ storage based on adsorption haven been well-identified but the studies on compressibility-based elastic storage of H₂ remain limited. The objective of this study is to evaluate different compressibilities of unconventional gas shale and related implications on geological hydrogen storage. Laboratory measurements were conducted to estimate the compressibilities of shale and adsorption capacity of H₂ in shale. Theoretical consideration on elastic storage of H₂ through the coefficient of fluid content by a poroelastic framework was developed to compare with the H₂ geo-storage based on adsorption mechanism. The results show that shale deforms linearly with gas pressure for helium, while more pronounced compression can be observed with the exposure of H₂. The modeled coefficient of fluid content is well able to predict the results calculated from measured data. By extending the framework of fluid content, the elastic storage capacity of H₂ was estimated as 0.0526 mmol/g at 8.5 MPa while the excess adsorption amount of H₂ in shale was measured as 0.03043 mmol/g at the same pressure. The results reveal that the compressibility-based elastic storage of H₂ in unconventional gas shale is not negligible due to its large potential with continuous gas injection. Additionally, strain hysteresis effects were observed after H₂ injection and depletion in shale, which is potentially caused by permanent structural alteration of shale. This structural variation promotes the elongation of gas pathway that enhances the permeability and further injectivity and recoverability of H₂.