Geochemical Integrity and Permeability Evolution of Wellbore Cement during Geological Hydrogen Storage and Natural Hydrogen Production

Summary Natural hydrogen, originating from accumulations deep within the Earth’s crust, represents a promising primary energy resource that can be harnessed through extraction wells. Simultaneously, underground hydrogen storage, as a core technology for large-scale hydrogen management, demonstrates broad potential for practical application. However, the interaction between hydrogen and cement can compromise well integrity to a certain extent and pose considerable safety risks to both natural hydrogen extraction and underground storage operations. The aim of this study is to systematically evaluate the effects of prolonged hydrogen exposure on G-grade cement under simulated subsurface conditions representative of hydrogen gas reservoirs. Hydrogen injection experiments were conducted on independent sets of G-grade cement core plugs for durations of 7 days, 15 days, 30 days, 60 days, and 120 days at 11 MPa and 85°C, alongside parallel nitrogen injection experiments lasting 120 days, to quantify changes in cement permeability and porosity. Additionally, mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) were used to characterize the pore structure, mineralogical evolution, and microstructural changes of the cement specimens. The results indicated that permeability changed in a nonmonotonic manner during 120 days of hydrogen exposure. After 60 days of exposure, the permeability of cement decreased by up to 68.42% compared with the initial measurements. The calcium-to-silicon atomic ratio (Ca/Si) exhibited a gradual decline over time. Overall, the findings of this study advance the understanding of hydrogen’s effects on cement permeability and reactivity, offering valuable insights into the sealing performance of wellbore materials in natural hydrogen extraction and underground hydrogen storage.