Hydrogen Blending in Natural Gas Networks: Embrittlement-Induced Leakage as a New Threat to Hydrogeological Integrity

The concept of hydrogen mixing into the pre-existing natural gas systems is a more and more actively encouraged approach to the decarbonization of the near term, but its integrity concerns go well beyond the traditional pipeline safety and subsurface environmental statement. This review summarizes existing information about hydrogen-material interactions in existing gas infrastructure and presents the leakage that occurs as a result of embrittlement as a new threat to hydrogeological integrity. We initially look at the processes involved in hydrogen ingress weakening metallic and non-metallic elements, and we mainly focus on how microstructural trapping, residual stress, weld heterogeneity, and pressure cycling accelerate crack initiation and subcritical crack propagation. We next consider how these degradation mechanisms can be converted to more diffuse and sustained leakage pathways than the more normal methane-based failures, and hydrogen is more likely to escape via microdefects and broken seals. The review also evaluates what happens to leaked hydrogen in soils and aquifers, and shows that it is highly mobile and capable of altering redox conditions, inducing hydrogenotrophic microorganisms to grow, and affecting the mobility of elements and co-contaminant redox-sensitive elements. Mixed scenarios of leakage of hydrogen and methane are considered as compound hazards, making it difficult to detect, attribute, and assess the risk. Lastly, we list the major gaps in research and governance, such as the necessity to have blended-gas embrittlement data in realistic operating conditions, hydrogen-sensitive leakage identification in buried infrastructure, and correlated transport reaction models, which relate the evolution of leakage to the effects of groundwater. In assuring decarbonization through hydrogen blending, a material-hydrogeology approach will be crucial to avoid any unintended effects of carbon on the subsurface environment.