Effects of Soil Properties on the Diffusion of Hydrogen-Blended Natural Gas from an Underground Pipe

The diffusion of hydrogen-blended natural gas (HBNG) from buried pipelines in the event of a leak is typically influenced by soil properties, including porosity, particle size, temperature distribution, relative humidity, and the depth of the pipeline. This study models the soil as an isotropic porous medium and employs a CFD-based numerical framework to simulate gas propagation, accounting for the coupled effects of soil temperature and humidity. The model is rigorously validated against experimental data on natural gas diffusion in soil. It is then used to explore the impact of relevant parameters on the diffusion behavior of HBNG under conditions of low leakage flux. The results reveal distinct diffusion dynamics across different soil types: hydrogen (H2) diffuses most rapidly in clay, more slowly in sandy soil, and slowest in loam. At the ground surface directly above the leakage point, H2 concentrations rise rapidly initially before stabilizing, while at more distant surface locations, the increase is gradual, with delays that grow with distance. In particular, in a micro-leak scenario, characterized by a pipeline buried 0.8 m deep and a leakage velocity of 3.492 m/s, the time required for the H2 concentration to reach 1% at the surface, 2 m horizontally from the leak source, is approximately 4.8 h for clay, 5 h for sandy soil, and 7 h for loam. The time taken for gas to reach the surface is highly sensitive to the burial depth of the pipeline. After 18 h of diffusion, the surface H2 molar fraction directly above the leak reaches 3.75%, 3.2%, and 2.75% for burial depths of 0.8, 1.1, and 1.5 m, respectively, with the concentration inversely proportional to the depth. Soil temperature exerts minimal influence on the overall diffusion rate but slows the rise in H2 concentration directly above the leak as temperature increases. Meanwhile, the effect of soil humidity on H2 diffusion is negligible.