Natural Gas Transport In Soil For Gas Pipeline Leakage

Natural gas is flammable and explosive,so leaks can cause significant economic losses and even casualties.Localization of the leakage source as soon as possible is the top priority in gas leakage accidents;therefore,natural gas transport in soils needs to be investigated for the safe management of underground pipelines and for rapid localization of leaks.This study used theoretical,experimental and numerical methods to research natural gas diffusion in soils resulting from gas pipeline leaks.Three models of the multicomponent gas transport in porous media were compared against existing experimental data for CH₄/O₂/N₂ in a 1 D geometry with the Dust Gas Model（DGM）shown to be the most appropriate model for simulating gas transport in porous media.A numerical model based on the DGM model was then used to predict the diffusive and advective fluxes and to determine which mechanism dominate the gas transport for various situations.The analyses investigated the effects of the pressure difference,pore diameter and water saturation.The results show that gas advection occurs even when there is no total pressure difference between the inlet and the outlet.The gas transport speed first increases with decreasing media pore diameter and then decreases.The pressure difference and the pore diameter both significantly impact which transport mechanism has the greatest effect on the gas transport.A full scale gas leak experimental system was set up to further study natural gas diffusion in soils for a small leak from an underground pipe.The evolution of the methane concentration was monitored at several positions in real time for various leakage flow rates and leakage directions.The experimental results show that a larger leakage flow rate results in the system more quickly reaching steady-state.The steady methane concentrations at points below the pipeline are higher than at points above the pipe,but the diffusion rate near the surface is faster.The concentrations at points below the pipeline only differ when the leak direction faces downwards while the concentrations above the pipe are sensitive to all leak directions.Another experimental setup was used to measure the diffusion coefficient of methane-air in soil with the methane concentrations in the gas mixture measured by micro thermal conductivity meters.The measured data was compared with analytical and approximate solutions of the transport equations to determine the effective diffusion coefficients.The diffusion coefficients from both the analytical had approximate solutions were smaller than the theoretical value for gas diffusion in free space with the analytical solution being closer to the theoretical value.The soil sample microstructures were scanned by a Micro-CT to measure the pore structures in the material by a three-dimensional visualization software which also showed the connectivity between the pores which was used to calculate the permeabilities of the soil samples.The natural gas transport in the full scale experiments was numerically simulated using a coupled multi-physics method,with the model results comparing well with the experimental data.The numerical model was used to analyze the effects of leak direction,heterogeneous media and convective mass transfer coefficient with the results showing that the mass transport from the soil to the atmosphere was largest for upward facing leaks and in homogeneous media.The random pore structure has little effect on the gas distribution and the total mass flow rate out of the ground surface.The total mass flow rates out of the ground for horizontally heterogeneous and vertically heterogeneous soil structures are less than for the homogeneous media.At the ground surface,the methane transport is primarily diffusive,while near the leak,advection is dominant.The advection is more important at the higher pressures.The surface advective mass transfer coefficient between the ground surface and atmosphere has little effect on the gas migration in the soil.