Characterization, Evolution and Physics-based Quantification of Methane Emissions from Leaking Underground NG Pipelines

Abstract:

Methane (CH₄) leakage from underground natural gas (NG) pipelines poses an environmental, safety and economic threat to the general public. While previous studies have focused on above ground infrastructure, the integrated analysis of methane concentration evolution and quantification of emission rate for underground NG pipeline leaks remains scarce. Therefore, the main objective of this study is to characterize the atmospheric behavior of methane emissions over a diurnal cycle from controlled underground pipeline leaks as well as assess the effectiveness of a physic-based remote downwind detection technique for estimating the leak rates.

Five controlled field experiments were conducted under a variety of underground leakage rates to (1) investigate how the CH₄ concentrations downwind change over 24-hours within the atmosphere surface layer; (2) determine the effectiveness of a Backward Lagrangian stochastic (turbulence) model to predict subsurface emissions by comparing modelled emission estimates to the known controlled release rates. The evolution of methane concentration is marked by a diurnal variability, specifically, lower concentrations were observed during the day and with much higher concentrations observed during the night. The magnitude of the measured downwind methane concentrations atmospheric conditions compared to changes in leak size, resulting in large leaks having lower concentration readings compared to small leaks under certain meteorological conditions. Diurnal variability of methane concentration is directly attributed to the atmospheric stability, which determines the intensity of turbulence within the atmosphere surface layer. Unstable conditions during the daytime enhances the dilution of methane plume through strengthening vertical mixing. On the other hand, nocturnal stable atmosphere conditions due to the inversion layer is favorable to methane accumulation as vertical mixing is reduced. The accuracy of the estimated emission rate from the model is in the range of 10-20%.The findings of this study illustrate a clearer physical understanding of methane evolution within the surface layer is essential for quantifying the emission rate of methane from NG leaks accurately.