Underground natural gas (NG) leaks pose an urgent safety threat, motivating ongoing efforts to improve leak detection methods. The objectives of this study were to investigate how realistic environmental conditions affect methane concentration distributions near leaking underground NG distribution pipelines and ultimately to inform protocols for leak detection by walking surveys. In the first study to do so to date, subsurface and atmospheric methane concentrations were measured at high spatial resolution at a field-scale testbed configured to allow controlled release of NG from an underground source. Our findings demonstrate the importance of considering the effects of subsurface processes with respect to aboveground methane concentrations measured in walking surveys. While subsurface methane concentrations from a large leak (0.52 kg/h of NG, 0.44 kg/h of methane) exceeded 80 vol % 20 cm below the ground, atmospheric concentrations dropped below 100 ppmv (0.01 vol %) within the first 10 cm above the ground when the average wind speed was >2 m/s, demonstrating substantial atmospheric dilution in a narrow boundary layer above the surface under moderate wind conditions. Our analysis indicates that detectors with minimum detection limits on the order of 10 ppmv may be required to detect large underground leaks under certain environmental conditions. While efforts to assess a broader range of leak rates and environmental conditions are ongoing, the findings of this study provide critical insight to practitioners regarding detector performance and placement requirements for walking surveys.
Mitigation of atmospheric emission of methane from leaky underground infrastructure is important for controlling the global anthropogenic greenhouse gas burden. Overexposure to methane may also cause occupational health problems in indoor/outdoor environments at the local scale. Subsurface soil conditions (e.g. soil heterogeneity) affect methane migration in soils while near‐surface atmospheric boundary conditions (e.g. wind and temperature) affect off‐site emissions across the soil‐atmosphere interface. This study investigated the above‐surface methane concentration boundary‐layer development under different soil conditions (homogenous and layered) and atmospheric boundary controls (wind and temperature). A series of controlled bench‐scale experiments was conducted using an open‐loop boundary‐layer wind tunnel interfaced with a porous media tank inside which a simulated methane point source was embedded to mimic a buried leaky pipeline. Results revealed pronounced effects of wind and, to a lesser extent, temperature on above‐surface methane boundary‐layer development. High atmospheric temperature contributed to the concentration boundary‐layer build‐up whereas high wind velocity caused erosion of the boundary layer. The boundary‐layer methane concentration profiles were adequately simulated within an advection‐diffusion modeling framework combined with a Navier‐Stokes free flow domain by coupling free air flow, heat flow and flow of a multicomponent gas mixture. Simulations further implied that the Fickian diffusion approach may have limited applications when pronounced non‐isothermal conditions prevail within the system. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.
With the increased use of natural gas, safety and environmental concerns from underground leaking natural gas pipelines are becoming more widespread. What is not well understood in leakage incidents is how the soil conditions affect gas migration behavior, making it difficult to estimate the gas distribution. To shed light on these concerns, an increased understanding of subsurface methane migration after gas release is required to support efficient leak response and effective use of available technologies. In this study, three field-scale experiments were performed at the Methane Emission Technology Evaluation Center in Colorado State University to investigate the effect of soil textural heterogeneity, soil moisture, and leak rate (0.5 and 0.85 kg/h) on methane migration caused by leaking pipelines. Subsurface methane concentrations, in addition to soil moisture and meteorological data, were collected over time. A previously validated numerical model was modified and used to understand the observed methane distribution behavior. Results of this study illustrate that the influence of soil texture, leak rate, and moisture on subsurface methane distribution is determined by the relative contribution of advection and diffusion and closely related to the distance to the leak source. Advection dominates gas transport within 1–1.5 m of the leak source, driving the migration of high concentration contours. Beyond this distance, diffusion dominates migration of lower concentration contours to the far-field. Although large leak rates initially result in faster and further gas migration, the leak rate has little influence on the diffusion dominated migration farther from the leak source. Soil moisture and texture complicate gas behavior with texture variations and elevated soil moisture conditions playing a dominant role in locally increasing methane concentrations. Scenarios highlight the importance of understanding the effects of soil moisture, texture, and leak rate on gas migration behavior in an attempt to unravel their contribution to the gas concentration within the soil environment.
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