Abstract
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.
Highlights
In the United States, there are over 2.2 million miles of natural gas (NG) distribution mains and service lines (Annual Report Mileage for Gas Distribution Systems, 2020) and subsurface pipeline infrastructure continues to grow to keep pace with the growth in NG usage
Using a field-scale test facility linked with numerical modeling, this study investigated the effects of soil moisture, texture, and leakage rate on NG transport from below ground pipeline leaks
Results show that for a homogeneous soil system, concentric subsurface methane concentration contours centered on the leak point are found both parallel and perpendicular to the pipeline
Summary
In the United States, there are over 2.2 million miles of natural gas (NG) distribution mains and service lines (Annual Report Mileage for Gas Distribution Systems, 2020) and subsurface pipeline infrastructure continues to grow to keep pace with the growth in NG usage. Advection and diffusion are the two primary mechanisms controlling methane transport in soil, in addition to biological activity (e.g., microbial oxidation) Their contribution to methane migration process varies based upon subsurface properties, as well as atmospheric conditions. There are many other factors influencing subsurface methane migration behaviors (e.g., Keskikuru et al, 2001; Poulsen et al, 2003; Patterson and Davis, 2009; Bahlmann et al, 2020), such as pipeline pressure, gas composition when pipelines carry heavier (C2þ) hydrocarbons, construction at/below ground, and soil– atmosphere interactions Despite these uncertainties, only a limited number of studies combine experimental and numerical approaches to investigate subsurface methane migration under different environmental conditions
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