Study of methane migration in the shallow subsurface from a gas pipe leak

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.

A closer look at underground natural gas pipeline leaks across the United States

“…This behavior suggests that for the soils tested, the contribution of advection and diffusion results in varying gas behavior. As demonstrated in Gao et al (2021), advective gas transport due to a pressure gradient caused by the gas release dominates the CH 4 transport close to a leak source, driving the migration of high concentration contours. However, in the far field (distances greater than 1.5 m), diffusion caused by the molecular concentration gradients dominates the migration.…”

“…The advection close to the source results in the gas escaping in the high permeability soils (Figure 2A; Gao et al, 2021). In the low permeability soils, diffusion was suspected to be the driving transport mechanism at distances greater than 5 m (Figure 2B).…”

“…Although slow, CH 4 slowly migrated farther away from the leak point, causing spreading of CH 4 gradually in soil over time. Diffusive behavior is also seen in high permeability soils but to a lesser extent due to the competition, per say, with the vertical transport of the gas to the soil surface and escape into the atmosphere (Gao et al, 2021). As the gas leak continues in-between LDAR, the area of influence can slowly become larger over time and reach steady state, influencing the leak classification.…”

“…However, when the moisture content is less than 0.15 g/g, the average concentration was much higher (225,861 ppmv). This behavior can be explained by two aspects as pointed out by Gao et al (2021). Water saturation has reverse effect on the contribution of advection and diffusion to CH 4 distribution.…”

“…NG pipeline systems encompass a wide range of both low-and high-pressure conditions, whereas low pressure conditions (1.5-2,000 kPa) prevail in distribution systems. CH 4 flow is typically dominated by advective flow near the leak point due to pipeline pressure and transitions to diffusion-dominated flow due to concentration differences at further distances away from the leak point (Forde et al, 2018;Gao et al, 2021). Furthermore, preferential flow paths from permeability variations in soil may cause increased gas migration distances, causing gas to emerge at surface locations considerable distances away from the leak location (Hendriks et al, 2010;.…”