Study shows soil moisture plays the biggest role in underground propagation of natural gas leaking from pipelines

Pipeline operators must consider how the amount of water found in the surrounding soil affects gas movement when trying to determine the potential hazards of a pipeline leak, said SMU’s Kathleen M. Smits, who led the study recently published in the journal elementa which examined the soil properties of 77 locations across the country where a gas leak had occurred.

“We don’t have to look further than Dallas or Georgetown, Texas, to see where leaks in underground pipelines can be catastrophic,” said Smits, Chair of Civil and Environmental Engineering at SMU’s Lyle School of Engineering and Solomon- Professor of Global Development. “We often see that such incidents are the result of a lack of clear leak detection or damage assessment protocols. Therefore, there should be more focus on the importance of environmental factors such as soil moisture and their proper consideration in leakage scenarios.”

In general, the team, co-led by Younki Cho, a research scientist at Colorado State University’s Institute of Energy, found that methane gas leaking from a pipeline does not travel as far when soil moisture content increases. That leads to a higher concentration of methane gas near the leak site in wetter soil, the study found.

The opposite was true for drier soils.

However, Smits emphasized that just knowing how wet the soil is at the time of the leak is not enough to draw conclusions about how soil moisture content affects gas movement. The soil’s moisture – or lack thereof – at the time of the leak triggers various complex behaviors in the soil as methane gas seeps into the same spaces as water and oxygen in the soil’s pores. Soil moisture content can also change over time due to weather and other factors such as seasonal water tables.

“You have to understand how humidity controls both movement and concentration together,” Smits said. “We can help with that [pipeline owners] with the further course of action in the management of leaks.”

The research team examined more than 300 soil samples from leaks across the country. Samples taken at the time of the leak and again after the leak was repaired were weighed wet. They were also weighed a second time after being dried in an oven.

“From the difference in dry and wet weights, combined with knowledge of the volume of the soil sample, we were able to calculate the soil moisture,” explains Smits.

Other soil qualities, such as texture and permeability, were also studied by the team but didn’t show as much of an impact on how natural gas moved underground.

Additional co-authors were Nathaniel L. Steadman of the Department of Civil Engineering at the University of Texas at Arlington; Bridget A. Ulrich of the Natural Resources Research Institute at the University of Minnesota Duluth; Clay S. Bell of the CSU Energy Institute; and Daniel J. Zimmerle, director and principal director of the Methane Emissions Technology Assessment Center at CSU.

Walk-through surveys are, under certain circumstances, better suited to finding pipeline leaks than mobile location

In another study to improve gas leak detection, Smits and researchers at the CSU Energy Institute found that there are instances where operating a mobile detection unit from the front or roof of a car wasn’t as effective as walkers using a wear wearable detection device.

In a mobile survey, the detection unit measures elevated levels of methane gas in the air while the car is driving. A higher driving speed or wind speed outside the car resulted in a lower likelihood of leak detection compared to foot patrol.

“For example, if you just isolate cruising speed – comparing a person walking at 2 to 3 miles per hour to a car driving at a slow speed of 20 to 30 miles per hour – the likelihood of spotting a leak goes down , from 85 percent for a walking survey to 25 percent for a car,” Smits said.

The study published in the journal Environmental pollution, showed that atmospheric stability also had an impact on mobile surveys.

Atmospheric stability essentially determines whether air rises, falls, or does nothing. Warm, less dense air rises (unstable) while cooler, denser air sinks (stable). Air that stays at the same altitude is considered neutral.

The researchers found that mobile surveys conducted at speeds between 2 and 11 miles per hour became progressively less effective (from 85 percent to 60 percent) at finding a leak as atmospheric stability went from extremely unstable conditions to extremely stable became. Walking surveys conducted under the same conditions did not reflect variability.

“Walking surveys find by far the most leaks, but they’re labor intensive and cost a lot of money,” Smits noted. “This study shows that if operators wish to use a different method such as mobile surveying, they need to carefully select an appropriate surveying speed under different weather conditions in order to achieve a detection probability equivalent to traditional on-foot surveying.”

The findings could apply to any type of underground pipeline, Smits said.

CSU’s Cho, Bell, Cho and Zimmerle supported this study, as did Stuart N. Riddick, a research scientist at the Energy Institute. Shanru Tian, ​​a Ph.D. student in the Department of Civil Engineering at the University of Texas at Arlington, was the lead author of this study.