Soil conditions affect the speed and distance of escaping natural gas

Dallas, Texas (SMU) – When natural gas leaks from an underground pipeline, soil cover from water/snow moisture, an asphalt coating or a combination of these factors can cause the gas to migrate up to three to four times farther from the leak than it would through dry soil, a new study found.

A research team led by SMU also found that these surface conditions can also affect the speed of the escaping gas: the gas moves 3.5 times faster than a comparable leak under dry soil conditions.

“This work is of great importance as it relates for the first time the impact of changes in surface conditions on subsurface gas transport times and distances,” said SMUs Kathleen M. Smitsone of the co-authors of the study published in the Journal Letters on Environmental Science and Technology. It is critical for first responders and gas and oil companies to consider ground surface structures when assessing the safety risk of a pipeline leak to nearby homes and businesses, said Smits, chair of the SMU Lyle School of Engineering Civil and environmental engineering and Solomon Professor of Global Development.

These pipeline leaks pose two dangers: Unburned natural gas, which consists mainly of methane (CH4), can cause an explosion, but methane is also the second biggest contributor to global warming after carbon dioxide (CO2). Global warming could be reduced if all the places where methane gas is leaking from leaky pipelines were found – and that gas was removed safely, Smits said.

“The findings of this study provide important insights into identifying and prioritizing leaks from both a safety and environmental perspective,” said Smits.

The SMU-led research team conducted controlled leak experiments at Colorado State University. Centre for the Assessment of Methane Emission Technologies (METEC) under the following ground surface structures: snow or rain on grass, grass-covered dry ground, or asphalt that was either dry, wet from rain, or covered with snow. Here, researchers were able to safely leak gas from a ruptured pipeline and then observe how far the gas leaked vertically and horizontally at specific times after the leak. For each experiment, natural gas was released continuously for up to 24 hours at predetermined leak rates to simulate the way gas would leak in a real-world scenario.

Navodi Jayarathne, a Postdoctoral fellow in the Department of Civil Engineering at the SMU Lyle School, led the study. Also assisted were Daniel J. ZimmerleDirector and Principal of the Methane Emissions Technology Evaluation Center at CSU; Richard S. Kolodziej IV, who is pursuing a master's degree at SMU and is part of Smits' Research teamand Stuart Riddick, a scientist at the Energy Institute at Colorado State University.

Key findings from the SMU-led study

The researchers found that rain, snow and asphalt block the escape of gas from the soil at the surface, causing the gas to migrate both downward and outward away from the leak.

Imagine the gas flowing through something like a slice of Swiss cheese, Jayarathne explained. The gaps or “holes” in the soil can be filled by water, gas or other particles.

“This causes the gas to repeatedly penetrate the ground over long distances, increasing the potential risk,” explains Jayarathne.

In addition, “we found that when there is asphalt, moisture or snow, the gas finally finds a way to escape from the ground, it moves very quickly and in high concentrations, which increases the safety risk,” said Smits.

Another finding surprised the researchers: Even after the gas supply was cut off or the leak was repaired, methane trapped under snow, wet soil or asphalt surfaces could still be detected in high concentrations for up to 12 days. And during this time, natural gas spread laterally from the source of the leak by up to 2 to 4 percent.

“Previous data show that gas escapes quickly from the ground after it is stopped,” Smits said. “But this study shows that gas escape is unique depending on the environment, especially the surface.”

First responders should be aware that the gas spot will continue to develop even after the leak is stopped, Smits said.

The researchers found that the migration distances they recorded were based on the soil type and composition of the METEC.

“The values ​​may be different when applied to other locations and environments. However, the patterns will also reflect expected behavior at other leak locations,” said Smits.

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