Soil conditions affect the speed and distance of escaping natural gas

If natural gas leaks from an underground pipeline, a layer of soil saturated with water or snow, a layer of asphalt or a combination of these factors can cause the gas to migrate up to three to four times farther from the leak than through dry soil, one study found.

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

A study led by SMU investigated under which ground surface conditions natural gas would travel the farthest from a leaking pipeline by conducting controlled leaks under various ground surface structures: Image: Environmental Science and Technology Letters.

“This work is significant because it is the first to link the impact of changes in surface conditions to subsurface gas transport times and distances,” says Kathleen M. Smits of SMU, one of the co-authors of the study published in the journal. Letters on Environmental Science and Technology.

It's critical for emergency responders and gas and oil companies to consider soil surface structures when assessing the safety risk of a pipeline leak to nearby homes and businesses, says Smits, chair of civil and environmental engineering at the SMU Lyle School of Engineering and Solomon Professor of Global Development.

These pipeline leaks pose two dangers: unburned natural gas, which consists mainly of methane (CH4), can cause explosions, but methane is also the second biggest contributor to global warming after carbon dioxide (CO2). Global warming could be reduced by finding all the places where methane gas is leaking from leaky pipelines – and removing the gas safely – says Smits.

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

The SMU-led research team conducted controlled leak experiments at the Methane Emissions Technology Evaluation Center (METEC) at Colorado State University under the following ground surface structures: snow or rain on grass, grass-covered dry ground, or asphalt that was either dry, rain-wet, or covered with snow.

Here, researchers were able to safely release gas from a ruptured pipeline and then observe how far the gas leaked vertically and horizontally at specific times after the leak. In 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 SMU Lyle School's Department of Civil Engineering, led the study. Also assisting were Daniel J. Zimmerle, director and principal of the Methane Emissions Technology Evaluation Center at CSU; Richard S. Kolodziej IV, who is pursuing his master's degree at SMU and is part of Smits' research team; and Stuart Riddick, a scientist at Colorado State University's Energy Institute.

Lead study author Navodi Jayarathne, a postdoctoral fellow in the SMU Lyle School's Department of Civil and Environmental Engineering, and Kathleen Smits, chair of civil and environmental engineering in the SMU Lyle School of Engineering and Solomon Professor of Global Development, at the Colorado test site.

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 says. 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 the gas finally finds a way to escape from the ground, it moves very quickly and in high concentrations, increasing the safety risk,” says Smits.

Another finding surprised the researchers: Even after the gas supply was cut off or the leak was repaired, methane trapped under snow, damp earth or asphalt surfaces could still be detected in high concentrations for up to 12 days. And during this time, natural gas spread sideways 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,” says Smits. “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 says.

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 also reflect the expected behavior at other leak locations,” says Smits.