A dull ache in your knee might mean a change in the weather. But changes in pressure systems can mean more than a trigger for joint pain—they can also encourage leaky natural gas to seep into surrounding soils.
A new study in Scientific Reports looks at how changing barometric pressures can influence escaping natural gas from wells. Researchers note that present-day estimates of escaping gas don’t consider barometric pressure changes, and this oversight might lead to underestimating the amount of greenhouse emissions leaking into the environment.
Mining for natural gas has environmental consequences, including leaky wells. Previous research has shown at least 7% of oil and gas wells lose some of their integrity, leaking gas into the ground and eventually into the air. Leaking methane can spread in all directions, leading to aquifer contamination, explosive conditions in soils, and greenhouse gas emissions.
Gas that has escaped into subsurface soils, called fugitive gas by scientists, can be tough to monitor. Gas leak monitoring is usually done by well operators, said Olenka Forde, a hydrogeology doctoral student at the University of British Columbia and lead author of the new study.
“If there’s bubbling in standing water, that’s a sign of a leak,” she noted, adding that other leaks aren’t as obvious. And what’s more, any leak is measured only once during the well visit, not over time.
Monitoring plans and approaches are still being developed, said Mary Kang, a hydrologist at McGill University who was not involved in the study. “Although new regulations for methane monitoring are being developed and implemented, there is a need for more research and development to accurately and efficiently monitor natural gas leaks.”
Looking for Leaks
Although the effect of barometric pressure on methane leaks was previously studied at landfill sites, Forde said, “we wanted to know if barometric pressure would control soil methane fluxes from leaking natural gas wells.”
The team picked a site near a natural gas play in northeastern British Columbia with a deep water table. Forde explained that a deep water table has a greater distance between the atmosphere and the saturated zone, allowing barometric pressures to push leaked natural gas deeper underground.
At the site, the researchers injected 30 cubic meters of natural gas into glaciolacustrine deposits (silts and clays, 12 meters below the surface). The injection lasted 5 days, and the team continuously monitored leaks within a 20- × 20-meter grid around the injection site for 24 days.
They found that when the barometric pressure fell, gas leaks would increase. “It’s like an accordion,” said Forde. Squeezing the accordion together compresses everything, essentially holding the gas leaks under the surface, she explained, “while stretching out the accordion will release the pressure and the gas can escape.”
Forde said the barometric pressure had a greater effect on fugitive gas than the rate of gas leakage. “Barometric pumping can hold gas in the subsurface, which means you see trends for weeks,” she said—and long after the leak has stopped.
“Leaks are not necessarily instantaneous events,” said Kang, adding that leaks can occur consistently for decades. Although the researchers found a link to barometric pressures and leaks, Kang cautioned that “it’s unlikely that barometric pressure is the only driver here.”
In addition to noting how barometric pumping can affect fugitive gas, Forde noted that leakage is site dependent. “Deep water tables and lithology can have a large influence on emissions,” she said.
“Lithology really influences gas movement and makes it difficult to predict gas migration,” said Forde. For example, low-permeability sediment like silts and clays can force lateral movement. Forde noted this means gas can migrate farther away from a leaky well than regulators expect.
Kang agreed, adding that wells that are geographically close to each other in a basin can have highly variable leak rates, up to 4–5 orders of magnitude in some cases.
“There are so many variables controlling leakage,” said Kang. “We should consider adding barometric pressure to the list of parameters being measured.”
“Without constant [barometric] monitoring, the fugitive gas emissions can be underestimated,” said Forde. She noted that considering barometric pressure changes and the site conditions can “give you the big picture of greenhouse gas emissions” from gas leaks.
—Sarah Derouin (@Sarah_Derouin), Freelance Journalist
Derouin, S. (2019), Fugitive gas abetted by barometric pressure, Eos, 100, https://doi.org/10.1029/2019EO136546. Published on 19 November 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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