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Atmospheric pressure controls greenhouse gas emissions from leaking oil and gas wells
Decreases in atmospheric pressure can lead to a 20-fold increase in fugitive greenhouse gas emissions (GHG) associated with leaking onshore hydrocarbon wells, a new study in Nature has shown.
A 30-day study led by Scottish and Canadian scientists showed that atmospheric pressure controls the timing and magnitude of fugitive gas emissions from leaky wellbores. The finding is significant, as changes in atmospheric pressure are not routinely taken into account when assessing fugitive gas leakage from onshore oil and gas wells.
Dr Aaron Cahill, from the Lyell Centre at Heriot-Watt University, says: ‘Fugitive emissions from hydrocarbon wells are a serious environmental issue. They release greenhouse gases into the atmosphere and can negatively affect the environment, including groundwater resources. Our experiment showed that atmospheric or barometric pressure changes, decreases in which can lead to rain, had a profound effect on fugitive gas emissions. When air pressure decreased by 0.2 PSI over a period of hours or days, a very small amount considering bicycle tires require 50 PSI or so, the release of natural gas from the soil to the atmosphere increased 20-fold.’
The study also shows that atmospheric pressure isn’t the only factor at work. Soil moisture, temperature, local geology and the type of well all play roles in the process, although change in air pressure is a key driving force.
Dr Cahill continues: ‘Current monitoring strategies, which generally involve one-time, simple measurements of soil gas concentrations around a wellhead, don’t consider barometric pressure changes or other environmental parameters that control leakage. This means some cases of leakage won't be detected, for example, if air pressure has been high for a period of time. It also means any estimates for the volume of fugitive gas escaping from a well will simply be wrong; either under- or over-predicting how much gas is coming out of the ground. We need frequent, continuous monitoring if we are to accurately detect and quantify fugitive gas emissions at oil and gas sites.’
‘Unfortunately, wellbore integrity failure and gas leakage can develop in a portion of all oil and gas wells, including active, abandoned, conventional and unconventional wells. Improving monitoring methods to optimally detect leakage is essential for identifying the wells that are leaking the most, and prioritising repairs as required. Hopefully, this work sets us on the right path.’
During the experiment, the research team simulated wellbore leakage in the ground at a field site in north-eastern British Columbia, Canada, a highly developed area with over 25,000 oil and gas wellbores. Some 30cm of natural gas was continuously injected 12 metres below the ground surface over five days. The injected gas was tracked through the soil and into the atmosphere, while atmospheric pressure and other environmental parameters were also monitored to assess relationships.