A study has recently found that boreholes in the North Sea constitute significantly one of the main sources of greenhouse gas, methane leakage than previously thought.
According to researchers from Helmholtz Centre For Ocean Research Kiel and the University of Basel 's calculations, shallow gas migration along wells may release around 3,000 to 17,000 tonnes of methane from the North Sea seafloor per year.
In the ocean, methane is usually degraded by microbes, thereby locally acidifying the seawater.
"We estimate that gas leakage around boreholes could constitute one of the main sources of methane in the North Sea", said first study author Dr. Lisa Vielstadte from GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany.
During expeditions to oil and gas fields in the central North Sea in 2012 and 2013, the scientists discovered a number of methane seeps around abandoned wells.
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Interestingly, the gas originates from shallow gas pockets buried less than 1,000 meters below the seabed.
They are simply penetrated when drilling into the underlying, economically interesting hydrocarbon reservoirs.
"These gas pockets usually do not pose a risk to the drilling operation itself. But apparently disturbing the sediment around the well enables the gas to rise to the seafloor", said another researcher Dr. Matthias Haeckel from GEOMAR.
Seismic data from the subsurface of the North Sea further shows that about one third of the boreholes perforated shallow gas pockets and may thus leak methane.
"Considering the more than 11,000 wells that have been drilled in the North Sea, this results in a fairly large amount of potential methane sources", Vielstadte added, who is currently based at the Stanford University in California, USA.
In the North Sea, about half of the wells are located in such shallow water depths that the methane leaking from the seabed can reach the atmosphere, where it is acting as a potent greenhouse gas - much more efficient than carbon dioxide.
The research appears in journal Environmental Science & Technology.
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