Methane leakage from abandoned wells in the Dutch North Sea

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Methane is a powerful greenhouse gas and the second most important when considering global warming due to anthropogenic added gasses. Global inventories of greenhouse gasses currently do not take into consideration methane emitted from the ocean and seas. The North Sea is an intensely exploited seas for oil and gas and it was recently suggested to be a major source for manmade methane emissions. All wells drilled through shallow gas (methane) were found to be leaking and one-third of all abandoned wells was found to be drilled through shallow gas. Here we present the results from a research expedition to investigate methane leakage at abandoned wells drilled through shallow gas in the Dutch North Sea. We surveyed 57 abandoned wells of which 33 were drilled through shallow gas. Nine locations showed bubble plumes (acoustic flares). We noted a distinct difference between gas leakage of abandoned wells and locations with natural gas seepage. Whereas well leakage consists of one or two bubble plumes at the wellhead itself and no bubble plumes in the surrounding area, natural plume fields are characterized by tens to hundreds of plumes and none at the wellhead. At six wells, we conclude that the plumes are caused by the well leaking shallow gas, whereas three observed plume fields classify as natural seepage. We found that 18% of wells drilled through shallow gas were leaking, with 11% of all abandoned wells being drilled through shallow gas. When we compensated for over-representation of shallow gas wells in our sample (58% of our sample is drilled through shallow gas), we find that less than 2% of all abandoned wells in the Dutch North Sea is likely leaking. Well leakage seems to occur when large quantities of shallow gas are present and the abandoned well apparently suffers from an integrity issue.