Natural or anthropogenic marine methane emissions – Investigating abandoned wells in the German North Sea

Abstract:

Abandoned oil and gas wells are one of the anthropogenic CH4 sources and onshore boreholes have been proven to release hydrocarbons from former wells. Recent studies found escaping CH4 also at marine abandoned wells in the North Sea and it was extrapolated that this may hold for one third of the ~11.000 wells in that region. Notably, not the thermogenic gas from the reservoirs but biogenic methane, potentially from shallow gas pockets, was detected. This could indicate that the methane has been mobilized by mechanical disturbance through the drill operation and the well serves as migration pathway. Methane emission from shallow seas such as the German sector of the North Sea may potentially be prone to CH4 leakages into the sea-air boundary layer, because bubbles may reach shallow water layers and the sea surface.

We studied seafloor characteristics, water column anomalies and sediment geochemistry in the German sector of the central North Sea and inspected nine abandoned well sites. First data demonstrate that at none of the studied well sites concentrations of dissolved CH4 at the upper water column was enriched. Extensive hydroacoustic mapping has been conducted to determine the presence and distribution of flares indicative for gas bubble releases. Spatial analysis revealed the relations of detected flares to subsurface salt diapir locations, seismically identified gas accumulations and abandoned well sites. In addition, measurements of dissolved CH4 concentrations in the water column were retrieved to support identification of seepage. At one area we observed CH4 abundances, which were ten times enriched compared to background. However, the enrichments occurred also 500 m away from the drill site and did not increase towards the center. Naturally seeping methane is presumably transported to the seafloor along sub-vertical faults, which have formed concurrently to the updoming salt. Our data argue for temporary active natural seepage and not a leaking well, but also underlines that natural seeps may challenge the identification of potentially leaking wells. Due to the shallow water depths of 30 to 50 m, flares were observed to reach close to the sea surface and a slight oversaturation of surface waters in the flare-rich area indicates that at least part of the released CH4 is contributing to the atmospheric inventory.