Anthropogenic emissions of greenhouse gases from the seafloor into the North Sea
The present thesis provides the first methane (CH4) emissions measurements from offshore abandoned wells in the North Sea and found that boreholes constitute unrecognized, but important conduits for the release of biogenic CH4 originating from shallow gas accumulations (<1,000 m sediment depth) in the overburden of deep reservoirs. Such kind of leakages is not currently considered in any regulatory framework or greenhouse gas emission inventory. In the North Sea and in other hydrocarbon-prolific areas of the world shallow gas pockets are frequently observed in the sedimentary overburden above the deep hydrocarbon reservoirs and aggregate emissions along numerous wells may be significant. This conclusion also has important implications for Carbon Capture and Storage (CCS) since it implies that leakage from a carbon dioxide (CO2) storage site can potentially occur along any type of well (production, exploration, or abandoned), as long as it penetrates the subsurface CO2 plume. The second part of this thesis focuses on the Sleipner CO2 storage site in the Central North Sea and investigates hypothetic, but probably realistic leakage of CO2 along a well that penetrates the subsurface CO2 plume and leaks into the ~80 m deep water column, using a combination of experimental field data and numerical modelling. Small footprints of CO2 leakage in seawater imply that the environmental consequences of a single well leaking CO2 are insignificant and finding those leaks may pose challenging. Considering the millions of oil and gas wells drilled world-wide and the prospective implementation of CCS at a scale that would have significant impact on global CO2 emissions, this thesis stresses that pressure-based testing of well integrity is not sufficient for identifying and quantifying gas emissions (CH4 or CO2) along hydrocarbon wells. Improving our surveying and monitoring efforts and adapting the respective regulatory frameworks (national and international) is important.