Shallow Gas Migration along Hydrocarbon Wells-An Unconsidered, Anthropogenic Source of Biogenic Methane in the North Sea.

Shallow gas migration along hydrocarbon wells constitutes a potential methane emission pathway that currently is not recognized in any regulatory framework or greenhouse gas inventory. Recently, the first methane emission measurements at three abandoned offshore wells in the Central North Sea (CNS) were conducted showing that considerable amounts of biogenic methane originating from shallow gas accumulations in the overburden of deep reservoirs were released by the boreholes. Here, we identify numerous wells poking through shallow gas pockets in 3-D seismic data of the CNS indicating that about one-third of the wells may leak, potentially releasing a total of 3-17 kt of methane per year into the North Sea. This poses a significant contribution to the North Sea methane budget. A large fraction of this gas (∼42%) may reach the atmosphere via direct bubble transport (0-2 kt yr) and via diffusive exchange of methane dissolving in the surface mixed layer (1-5 kt yr), as indicated by numerical modeling. In the North Sea and in other hydrocarbon-prolific provinces of the world shallow gas pockets are frequently observed in the sedimentary overburden and aggregate leakages along the numerous wells drilled in those areas may be significant.