Fate of Hydrocarbon Leaks From Plugged and Abandoned Wells Compared to Natural Seepages

As the hydrocarbon fields mature and reach the end of their productive lives, their Permanent Plug and Abandonment (PP&A) become inevitable. Even though new technology and verification methods are being researched, it is evident that operational, barrier material and qualification challenges together contribute to a risk of leaks from abandoned wells. Well integrity standard NORSOK D-010 constitutes zero leak acceptance criteria to protect the environment; however, natural hydrocarbon seepages are occurring all over the world on a daily basis. In this study, we introduce the comparison between leaking wells and natural seeps and suggest conducting a fate analysis is appropriate to provide necessary data for evaluating environmental implications of leaking wells. Two case studies were analyzed using SINTEF Ocean’s OSCAR (Oil Spill Contingency And Response) software; one historical gas leak (Field A) and a theoretical oil leak (Field B). It is found that for releases of natural gas at 70 m water depth, 95 to 99 % dissolve in the ocean, and the fraction of gas reaching the atmosphere is dependent on the initial gas bubble size. Fate of oil is more complex than gas, but evaporation, sedimentation and biodegradation are the main contributing mechanisms in the fate analysis.