Plug & abandonment of offshore wells: Ensuring long-term well integrity and cost-efficiency

There is an upcoming "P&A wave" of wells that need to be permanently plugged and abandoned, especially in mature, offshore areas such as the North Sea and Gulf of Mexico. It is important to ensure that plugged wells do not leak after abandonment, as there could be several potential leak paths such as microannuli in plugged wells. To ensure well integrity after abandonment, permanent well barriers must extend across the full cross section of the well. That includes establishing barriers in all annuli, which could however be quite time-consuming and thus costly. This paper is a review of challenges and technologies for P&A of offshore wells, with an emphasis on cost-effective solutions while establishing permanent well barriers. An overview of cement and other plugging materials is given, as well as a discussion of different types of potential leak paths and failure mechanisms in permanently plugged and abandoned wells. Moreover, recent technology developments such as utilizing shale as barrier for P&A are described. A discussion on the special considerations related to P&A of subsea wells is also included.