Long Term Well Plug Integrity Assurance – A Probabilistic Approach

Well plugging and abandonment (P&A) poses significant long-term risks for the industry with thousands of onshore and offshore wells requiring P&A in the coming decades. Historically, operators have typically followed a prescribed P&A approach to meet regulatory requirements. However, a risk-based approach allows bespoke abandonment strategies for individual wells, with the potential to reduce the time and effort needed for low-risk wells, while justifying any necessary additional P&A resources for managing higher risk wells. An advance computational risk-based, predictive well integrity model, STEM-flow, has been developed to support risk-based P&A and new plug technology qualification, and to compliment an extensive material test programme for structural bismuth alloys for plugging and abandonment and carried out as part of a collaborative research program. The model enables assessment of leak paths and leak rates with time, addressing multiple plugs and well barrier elements, degradation of barrier materials, cross-flow potential and reservoir re-pressurization. The mathematical model is based on a connectivity matrix, whose elements are effective permeability values, derived from well barrier models, which separate isolated pressurized zones, along a potential leak pathway. Key elements of the approach include the focus on fundamental models for barrier failure mechanisms and the handling of uncertain model parameters through Monte Carlo Simulation to calculate statistical life and probability of failure of plugs and complete well P&A designs. The paper outlines the approach employed by STEM-flow for evaluating and comparing the effectiveness of different P&A designs, plugging technologies, cement plugs and new sealing materials such as creep resisting structural bismuth alloys. Two cases are studied, involving the prediction of plug life and overall life of a specific well P&A design. These cases compare a typical well P&A design for a subsea well in the Central North Sea using conventional cement barriers with that of structural bismuth alloy plugs under development.