Developing a Well-Centric Flow Model - the First Step in a Risk-Based Approach to Oil and Gas Well Decommissioning

The final stage in the life of a hydrocarbon well is decommissioning, which aims to ensure that the original integrity of the cap rock is restored and all penetrated sub-surface formations with flow potential are isolated with an “eternal perspective” (Oil&Gas_U.K., 2018). Well plug and abandonment (P&A) costs constitute a significant proportion of the total decommissioning expenditure. Careful planning and execution is always required, because a failed P&A often requires costly and complex remedial operations, and can potentially result in significant environmental costs, reputational damage and financial loss. The high cost of well P&A operations and the increasing number of North Sea wells approaching the end of their economic life have resulted in a growing need to identify and develop new approaches to well P&A that are both fit-for-purpose and cost-effective. The development of a tool to support a risk-based analysis of P&A designs has been identified as one of the means for achieving these twin objectives. This paper describes the first stage in this process - the development of a novel modelling framework that can provide a quantitative assessment of the leakage risk associated with each well P&A design scenario over a specified period of time. The framework is built on a commercial, grid-based, numerical, reservoir simulator and can be used to model most North Sea wells scheduled for P&A during the coming years. Preliminary results are presented to illustrate the value that can be derived from the framework. The importance of understanding the ranking of model inputs on the basis of their impact on modelled leakage rates, and hence the significance of any associated uncertainties, is also illustrated using an example which shows cement absolute permeability (k) rather than relative permeability (kr) to be a more critical determiner of hydrocarbon leakage rates for the P&A cases examined. The modelling results also demonstrate the robustness of a grid-based numerical simulation approach to well P&A system modelling.