A Holistic Approach to Consider for Slot Recovery Based on a Failure Incident in North Sea Operations

Abstract

Abstract An oilfield service company was awarded the scope by an operator in the North Sea to recover three slots from an existing offshore platform and redrill the HPHT wells to tap into unreached reservoir resources on a gas field. This paper covers the holistic approach of analysing the slot recovery operations strategy, plug and abandon the original wellbore, and how to avoid tunnel vision in the decision-making process to achieve the goals. Cut and pull operations have one of the highest financial impacts on slot recovery and can lead to loss of the well in unfortunate circumstances. After slot recovery operations, including the cutting and milling of the 10-3/4″ × 10″ casing, the 13-3/8″ casing failed the pressure test. The incident triggered a Cause Analysis Tree Diagram of the well conditions that led to the discovery of a hole in the 13-3/8″ casing. The investigation highlighted the tunnel vision of the decision-making process where ‘stop points’ were bypassed or not efficiently identified, contributing to operational delays and potential loss of the wellbore. The investigation concluded that insufficient information on the well was obtained during the planning phase and the financial decision tree analysis was incorrectly defined to proceed with the preparation of the contingency plans. In this process, all existing logs were re-evaluated to identify the solids behind casing, ovality, centralization, and casing-on-casing contact points. The original plan was to cut and pull the casing and then mill ~374ft of casing, with a shallow sidetrack option as contingency. However, during the execution, due to height of the top of solids encountered behind the casing, it was required to start milling from a shallower depth, with a total of ~ 1,500 ft of casing to be milled. At this stage in the operations, a fishbone diagram should have been created to perform a detailed analysis of the technical risks and cost impacts and to decide whether to continue milling or to proceed with the contingency option to sidetrack from a shallower depth. However due to peer pressure, tunnel vision goal seeking (Gasaway, n.d.), and phycological safety the multi-factor decision ‘stop points’ were not considered leading to an expensive, lengthy, and unsuccessful operations. This paper summarizes the decisions and ‘stop points’ in the process of slot recovery operations and highlights the potential outcomes of incorrect technical risks and cost analysis. Further reviewing the utilization of the engineering analysis, and economical aspects through preparation of a fishbone diagram to manage stakeholders’ expectation on upfront commitment to continue with challenging slot recovery operations on a major decision-making point. The example and process provided in this paper will benefit the industry by helping to recognize a ‘stop points’ in the operations, highlighting the risks of having catastrophic event.