Digital 4D Modelling for De-Risking High-Integrity-Risk Wells: A Case Study

The effective risk assessment of wells with high integrity risk is a crucial step for safe operations in workovers and decommissioning. Traditional methodologies, such as qualitative risk matrices or BowTie analyses, do not provide robust and reliable assessments due to their subjective nature and susceptibility to misinterpretation and bias. This paper illustrates how a digital 4D modelling methodology (i.e. integrated 3D well P&A and subsurface models over time) provides a superior, data-driven approach, that delivers quantitative insights to support operational and regulatory decision-making. The study presents application of an innovative modelling approach for well integrity assessment in high-risk wells, integrating numerical simulation to enhance the accuracy and reliability of risk evaluations. The proposed model quantifies well-specific leakage risks to surface and crossflow under various reservoir recharge and leaking fluid scenarios. By incorporating probabilistic assessments and accounting for uncertainties such as cement flow properties and casing corrosion, the workflow enables comprehensive risk evaluation and comparison. The detailed and auditable results support effective stakeholder communication and informed decision-making. The application of the modelling approach is demonstrated using a case study featuring a high-risk well situated in a depleted reservoir with multiple flow zones. A probabilistic analysis, while accounting for uncertainties in cement flow properties, reveals the prevalence of crossflow risks into overburden flow zones while the risk of leakage to surface is expected to be small. The model is also able to quantify the impact of various uncertainties such as casing corrosion, as well as formation porosity, permeability, net-to-gross and reservoir recharge pressure on the subsequent risk of crossflow. By providing a more robust, data-driven risk assessment, the simulation enables effective comparison of alternative scenarios to ensure compliance with local regulations and company standards. Additionally, it enhances transparency in decision-making by clearly communicating assumptions and findings to regulators and other stakeholders. Ultimately, this approach facilitates the selection of the optimal "As Low As Reasonably Practicable" (ALARP) scenario for remedial actions, alternative P&A designs, monitoring strategies and project planning. In conclusion, the presented workflow offers significant advancement in well integrity assessment, equipping stakeholders with a powerful tool for informed decision-making, regulatory compliance, and effective risk mitigation in well operations.