Risk-Based Assessment of Leakage Rates from Legacy Wells for CO2 Storage

As the world strives to meet global climate targets, carbon capture and storage (CCS) has emerged as a critical technology in reducing greenhouse gas emissions from hard-to-abate sectors such as cement, steel, and oil and gas. While capturing CO2 is an important step, long-term geological storage of CO2 is essential to ensure that captured emissions are kept out of the atmosphere permanently. CO2 storage typically involves injecting the gas into deep underground rock formations, such as depleted oil and gas reservoirs or deep saline aquifers, at depths greater than 800 meters where CO2 remains in a dense, supercritical state. Norway is a global leader in this field, with projects like Sleipner and Northern Lights demonstrating that large-scale offshore CO2 storage is both technically feasible and safe. To gain public trust and regulatory approval, it is crucial to understand and manage the risks of CO2 leakage. Although storage formations are selected for their natural sealing properties, often the same that held oil and gas for millions of years, several potential leakage pathways must be evaluated and monitored. Leakage pathways can be classified in two main categories; paths through wells due to lack of well integrity and paths due to geological leakage such as caprock failure, leakage through faults and fractures and lateral migration. While a complete evaluation of CO2 leakage risk is a comprehensive task with the main steps shown in Figure 1, we will focus on leakage pathways through wells only in this paper. The domain of well integrity covers applications of technical, operational, and organizational solutions making sure that oil & gas wells are operated in a safe, efficient, and environmentally responsible manner. In Norway, NORSOK D-010 (Standard Norge 2021) is often used as the basis for well integrity solutions throughout the lifetime of a well, from well design to permanent plug & abandonment (P&A). With respect to CO2 storage, it is in particular the NORSOK D-010 requirements for P&A of wells that are crucial, where the key principles are a) long term barrier element properties, b) two independent barriers and c) verification of barriers and corresponding documentation of a,b and c. Barriers designed using these principles shall in theory restrict any fluids below the barrier to flow upwards, be it liquids or gases for a conservative (high) reservoir pressure. The core takeaway from this paper is that leakage risk assessments developed for oil & gas wells with respect to primarily natural gas leakage and with barriers in accordance with NORSOK D-010 can be modified to assess leakage risk from legacy wells in a CO2 storage context.