Carbon Capture and Sequestration (CCS) is one of the most important technologies that can help many countries to reduce emissions into the atmosphere and lower their carbon footprint, which in turn can help to achieve the net-zero goal. However, when CO2 is injected into a suitable geological formation in the subsurface during CCS operations, it is essential to ensure that the well integrity of the legacy well within the Area of Review (AoR) is maintained so that the injected CO2 will not make its way to the shallow formation, which can ultimately contaminate the Underground Source of Drinking Water (USDW) and make it to the surface, which can have harmful effects on the environment and human health. Hence, this paper presents a semi-quantitative risk assessment framework for legacy wells within a CCS Area of Review (AoR) and for an Underground Injection Control (UIC) Class VI injection well. The method converts a Feature Event and Process screening into an interaction matrix (IM), assigns probability × severity scores using an incident potential matrix (IPM), and derives cause-and-effect metrics to rank barrier elements and wells. The legacy wells are evaluated using a 5 × 5 IM (casing, cement sheath, water composition, gas/CO2, and USDW), and the injector is assessed using a 7 × 7 IM (adding tubing and packer). From the results, it was found that the risk levels of the Types 2 and 3 wells were the highest, while Types 4–6 clustered in the medium-to-low range, and Types 7–9 and the Class VI well were dominated by low/very low classes. Therefore, it was concluded that the level of risk associated with legacy wells in the AoR depends upon well completion, the well configuration, the number of well barriers, and the depth to which the well penetrates. It was further found that, within the multi-barrier well, the risk level of the first barrier is lower; the risk score continues to increase with each subsequent barrier above it. The most critical elements in the given risk assessment framework for legacy wells in the AoR remain the well cement sheath, casing, and USDW. Meanwhile, the components that affect the well are water composition and the presence of gas/CO2.
Abid et al. (Wed,) studied this question.