Development of a Risk-Based, Modelling Framework for Integrity Assessment of Legacy Wells in CO2 Storage Applications

Demonstrating the integrity of Plugged and Abandoned (P&A'd) legacy wells during CO2 storage projects is a crucial requirement for regulators, stakeholders, and operators. The corrosive nature of CO2 may affect the integrity of such wells, jeopardising the long-term containment of the CO2. This study illustrates the new capabilities, tailored for CO2 storage applications, of a modelling framework (Johnson et al, 2021a&b) that provides a quantitative, risk-based assessment of the long-term integrity of legacy P&A'd wells. The following three new modelling modules are added to this integrated framework to account for the key concerns due to the presence of CO2 in the system:

- The cement-CO2 geochemistry "add-on" module evaluates the phenomenon of cement's self-sealing/self-degradation behaviour. Carbonated water reacts with cement minerals; some of which are initially dissolved followed by a possible precipitation process that is controlled by the fluid's flow rate/residence time in the cement defects such so microannuli. The self-sealing mechanism can reduce the microannuli's width, potentially stopping CO2 leakage, while self-degradation has the opposite effect.

- The geomechanical "add-on" module quantifies changes in the size of defects within the well P&A system when the pressure/stress changes during the CO2 storage process. Pressure/Stress variations change the radius of the casing and the size of the adjacent microannuli, altering the leakage rates within the legacy well system. The elastic/plastic stress behaviour of cement can potentially contribute to increasing risks.

- The CO2/casing corrosion "add-on" module estimates the rate of casing corrosion and subsequent reduction in casing thickness over time based on experimental data. Excessive corrosion can lead to casing breach. These processes increase the size of defective leakage paths and increase the leakage risk from the legacy well.

On completion, the modelling framework will be capable of being an integral part of the risk-assessment process when (a) selecting between possible CCUS projects and (b) supporting operators, regulators and other stakeholders in their decision-making process when analysing CCUS proposals.