Impact of chemical and mechanical processes on wellbore integrity in CO₂ storage systems

Carbon Capture and Storage (CCS), involving the capture of CO₂ at large point sources, such as power plants, followed by long-term storage in depleted hydrocarbon reservoirs or saline aquifers remains a key option for reducing CO₂ emissions while fossil fuel use continues. For CCS to be effective, the injected CO₂ must remain confined to the target reservoir by a suitable caprock formation. However, the wellbores needed to access the reservoir inevitably penetrate this natural caprock seal, forming obvious pathways for potential leakage of CO₂. Moreover, the artificial barriers created in wellbores to prevent unwanted fluid migration typically consist of steel and Portland-based cement, which both are prone to chemical attack by CO₂-rich fluids. Wellbores also experience changes in temperature and stress state during CO₂ injection, which can lead to the development of mechanical damage, such as fractures in cement seals or debonding at material interfaces, creating pathways for CO₂-rich fluids to penetrate the wellbore. This PhD thesis is a predominantly experimental study, aimed at investigating the effect of CO₂ on the mechanical and chemical integrity of wellbores in geological storage systems for CO₂.
In particular, this study addresses effects of mechanical damage and chemical alteration (and their interplay) on the transport properties of fractured cement and of casing-cement interfaces, which present widely perceived leakage risks. The main objectives include a) determining the effect of CO₂-induced reactions on the mechanical properties of fractured wellbore cement, b) characterizing the key chemical reactions in cement-steel-CO₂-brine systems, c) determining the effect of these reactions on the transport properties of debonding defects at casing-cement interfaces in wellbores, d) understanding the underlying reactive transport processes on the metre length-scale, and e) determining whether chemical reactions that involve a solid volume increase and develop a force of crystallisation may be used to remediate or prevent wellbore leakage (e.g. by mechanically eliminating or reducing defect apertures to the point where CO₂-induced reactive transport leads to self-sealing behaviour). These aims are addressed using batch reaction experiments, permeametry experiments, conventional triaxial compression tests, long-range reactive flow-through experiments, numerical reactive transport simulations, and uniaxial CaO hydration-deformation (force of crystallisation) experiments. Using the results obtained, implications for wellbore integrity in geological storage systems for CO₂ are discussed, and remaining and newly surfaced knowledge gaps are identified, delineating possible directions for future research.