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