Numerical sensitivity analysis of cement sheath bond integrity for CO2 injection wells under pressure and thermal loading

• CO₂ injection well cement sheath bond damage is modelled and characterised. • 144 combinations of CO₂ injection pressure and temperature are simulated. • Microannulus development is more sensitive to thermal loading than pressure loading. • Once initiated, cement sheath bond damage occurs rapidly to form a microannulus. • Caprock stiffness is a key regulator of cement sheath bond damage development. This paper presents a numerical analysis of CO₂ injection well integrity, focusing on degradation of cement sheath bonds with the casing and caprock. The cement sheath and caprock are modelled as thermo-poroelastic materials subject to coupled thermal, hydraulic, and mechanical behaviour. Debonding at the cement-casing and cement-formation interfaces is explicitly modelled in the finite element formulation using a cohesive zone model. A mixed-mode traction-separation failure criterion is employed to capture progressive failure under tension and shear. 144 simulation scenarios are considered for practical ranges of CO₂ injection pressure (15–23 MPa) and temperature (0–15 °C) sustained for 30 days in a well system at 1.5 km depth. Predictions are compared based on the timeframe of damage development and the apertures of any resulting microannuli. For the system studied, CO₂ injection conditions align with the ‘window’ of damage initiation and development at the cement-casing interface, whilst no damage is predicted at the cement-formation interface. Thermal loading has a greater influence on damage development than pressure loading, with lower injection pressures and temperatures producing earlier damage onset and larger microannulus apertures. Higher injection pressures somewhat mitigate damage by counteracting thermal contraction of the system, although this pressure effect would be less pronounced for a real well completion considering the injection tubing and A-annulus fluid. Once initiated, damage develops rapidly and has typically fully evolved within one day. These findings contribute to robust CO₂ storage risk assessments and support planning of corrective measures to ensure long-term wellbore integrity during geological CO₂ storage.