Effects of CO2-induced Reaction on the Transport Properties of Debonded Well Cement-Casing Interfaces: Reactive Flow-through Experiments on the Metre Scale

Abstract:

Debonding between casing and cement may create interfacial leakage pathways, compromising well integrity in CO₂ storage systems. Our previous work on such debonding-defects showed that chemical reactions between cement, steel and CO₂-fluids provide only limited potential for local reaction-induced sealing, allowing short-range defects to remain open. Changes in temperature and stress state may subsequently cause these defects to grow and connect, potentially forming long-range interfacial leakage pathways. This study investigates how CO₂-induced reaction affects the transport properties of such interconnected defects by means of reactive flow-through experiments.

Metre scale sections of debonded cement-casing interface were simulated using composite samples, prepared as follows. Cement was cast into steel tube coils (L 1.5–3.0 m, ø 6–8 mm). After curing, the coils were pressurized using water, causing the steel tube to deform permanently and lift off the cement, creating casing-cement samples that contain sections of (partially) debonded cement-steel interface.

Reactive flow-through experiments were performed in a permeameter, capable of running at temperatures (80°C ± 0.5 °C) and fluid pressures (10–12 MPa) representative for downhole environments. The coil samples were one-sidedly flooded with CO₂-bearing fluid, continuously measuring apparent sample permeability and periodically sampling the downstream fluid. Additionally, post-experiment microstructural analyses were performed. Results show permeability decreases of several orders, indicating reactive-transport phenomena that occur on the metre scale contribute significantly to the sealing-potential. As such, our experiments can be used to understand the long-range behaviour of annuli in wells, beyond the commonly used lab-scale.