Interface debonding driven by fluid injection in a cased and cemented wellbore: Modeling and experiments

• A model for the debonding of interfaces of cemented wells due to fluid injection.
• The initial stress on the interface and its toughness control debonding occurrence.
• Laboratory experiments confirm model predictions of the debonding growth rate.
• Vertical propagation is found either self-sustaining or self-limiting.
• Partial debonding around the annulus is found to be always favored.

The integrity of the wellbore completion under injection conditions is vital for the effective, long term storage of carbon dioxide. Here we experimentally demonstrate and mathematically model fluid-driven debonding of the wellbore annulus in order to provide a fundamental basis for well design. We show that self-limiting versus self-sustaining propagation of the annular debonding are distinguished by the sign of a fluid buoyancy parameter that involves a non-trivial relationship between the hydrostatic pressure variation of the fluid with depth and the clamping stress provided by the internally-pressurized casing/cement system. The theory also gives a series of scaling relationships that can be used to predict the rate of growth of the debonding and the fluid flux through the annulus for various growth regimes. The experiments confirm the theoretical predictions of debonding growth rate for the limiting case of zero-buoyancy. We also observe azimuthal debonding extending around 1/2 to 3/4 of the well annulus in the experiments, which is shown to be consistent with physical insights that can be derived from the theoretical model. We conclude that the clamping stress on the well annulus is a critical quantity for hydraulic isolation of the well, and therefore appropriate design of the casing/cement system relative to the intended injection conditions is necessary for the integrity of CO2 injection wells.