Cement sheaths are among the most important well barrier elements and cement sheath integrity is thus important to maintain zonal isolation. Repeated pressure cycling in a well might lead to radial cracks and/or formation of microannuli in the cement sheaths. However, most experimental tests and model simulations of cement sheath integrity do not include mudfilms at the cement-rock interface, and the effect of such mudfilms on cement degradation during pressure cycling is thus not well understood.
In this paper, we have used our custom-made laboratory set-up to study how mudfilms influence cement sheath integrity during pressure cycling, where X-ray computer tomography (CT) is used to visualize microannuli and cracks formed in the cement. Our results show that the bonding towards the formation is of importance for the cement sheath integrity during pressure cycling, both in the case of a soft and a stiff surrounding formation. Additionally, we see that with mudfilm present at the cement-rock interface the cement sheath is able to withstand less casing pressure before failure compared to a cement sheath without mud. For samples with a mudfilm present, radial cracks were not observed to propagate from the cement sheath and into the surrounding rock in the area covered by mudfilm.