Cement sheaths are among the most important barrier elements in petroleum wells. However, the cement may lose its integrity due to repeated pressure variations in the wellbore, such as during pressure tests and fluid injections. Typical cement sheaths failure mechanisms are formation of radial cracks and microannuli, and such potential leak paths may lead to loss of zonal isolation and pressure build-up in the annulus. To prevent such barrier failures, it is important to study and understand cement sheath failure mechanisms.
This paper describes a series of experiments where we have used a tailor-built laboratory set-up to study cement sheath integrity during pressure cycling, where the set-up consists of down-scaled samples of rock, cement and casing. Cement integrity before and during casing pressurization is characterized by X-ray computed tomography (CT), which provides 3D visualization of radial cracks formed inside the cement and rock. We have studied how contextual well conditions, such as rock stiffness, casing stand-off and presence of mudfilm, influence cement sheath integrity.
The results confirm that the rock stiffness and casing stand-off determine how much casing pressure the cement can withstand before radial cracks are formed in the cement sheath, where the rock stiffness is significantly more important than casing stand-off. Furthermore, it is seen that the radial cracks in the cement sheath continue into the rock as well. However, when a thin mudfilm is present at the rock surface, the cracks stop at the cement-rock interface, and the cement sheath withstands less pressure before failure. The bonding towards the rock is thus of importance.