Failure analysis for the cement with radial cracking in HPHT wells based on stress intensity factors

Abstract

Wells are often cemented to provide zonal isolation and structural support through their entire service lifetime. Even if a cement sheath is set properly, it may inevitably contain small cracks which tend to propagate under harsh environmental and operational conditions. This work presents a mechanical model to predict the stress intensity factor at a radial crack tip in the cement under high pressure/high temperature (HPHT) conditions. The well section comprised of casing-cement-formation is modeled as three isotropic material domains under plane strain condition. The model calculates the stress field in the well section and estimates the stress intensity factor by a distributed dislocation technique. Two failure factors corresponding to the material strength criterion and fracture propagation criterion are introduced to evaluate the cement integrity. The influence of loading conditions and cement's mechanical and thermal properties on the failure factors is also investigated. The results indicate the predominant failure mode under HPHT conditions. The proposed model provides guidelines for the assessment of cement sheath integrity based on fracture mechanics.