Effect of casing eccentricity on cement sheath integrity

Abstract

Cement sheaths play an important role in providing zonal isolation and preventing the migration of formation fluids to aquifers and the surrounding environment. The condition of a cement sheath may change because of the imposed pressure and temperature alterations during a wellbore lifetime. Cement sheath mechanical failure may happen because of poor cement placement, development of cracks within the cement sheath and debonding at the cement sheath, casing and rock interfaces. A three-dimensional finite element framework, employing an appropriate constitutive model (Concrete Damage Plasticity, CDP) for cement sheath and a surface-based cohesive behaviour for the interfaces, is developed for integrity investigations. The incorporation of the CDP is very advantageous to model quasi-brittle materials due to its capabilities to simulate both compression and tensile damage. The effect of casing eccentricity on stress distribution within the cement sheath and the integrity of the cement sheath is investigated while enhancing the wellbore pressure. Three different degrees of casing eccentricity (30%, 50% and 70%) were considered. The huge stress concertation within the narrower part of the cement sheath makes this section susceptible to compression and tensile damage. The high magnitude of compression and tensile damage in the scenario with 70% casing eccentricity highlights the importance cement sheath centralisation.