Creep microindentation of low-density oil well cement and the implication on radial cracking risk of cement sheath

To better provide zonal isolation for the production of oil and gas, low-density oil well cement (LD-OWC) filling a deep oil well was developed to reduce the high hydrostatic pressure caused by the cement slurry. The key mechanical properties of LD-OWC and related cracking susceptibility, nevertheless, have not been fully understood. This work was to characterize the elastic and creep properties of LD-OWC using microindentation and assess the radial cracking risk of the cement sheath. With two types of LD-OWC cured at different temperatures, our measurement through mercury intrusion porosimetry (MIP) showed that their porosities depended on the slurry densities level, and the basic creep exhibited a logarithmic increase in the long term. Mechanical properties such as Young's modulus (E), indentation hardness (H), and creep modulus (C) of LD-OWC were statistically characterized, in particular, E and C were in line with those obtained by macroscopic tests. Moreover, a viscoelastic sheath model was developed to evaluate the stress redistribution in the steel casing, the cement sheath, and the formation in the long term. We found that lower slurry densities of OWC reduced the risk of radial cracking. Taking advantage of parametric analysis, the power-function correlation between the safety factor of radial cracking and the mechanical and geometrical sheath properties was also demonstrated.