Effect of loading history on mechanical properties of hardened oilwell cement

Cement sheath in oil/gas wells plays an important role by providing zonal isolation of different fluids; it protects also the tubular against corrosion and provides mechanical support. In a well, cement paste that forms the cement sheath, hydrates under different conditions of temperature and pressure and is submitted to various mechanical and thermal loadings. Some of these loadings are applied at very early-age and may affect the long-term properties of hardened cement paste. It is then important to analyze the impact of hydration conditions and loading history on the long-term properties of hardened cement paste. To this end, an oilwell class G cement paste was used, with w/c of 0.44. Three cement paste samples, hydrated under different pressures, were submitted at 144 hours and after three months to similar mechanical loading paths. The results showed the hardened cement paste are stiffer and also stronger when hydrated under higher pressure. Significant irreversible strains occurred on samples hydrated under lower pressure. At shallow depth in the oil field, a particular attention may be paid during a casing test, as micro-annulus could be created.