Dilatometric behavior and crystallographic characterization of Portland-polyurethane composites for oilwell high-temperature cementing applications

In order to improve the behavior of Portland-based oilwell sheath to high-temperature applications and the corresponding thermal gradients, polymeric admixtures can be added. Steam injection, for instance, raises the cement sheath-steel casing interface temperature to ∼300 °C which can be deleterious to the mechanical stability and zonal isolation provided by the cement sheath as a result of cracking of the cement. Polyurethane can be added to the cement to improve its plastic behavior and reduce the thermal expansion mismatch between casing and sheath. However, the use polymeric admixtures, especially as aqueous dispersion, can affect the kinetics of the hydration reactions and, as a consequence, the crystallographic composition and coefficient of thermal expansion of the material. Therefore, the present study aimed at evaluating the effects of the addition of different concentrations of polyurethane on the thermomechanical properties of Portland cement pastes cured at 300 °C from 14 to 90 days. Formulations containing 1 gpc (0.052 kg), 2 gpc (0.102 kg) and 3 gpc (0.153 kg) of polyurethane, respectively named PU1, PU2 and PU3 were prepared according to the American Petroleum Institute guidelines. A standard PU-free slurry was also prepared. X-ray diffraction and dry dilatometric tests were carried out. The results revealed that the curing time does affect the kinetics of hydration of Portland-polyurethane composites, retarding the formation of high-temperature stable phases, i.e., tobermorite and xonotlite. Moreover, longer curing times were necessary to reveal the improved expansion of the composites with respect to the standard sample. The addition of polyurethane can be pointed as an approach to high-temperature oilwell cement applications, since it reduces the thermal mismatch between cement sheath and steel casing for and retards but do not prevent the formation of xonotlite.