Formation of tobermorite and xonotlite fiber matrices in well cementing and impact on mechanical properties

Abstract

Over the last two decades, there has been an increased focus on the stability of the cement sheath under the influence of shear stresses imposed during the lifetime of the well. However, there has been no investigation, on the impact to the cement sheath matrix, on the formation of fibrous crystalline calcium silicate hydrates that occurs under temperature and pressure. In well cements, tobermorite (Ca5Si6O16(OH)2·4H2O) has been reported to form as the predominant phase at temperatures in the range of 120–180 °C and has a platelet, lath, or fibrous habit while xonotlite (Ca6Si6O17(OH)2) forms at temperatures above 150 °C and has a needle-like habit. Tensile strength data were obtained at 150 and 230 °C with phase assemblage of the hydrated cement determined using XRD, crystal habit by SEM analyses, and porosity by mercury intrusion porosimetry. The effect of retarder on crystal formation was also investigated and correlated to thickening time and calorimetry.