Thermal Stresses in Annular Cement

Abstract

Heating of casing, e.g. by the drilling fluid returning to surface, expands the casing string. This results in the hoop stress in the cement sheath becoming less compressive (more tensile). Similarly, cooling of casing, e.g. by injecting cold water down the well, makes the casing contract. This results in the radial stress in cement sheath becoming less compressive (more tensile). These stress changes may induce radial cracks or debonding at cement-casing and cement-rock interfaces. Finite-element simulations are performed in order to estimate the magnitude of the stress variations in cement sheath caused by the temperature variation at the inner side of the casing. Simulations are performed for different combinations of thermal expansion coefficients of cement, steel, and rock. It is shown that, at least in some cases, it is beneficial to have cement formulations that result in lower Young’s modulus and higher tensile strength of cement upon hardening. The role of initial stresses in cement sheath for practical evaluation of cement sheath stability during wellbore heating/cooling is discussed.