Effects of Long-Term Exposure to Ultrahigh Temperature on the Mechanical Parameters of Cement

Abstract

Abstract Long-term mechanical durability of a cement sheath exposed to stresses in a wellbore environment is critical to maintaining zonal isolation for the life of a well. The ability of a cement sheath to remain intact when exposed to changing wellbore stresses is highly dependent upon the parameters of Young's Modulus, Poisson's Ratio and tensile strength of the cement and the surrounding rock. Wellbore stresses sufficient to cause failure of a cement sheath may result from changes in temperature, casing pressure, formation pressure, or near-wellbore tectonic stresses. Wells subjected to Cyclic Steam Stimulation (CSS) for heavy oil recovery undergo extreme temperature changes that impart substantial stresses on the cement sheath. During CSS, wellbore temperatures can fluctuate between 77°F and 645°F, potentially leading to cement sheath failure. Various Finite Element Analysis models exist that are useful in predicting the failure potential of a cement sheath. These models require input of cement mechanical parameters. Mechanical parameters of numerous cement formulations have been measured in recent years but none of those measurements was made after long-term exposure of the cement to the ultra-high temperatures encountered in CSS wells. Development of such data was required to model the risk of cement sheath failure in wells subjected to CSS. This paper presents a test methodology for measuring the mechanical parameters of five cement formulations after exposure to a temperature of 645°F. Parameters of Young's Modulus, Poisson's Ratio and tensile strength were measured. Evolution of these parameters over a two-year period is presented. The temperature conditions to which the cement was exposed was found to have a significant impact on the mechanical parameters. The Young's Modulus, Poisson's Ratio and tensile strength of systems exposed to 645°F varied significantly from systems exposed to an ambient temperature of 77°F. The mechanical parameters of each individual cement formulation were also markedly different. Specialized cement systems, such as foamed cement or blends containing flexible particles, did not necessarily have mechanical parameters that were better suited for use in CSS wells than some of the more conventional formulations.