New measurement method for the tensile strength of cement sheath

Even though cracks always initiate at free surfaces or interfaces, the Brazilian test does not include the interfacial/free surface effects on the tensile strength measurement. For sandwich-structured systems, for instance, measurement methods that merely focus on the tensile strength of the bulk material, may not present a thorough understanding of the integrity of the system. It becomes even more important for systems, like steel casing–cement sheath–rock formation, where the tensile strength of the sandwiched material, cement in this case, is considered the main drive behind many system integrity problems. In this study, a novel yet straightforward approach, illustrated in Fig. 1, is proposed to incorporate the ring geometry of the cement sheath into the evaluation of its tensile strength. Initially, an analytical solution is formulated to accurately account for the influence of boundary conditions on the stress distribution within the cemented wellbore. Subsequently, the tensile strength of the cement sheath is estimated by establishing a correlation with the tensile strength determined from the Brazilian test. The analytical solution helps to quantify the risks and provide more informed casing and cement job designs.