Cement mechanical-property measurements under wellbore conditions

Cement integrity preservation during completion, stimulation, production, and even well abandonment is of critical importance for an operator from long-term economic, productivity, and safety perspectives. Traditionally, compressive strengths have been considered indicators of cement integrity. However, numerous squeeze cementing jobs regularly performed on completed wells are testament to the poor correlation between compressive strengths and cement integrity. Additional mechanical properties such as tensile and flexural strengths, elastic modulus, and Poisson's ratio are being taken into account with increasing frequency for maximizing the cement sheath performance during the life of the well. Unfortunately, all such measurements are performed on samples that have been cured, either under wellbore conditions (for example, pressure and temperature) or laboratory conditions (for example, atmospheric pressure), but tested at atmospheric pressure and temperature. Such properties may at best be useful for comparing different formulations in the selection process, but do not provide information about cement properties under downhole conditions. Using a combination of ultrasonic shear wave and compression wave measurements, dynamic mechanical properties such as elastic modulus, bulk modulus, Poisson's ratio, and compressive strength are measured under pressure and temperature. These measurements are compared with mechanical properties obtained from load vs. displacement tests under static conditions and acoustic compression and shear wave measurements at atmospheric pressure and temperature. Correlations are calculated for several slurries, and the results are presented. These results include cases where the measurements made using this method demonstrated unique advantages over the conventional load vs. displacement techniques. Copyright © 2007 Society of Petroleum Engineers.