Tensile and shear failures are two common cement-sheath failure criteria. Tensile failure occurs when a stress on the cement surpasses the tensile strength of the cement, and shear failure happens when a stress exceeds the shear strength of the cement. The objective of this study is to introduce a laboratory method that investigates the effects of fatigue (cyclical loading) on the cement sheath. Chances of cement-sheath failure also increase when the application involves high-pressure/high-temperature conditions; therefore, this test setup is rated to 25,000 psi and 450°F. In this method, fatigue-failure cycle and stress magnitude could reveal the extent of the fatigue tolerance of different slurry designs relative to each other. One could program and adjust casing pressure (Pi), pore pressure (PP), temperature, cycle intervals, and ramp rates to conduct a cyclical-loading test automatically.
Failures caused by thermal and stress loading may occur as a result of temperature and pressure-induced stresses created by well events such as hydraulic fracturing, cyclic steam injection, steam-assisted gravity drainage, production, completion, or remedial treatments. Thermal and stress cycles could impose a risk to zonal isolation and lead to a delay in hydrocarbon production. Studying such effects could be helpful in developing a better understanding of the cement-sheath-failure threshold and, in fact, could be complementary to mechanical-properties measurement. The cement sheath fails after a certain number of cycles when reaching its fatigue-endurance limit. This research showcases examples in which the fatigue-failure cycles were identified for 1,000-, 2,000-, and 5,000-psi pressure differentials between the pore pressure (exposed to the outside of the sheath) and the casing pressure. For these experiments, the pore pressure and temperature were held constant at 15,000 psi and 330°F whereas the casing pressure varied cyclically between 15,000 and 16,000 psi (Set A), 17,000 psi (Set B), and 20,000 psi (Set C).