Zonal isolation is critical for producing hydrocarbons in an economic and environmentally conscious manner. The isolation of wellbore fluids from the surrounding formation is achieved through effective cement placement. Cement is a brittle material which can fail when subjected to repeated application of stresses lesser in magnitude than the statically determined strength. In the field of mechanics of materials, repeated application of stress is referred to as fatigue loading. Well events such as hydraulic fracturing, completions or workover apply fatigue loads to the wellbore system, so it is important to understand the impact of fatigue on the mechanical behavior of the cement sheath. Many papers have investigated cement failure in wellbore systems under static load; however, few studies reviewed the effect of fatigue on cement mechanical properties. In this study, we investigated the evolution of cement elastic and strength properties under fatigue loading and evaluate the impact of the resulting changes in mechanical properties on cement sheath integrity.
A cement slurry design commonly used for wells in a US shale play was mixed and cured under downhole conditions. Three samples were failed to capture the unconfined compressive strength. Other samples from the same batch of cement were subjected to unconfined compressive fatigue cycles of 5, 20, 40 and 100 cycles respectively with a stress amplitude equal to 85 percent of the baseline unconfined compressive strength.
Axial stress and axial strain data were analyzed to determine the energy dissipated due to permanent deformation. Following fatigue loading, unconfined compressive strength and the elastic properties; Young's modulus and Poisson's ratio were measured. The cement strength, elastic properties, and dissipated energy were analyzed in reference to the number of fatigue cycles. The elastic properties determined during fatigue testing were applied to an analytical model which calculates cement stress in a multi-layer wellbore system. Results show that cement elastic properties can evolve under fatigue loading and lead to otherwise un-anticipated cement sheath failure.