Mechanical Behavior of Berea Sandstone under Cyclic Loading: An Application to Dynamic Loading of a Wellbore

Abstract

Summary During the drilling process, many operations will cause cyclic loading of the wellbore, including surge/swab pressure generated by tripping operations and pressure fluctuations induced by shutdowns/startups of the mud pumps. With cyclic loading, the wellbore rock may fail even if the stress level is lower than the predetermined static rock strength because of fatigue. In this paper, comprehensive triaxial cyclic compression tests have been conducted under different maximum cyclic loading levels, confining pressure, amplitude, and frequency. New methods have been proposed and successfully applied to predict the monotonic strength of the rock samples. These approaches overcome the uncertainty caused by the standard average method. Using these experimental results, some empirical models (amplitude and fatigue life, frequency, and fatigue life) are proposed. Also, the evolutional laws of rock parameters (strain, residual strain, energy dissipation, etc.) during the cyclic loading stage are generated. A clear three-stage behavior is observed: early stage, steady stage, and late stage. Finally, the experiments are combined with two field-drilling cases. Results show to avoid rock fatigue because of tripping operations and shutdowns/startups of the mud pump, the maximum loading level should be less than 95% of the static strength for Berea Sandstone used in this study. Therefore, the maximum equivalent loading stress should avoid being close to the failure boundary. A safe buffer zone should be designed between the wellbore collapse boundary and actual bottomhole pressure.