Methodology Prevents Drillstring Fatigue and Failure in Deep Well With Large Shallow Dogleg

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 23191, “Methodology for Preventing Drillstring Fatigue and Failure in a Deep Well With Large Shallow Dogleg,” by Jiawei Zhang, Hongchun Huang, and Lingzhan Zou, CNPC, et al. The paper has not been peer reviewed. Copyright 2024 International Petroleum Technology Conference. _ Fatigue-induced failure is recognized as the primary cause of tubular twistoff. Repeated cyclic bending loads and stress caused by dogleg severity, pipe weight, and buckled drillpipes can result in drillstring fatigue and failure. The complete paper deals with the incidence of seven twistoff accidents in large shallow-dogleg sections in three deep wells with a true vertical depth (TVD) of greater than 5000 m. The authors propose a methodology for preventing drillstring fatigue and failure in deep wells with large shallow doglegs. Introduction Three deep exploration wells with TVD of greater than 5000 m have suffered from repeated (two or more) twistoff incidents in large shallow-dogleg sections while drilling. All drillstrings used for exploration wells were newly manufactured and evaluated to be flawless by ultrasonic testing and magnetic particle inspection before entering the wellbore. The cumulative working time of this patch of drillstring was expected to be between 900 and 1,000 hours before maintenance; however, total working hours for each failure case were far from the preset limit. Enormous time and financial loss were incurred during fishing jobs after these drillstring failures. Moreover, secondary downhole accidents, including stuck pipe, also were encountered during fishing jobs. Well B was sidetracked at approximately 5900 m after 11 unsuccessful fishing jobs and large, deep doglegs were created during sidetracking, which greatly affect wellbore integrity and increase the difficulty for later well-completion operations. Under such circumstances, plugback and redrilling with better trajectory-dogleg control may be an alternative, but this solution generally comes with huge financial cost. Prevention of drillstring failure is crucial for successfully reaching target depth if drilling is to be continued. The relative fatigue index-prediction model, used to calculate the cumulative fatigue distribution of the entire drillstring in drilled or predrill intervals, is established by considering drillstring-stress characteristics and crack-propagation behavior. A section of the complete paper is devoted to a discussion of principles behind these considerations, including related equations. Procedures and Workflows Drillstring-Fatigue and -Failure Prevention. In general, deep or ultradeep wells have wellbore structure designs of three or more casing layers. The drillstring may be reused in different well spuds or sections in actual drilling operations. Therefore, the actual usage record of the drillstring shall be considered when calculating the cumulative fatigue index of the drillstring in predrill intervals.