Case Study: Drillstring Failure Analysis and New Deep-Well Guidelines Lead to Success

Three drillstring fatigue failures Occurred while drilling two deep wells below 16,500 ft true-vertical depth (TVD) in the US mid-continent region. All the failures Occurred across 2 degrees/100 ft- to 3 degrees/100 ft-dogleg severity (DLS) intervals from 6,000 to 8,000 ft., g The well conditions (i.e., pipe condition and directional plan) were not significantly different from other deep wells in the area, which had not failed. A deep-well drillstring failure study was conducted, which included a review of drillstring-inspection reports, daily drilling reports, digital data, technical literature, and engineering analysis for the two wells. A Cumulative fatigue analysis (CFA) modeling technique taking into account specific well conditions [i.e., wellbore geometry, rotary speed, rate of penetration (ROP), hook load, and drillstring configuration] was applied. The model indicated that drillstring failures would Occur across shallow doglegs mainly because of high hang-down loads combined with slow ROP. The results of the Study led to the development of new deep-well design criteria and implementation of new drilling guidelines. The new guidelines included the use of look-ahead CFA modeling when approaching drillstring endurance limits to minimize drillpipe-fatigue failures. Look-ahead CFA modeling and the new drilling guidelines were used on two Subsequent deep wells in the area, leading to successful drilling to total depth (TD) of 18,000 ft TVD without failure. One of the wells had a 1.4 degrees/100-ft DLS (calculated based on 100-ft survey spacing) at 1,500 ft and drillpipe shuffling Was required to prevent drillstring failure in the deep-hole section. The drillstring-fatigue failure prevention guidelines apply to deep wells drilled worldwide.