Catastrophic Drill String Failures Caused by Downhole Friction Heating -An Increasing Trend

Abstract

Abstract Drill string failures caused by friction heating of bottom hole assembly (BHA) components and drillpipe have increased dramatically over the last several years. Although drilling engineers are familiar with heat checking caused by downhole heating due to borehole friction, catastrophic overheating failures were rarely experienced prior to the last several years. The consequences of severe downhole heating can be dire often resulting axial separation of the drill string creating potential well control safety issues, costly fishing jobs and other remedial efforts. In one failure mode, the drill pipe is heated above a critical transformation temperature accompanied by a rapid decrease in tensile strength. Subsequently, the component fails under a tension loading, well below the rated strength of the drill string. Recently, another failure mode of heavyweight drill pipe has been documented on three different wells where the pipe parted in a purely brittle mode. These fractures occurred as direct consequence of the steel being heated above its critical temperature, followed by rapid cooling (quenching) by the drilling fluids resulting in a very brittle, low toughness steel. The fracture surfaces that occur from this failure type often cause confusion during failure investigation due to the presence of flat fracture surfaces which are rarely seen in drill pipe and BHA components. Due to increasingly harsh drilling conditions it is likely these types of failures will become more common. This paper addresses the features of a downhole heating failure including the material attributes that can be used to identify the phenomena. Guidelines and operating practices that can be employed to minimize the occurrence of these costly and potentially dangerous failures are discussed. Several case histories are presented that illustrate these issues and provide lessons learned to improve future performance. Metallurgical data taken from failed specimens will be presented which demonstrate the sequence of events leading to these failures. Introduction The last few years have seen a dramatic increase in extreme friction heating induced failures of oilfield drill string components. Although surface friction heating damage in the form of heat check cracking has been known to occur since the late 1940's1, extreme friction heating failures due to the steel being heated above its critical temperature of 1,300–1,500 °F are now becoming more frequent. It can be argued that the leading contributor to the recent rise in occurrences of these catastrophic failures is the increasing use of top drives that possess higher (revolutions per minute) RPM and torque capacities than kelly drives. As such, it possible to drill deeper, more highly deviated and extended reach horizontal wells with top drives that could not otherwise be drilled with a kelly. This increase in drilling capability has resulted in greater contact pressures between the drill string and the wellbore at higher RPM. Overheating of drill string materials in contact with the wellbore can be particularly acute if there is no vertical movement of the drill string causing the drill string to rotate up against the wellbore in the same position. In addition, because drilling with complete treble stands of pipe is possible with top drives, conditioning the hole and working tight spots can be done more efficiently. However, when these operations are carried out, the bit is off bottom, and the entire weight of the drill string is brought to bear resulting in higher lateral forces between the drill string and the wellbore, especially when wellbore curvature exists high up in the well. Greater contact forces translate into higher friction, and if the drill string is not continually moved vertically up and down, overheating will occur more readily and with greater frequency. The same phenomena occur when trying to free pipe that is stuck, but can still be rotated. Continued rotation while pulling against keyseats will generate greater side loading and commensurate amounts of heat to the drill string that is in contact with the wellbore.