Severe Downhole Frictional Heating Driving Increase in Drillstring Failures
Authors: Jellison Michael J. , Hehn Lucien , Chandler Raymond Brett , Shepard Jeffrey Scott 
Published: December 2007

Abstract
Drill string failures caused by friction heating of bottom hole assembly (BHA) components and drillpipe have risen 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 severe 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 drillpipe is heated above a critical transformation temperature accompanied by a rapid decrease in tensile strength. Subsequently, the component fails under tension loading, well below the axial strength rating of the drillstring. Recently, another failure mode of heavyweight drillpipe (HWDP) has been documented on three different wells where the pipe parted in a purely brittle fashion. These fractures occurred as direct consequence of the steel being heated to very high temperatures (1,300 ºF and above), followed by rapid cooling (quenching) by the drilling fluids resulting in very brittle, low toughness steel. The fracture surfaces that occur from this failure type often cause confusion during failure investigations due to the presence of extremely flat fracture surfaces that are rarely seen in drillpipe and BHA components. Increasingly harsh drilling conditions will likely make these types of failures more common.
This paper discusses the characteristics 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 presented. Several case histories are included that illustrate these issues and provide lessons learned to improve future performance. Metallurgical data taken from failed specimens is presented that demonstrates the sequence of events leading to these failures.

Introduction
As mentioned above, 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's 1, 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 rotational speeds and torque capacities than rotary tables. As such, it possible to drill deeper, more highly deviated and extended reach horizontal wells with top drives than could otherwise be drilled. 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 axial movement of the drill string causing the drill string to rotate up against the wellbore in the same position. In addition, with top drives it is possible to pick the bit up off bottom to condition the hole, work through tight spots and perform other operations that can't be accomplished with a rotary table. However, when these operations are carried out, the bit is off bottom, and the entire weight of the drill string increases tension loading and creates higher side loads between the drill string and the wellbore, especially when wellbore curvature exists in the upper portions of the well.
Greater contact forces translate into higher friction, and if the drill string is not continually moved axially up and down axially, overheating can occur rapidly and with greater frequency. The same phenomena can occur when trying to free pipe that is stuck, but can still be rotated. Continued rotation while pulling against keyseats, ledges or other obstructions can generate high side loads and commensurate heat input to the drillstring that is in contact with the wellbore.