Severe Downhole Frictional Heating Driving Increase in Drillstring Failures

摘要

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.