Mitigating Vibration in Bi-center Drilling Programs

Abstract

Despite extensive investigation by the drilling and research community, vibration-related bottom-hole-assembly (BHA) failures are still common today. There are many theoretical forms of vibration that have been described in industry literature. Having to address multiple forms simultaneously, vibration mitigation procedures are often overly complex and ineffective. An extensive study of high-resolution vibration data was undertaken to identify primary forms of destructive motion. Consistencies in BHA behavior were identified from this study and revealed that specific BHA's regularly demonstrate fundamental vibration tendencies inspecific environments. As a result, a more focused procedure for corrective action has been adapted and implemented. This procedure has been proven effective without significant negative economic impact to the drilling operation. This paper discusses the findings of the study. More specifically it addresses the destructive motion of bi-center assemblies. Data from two field examples will be highlighted for practical application. Introduction Reaming-while-drilling (RWD) with bi-center bits has proven challenging with regard to avoiding vibration-related inefficiencies. BHA component failures are common despite industry's understanding of the problem. There are two characteristics that are commonly viewed as being responsible for vibration problems with bi-center assemblies. Due to the asymmetrical geometry of a bi-center bit, imbalances in force and mass have been viewed to be one of the major challenges with this technology. Secondly, due to minimum pass-through requirements, these assemblies are virtually always run with significantly under-gauge stabilizers. This text will focus on the ramifications of the latter. Under-gauge stabilizers will allow for a certain degree of lateral movement of the BHA within the constraints of an enlarged wellbore. With the large number of mechanical failures of BHA components still common today, it seems safe to assume that whatever the root cause of the unwanted lateral motion, the BHA ultimately succumbs to oscillations of lateral acceleration. The situation must only be exacerbated by the freedom these BHA's have to displace laterally. When LWD equipment is run in the BHA, it is common practice to record down-hole accelerations to a limited degree. Most commonly, these measurements record only lateral and axial accelerations or shocks over various sample periods. Additionally, some form of torsional acceleration measurement is recorded via magnetometers used to directly measure down-hole revolutions per minute (RPM), or with accelerometers used to measure tangential and radial accelerations. Conventionally, destructive BHA vibrations have been identified using short sample periods. However, even though many of these measurements are sampled as often as every one second, they do not commonly provide a mechanism for the measurement of the frequencies of motion. A company may employ mechanisms to measure frequencies and even diagnose them "down-hole" but do not provide for the recording of high resolution frequency data.1 At least one company has the capability to take high resolution vibration measurements and provide comprehensive frequency analysis3. In turn, this data has been used to identify the occurrence of specific forms of vibration in laboratory experiments and field cases.2,4,8