BHA Lateral Vibrations: Case Studies and Evaluation of Important Parameters

Abstract

SPE Member Abstract Recent efforts to compute and measure drillstring and bottom hole assembly vibrations have shown that vibrations occur in the BHA which do not coincide with computed natural frequencies for axial and torsional vibrations. Analysis of BHA failures has indicated that lateral vibrations in the BHA was the probable cause of some failures. This paper presents a collection of bottom hole assembly failures. The possibility of lateral vibrations being the cause of a failure has been evaluated by considering circumstances surrounding the failure and making comparisons to a lateral vibration computer model. Similarities between lateral vibration failures are analyzed to determine what parameters are important in identifying the susceptibility of a bottom hole assembly to large lateral vibrations. The parameters include depth, hole size, ratio of hole size to drill collar diameter, and stabilizer placement. The results of this paper allow the drilling engineer to avoid BHA designs which have been shown to be susceptible to lateral vibration failures and identify designs which should be evaluated in more detail such as with a lateral vibration analysis. Introduction Excessive vibrations of drillpipe and bottom hole assemblies can cause fatigue failures, excessive wear, washouts, and in extreme cases can result in destruction of drillstring components. Identification of operating speeds that may result in destructive longtitudinal harmonic vibrations has largely been accomplished with analyses similar to those of Dareing and Livesay. Speeds that may result in destructive lateral vibrations have been calculated with equations 9.11 and 9.12 of API RP 7G. Although these methods for calculating critical speeds may be adequate for most wells, recent studies have shown that more accurate models are necessary to determine the critical speeds of a bottom hole assembly. These complex models currently require significant computer resources and are therefore not used on a day to day basis by drilling engineers. It would be beneficial to be able to determine, based on operating parameters, whether a particular BHA is likely to be susceptible to destructive lateral vibrations. The drilling engineer could then evaluate the necessity of using the more advanced BHA vibration models to design the BHA or select operating speeds. To investigate parameters that may be common to lateral vibration BHA failures, a number of bottom hole assemblies, which have had numerous failures attributed to lateral vibrations, have been collected and compared. Description of Field Data Data on thirteen bottom hole assemblies which have experienced failures attributed to lateral vibrations are presented in Table 1. The presented data include hole size, depth of the failure, weight on bit, operating speed, mud weight, number of stabilizers and their placement, and drill collar sizes. These assemblies come from eight different wells and four operating companies. Although details such as well location and formation remain confidential, all of the BHA's presented were used in drilling hard formations or alternating hard and soft layered formations with a tricone bit. In each case, the BHA suffered numerous failures (washouts, twist-offs, or excessive wear) and the operator concluded from postfailure inspections that lateral vibration of the BHA was a likely cause of the failures. In most of the cases, the computer model presented in [3] and [4] confirmed the presence of destructive lateral vibrations and the absence of destructive longtitudinal or torsional vibrations. P. 531^