Surface mud logging systems have detected a severe form of torsional vibration that causes rapid destruction of polycrystalline diamond compact (PDC) drill bits. These failures can occur when combining a limber bottomhole assembly (BHA) with a PDC bit lacking some form of anti-whirl stabilization. On the basis of experience to date, these vibration-induced failures occur only in larger hole sizes. This paper describes the nature of these bit failures, their dependence on operating conditions, and practical recommendations for avoiding failure. Experience from one operation, in which revised drilling practices and bit selection criteria eradicated the failures, will be presented. Introduction The nature of various vibration modes, their damaging effects on downhole equipment, and the measurement and suppression technology developed to combat vibration, is well-documented in the literature.1,2,3,4,5 In drilling operations, one of the more common modes is torsional vibration, which is recognized by surface torque fluctuations with a frequency close to the fundamental torsional mode of the drillstring. The frequency of torque oscillations under such conditions is given by: f=12πkJb(1) Fig. 1, which was taken from an actual drilling operation, is an example of this frequency. These data, and all other vibration data described here, were acquired with a previously described detection system.5 In its extreme, torsional vibration can advance to the point that the bit and bottomhole assembly come to a complete stop-start (stick-slip) motion.6 Various characteristics in surface torque behavior can detect this extreme condition. First, the frequency of surface torque oscillations will decrease below the fundamental frequency, thereby reflecting the sticking time downhole. Second, peak surface torque will increase to a value approximately equal to the static torque required to break the bit/BHA free, plus the torque required to rotate the drillstring. In fact, if a drop in surface rotary speed accompanies this rise in torque, which is typical for topside rig equipment, the speed minima concurring with the torque maxima creates an inertial force opposite to string rotation. This adds another component to peak surface torque. Finally, the effect of inertia will also reduce surface torque below the level that is required to rotate the drillstring as the bit and BHA break free downhole. Fig. 2 is an example of this fully developed ‘slip-stick’ behavior. Of particular significance is the frequency of the torque oscillations compared to the calculated fundamental frequency. The rise in torque is linear to the observed maxima, reflecting the deceleration of the BHA during the stick phase. The subsequent torque drop suggests a rapid acceleration of the BHA during the slip phase. The purpose of this paper is to report the operator's experience with this form of slip-stick vibration in one major drilling program. The circumstances under which it occured, its effect on drilling performance/economics, and measures that have been developed to control it, will be described. In addition, the authors will use detailed analyses of surface drilling parameter and other operational data, and simulations using a finite element model of the drillstring to interpret the downhole behavior that provokes slip-stick.

Excessive torque and drag can be critical limitations in extendedreach drilling (ERD). This paper details issues related to torque-anddrag prediction, monitoring, and management in ERD wells. Results are presented from sensitivity analyses of extreme ERD trajectories such as 7-to 8-km departures at 1600 m true vertical depth (TVD). Several such wells have now been successfully drilled at the Wytch Farm oilfield using results from these studies. In such high-angle ERD wells, compression generated in the drillpipe during tripping and sliding operations can exceed the critical buckling load and cause the drillpipe to buckle. As a result, buckling initiation and post-buckling analyses are used to quantify the extent and severity of buckling and the associated increases in drag forces and pipe stresses. The paper addresses the importance of drilling data in calibrating torque/ drag models to capture the continual changes in drilling parameters and operating conditions. The paper presents a number of field case studies where analyses have been conducted to directly assist drilling operations. This paper should be of high interest to engineers executing, planning, or evaluating ERD operations. General TorqueĆandĆDrag Considerations for ERDFrictional and Mechanical Torques. The theory behind the "softstring" model for basic torque/drag prediction is well-known in the industry. 3 Proper application of the model requires a full understanding of the factors influencing torque and drag in the field. Total-surface torque is comprised of frictional string torque, bit torque, mechanical torques, and dynamic torques. Separating these compo-

Excessive torque and drag can be critical limitations in Extended-Reach Drilling (ERD). This paper details issues related to torque and drag prediction, monitoring, and management in ERD wells. Results are presented from sensitivity analyses of extreme ERD trajectories such as 7 to 8 km departures at 1600 m TVD. Several such wells have now been successfully drilled at BP's Wytch Farm oil-field using results from these studies. In such high-angle ERD wells compression generated in the drillpipe during tripping and sliding operations can exceed the critical buckling load and cause the drillpipe to buckle. As a result, buckling initiation and post-buckling analyses are used to quantify the extent and severity of buckling and the associated increases in drag forces and pipe stresses. The paper addresses the importance of drilling data in calibrating torque/drag models in order to capture the continual changes in drilling parameters and operating conditions. The paper presents a number of field case studies where analyses have been conducted to directly assist drilling operations. This paper should be of high interest to en~ineers executing, planning, or evaluating ERD operations.General Torque and Drag Considerations for ERD Frictional and Mechanical Torques. The theory behind the "soft-string" model for basic torque/drag prediction is well known in the industry [3]. Proper application of the model requires a full understanding of the factors influencing torque and drag in the field. Total surface torque is comprised of frictional string torque, bit torque, mechanical torques, and dynamic torques. Separating these components allows more accurate definition of friction for torque projections and allows proper prioritization for torque reduction measures. Frictional torque is generated by contact loads between the drillstring and casing or open-hole. The magnitude of contact loads is determined by drillstring tension/compression, dogleg severities, DP and hole size, drillstring weight, and inclination. Profile optimisation and tortuosity control are therefore im~ortant measures to minimize contact loads. Lubricity is a maJor factor controlling friction, and is itself largely controlled by mud and formation types. With means of predicting bit torque, the implications of using different bit types can be assess~d. Dynamic torques can also significantly impact operatiOns and should be minimized [4]. Mechanical torque sources, such as cutting beds, borehole ledges, and stabilizer effects can be very significant and must also be minimized.

Vibration detection from mud logging systems has revealed that torsional vibration is common in harsh drilling environments, and is a major cause of bit and drillstring failures. Suppressing this type of vibration with an automated vibration detection system, torque feedback, and rigsite vibration suppression guidelines has produced a significant improvement in drilling performance.

SPE Members Abstract This paper introduces the novel concept of a "flexible" polycrystalline diamond compact (PDC) bit, and its capability to reduce detrimental vibration associated with drag bits. The tilt flexibility, introduced at the bit, decouples the dynamic motion of the bottom hole assembly (BHA) from that of the bit, thus providing a dynamically more stable bit. The paper describes the details of a prototype 8-1/2" flexible bit design together with laboratory experiments and field tests which verify the concept. Introduction Vibration is the main cause of premature PDC bit failure. The bit-formation and drillstring-wellbore interactions are the two major sources of downhole vibration. The response of the bit and drillstring to these excitation sources is very complex. In particular two main mechanisms of damaging downhole vibrations, namely slip-stick and bit whirl have been attributed to the cutting action of PDC bits. The complexity of downhole vibration is largely due to the coupling which inherently exists between the bit motion and that of the string. The bit is rigidly connected to the bottom hole assembly and hence any movement of the bit directly influences the response of the string and vice versa Providing a degree of flexibility at the bit-BHA connection would therefore reduce the dynamic interaction between the bit and rest of the string. This in turn would tend to reduce the severity of vibrations. The paper introduces the concept of "flexible bit". The flexible bit is attached to the BHA via a connection sub rigid in axial and torsional motions but allows flexibility in bending or tilt motion. Details of a development programme involving laboratory experiments and field tests are described which demonstrate the potential benefit of the flexible bit concept in reducing detrimental vibrations associated with PDC bits. BIT-BHA DYNAMIC COUPLING Bit and Downhole Dynamics Downhole vibration can induce severe torsional, lateral, w0d axial dynamic motions at the bit and in the drillstring This occurs in terms of particular vibration events or mechanisms such as slip-stick, bit/BHA whirl, bit bounce and BHA resonance. These vibration events can be originated as a result of frictional and reactive forces generated at the bit, stabilisers and elsewhere along the string. Due to the rigid connection between the bit and the BHA, vibration events originated in the BHA can influence the dynamic motion of the bit and vice versa. As a consequence of this dynamic coupling, a given vibration mechanism, which involves the bit, can trigger one involving the BHA. For example severe bit slip-stick torsional vibrations have been observed to cause BHA lateral instability which can in turn trigger whirl as a result of increased BHA/wellbore interaction. Conversely, BHA whirl can induce lateral bit instability. Fig. 1, adapted from ref. 7, schematically illustrates the interaction between various vibration mechanisms. P. 263