The effects of downhole vibrations on polycrystalline diamond compact (PDC) bit performance have been well discussed in recent years. However, most research on the downhole dynamics of roller cone bits has focused on axial vibration and its effects on bit performance. The questions which remain to be answered are: What kind of vibrations does a roller cone bit experience, and what are the effects of these vibrations on roller cone bit performance? To investigate these questions, a series of field tests was conducted in a full scale drilling research rig, with downhole vibration data recorded by an advanced instrumented sub. Soft to medium hard formations were drilled using four 81/2-in. roller cone bits (IADC Code 517. Three kinds of harmful vibrations were observed, and their effects on bit performance in terms of rate of penetration (ROP), insert breakage, and bearing/seal and bit life are discussed.

Summary The effects of downhole vibrations on the performance of polycrystalline diamond compact (PDC) bits have been well discussed in recent years. However, most studies on the downhole dynamics of roller-cone bits have focused on axial vibration and its effects. The questions that remain to be answered are what kinds of vibrations a roller-cone bit experiences and what the effects of these vibrations are on roller-cone bit performance. To investigate these questions, a series of laboratory and field tests was conducted. Downhole vibration data were recorded by an advanced instrumented sub. Soft- to medium-hard formations were drilled with four 8 1/2-in. roller-cone bits (IADC 517). Three kinds of harmful vibrations were observed, and their effects on bit performance in terms of rate of penetration, insert breakage, and bearing/seal life are discussed. Introduction Based on their directions, downhole drilling vibrations may be divided into three types - axial, torsional, and lateral. Furthermore, two specific types of vibrations are often observed during drilling - stick-slip and whirl. Stick-slip vibration is a specific kind of torsional vibration. By definition, bit stick-slip vibration involves periodic fluctuation in bit rotational speed, ranging from near zero to a high value, often more than twice the rotational speed measured at the surface. By contrast, other kinds of torsional vibrations occur in which bit rotational speed may be near-constant. Similarly, bit whirl (backward or forward) is a specific kind of lateral vibration. Similar to the case of stick-slip vibration, bit whirl is not the only form of bit lateral vibration. That is, a bit may vibrate laterally but may not necessarily be whirling. Only when the bit's geometric center moves around the hole's center line (either forward or backward) is the bit truly whirling. In addition, in most cases, bit whirl is coupled with other forms of lateral vibration. Therefore, the trajectory of the bit center is usually not a circle. For PDC bits, it has been found that these types of vibrations - stick-slip and whirl - have a significant negative effect on several aspects of bit performance, including rate of penetration (ROP) and bit life. It has been also found that the cutting structure plays a major role in the onset of backward whirl. The antiwhirl and track-lock design concepts of PDC bits1,2 are two examples of how the onset of bit whirl can be successfully discouraged through the cutting structure's design. To date, most research in the field of roller-cone bit downhole dynamics has focused on axial vibrations, or "bit bounce."3,4 The stick-slip and backward-whirl vibrations of roller-cone bits were observed only in recent years and mainly in laboratory tests.5,6 Issues like the effect of stick-slip and whirl vibration on bit performance, the influence of the bit cutting structure on bit vibration, and the effectiveness of downhole tools to suppress vibrations are still unclear and need to be studied further. Test Conditions To investigate how roller-cone bits vibrate during drilling and what the effects are on bit performance, a series of laboratory and field tests was designed. The field tests were conducted in a full scale research test rig at the Advanced Drilling Intl. Facility located in Montrose, Scotland. Four different 8 1/2-in. roller-cone bits were each tested with 12 different combinations of weight on bit (WOB) and rotational speed. A comparison of the four bits is shown in Table 1. In the field tests, drilling with rotary assemblies began at a depth of 2,000 ft. The formation is homogeneous consolidated sandstone made of volcanic detritus minerals. This is a soft to medium formation. The inclination angle of the well was approximately 20°. Surface data were monitored with conventional techniques. Downhole vibrations were recorded by an advanced, instrumented dynamic sub that uses accelerometers, strain gauges, and contact sensors to measure axial, torsional, and lateral vibrations, as well as instantaneous bit rotational speed, bit center trajectory, and actual WOB and torque on bit (TOB). A detailed description of the instrumentation may be found in Ref. 7. Stick-Slip Vibration of Roller-Cone Bits In Fig. 1, the downhole instantaneous rotational speed of Bit A is charted vs. time for a portion of its run and demonstrates a typical case of stick-slip vibration. Bit A is a conventional design. The surface rotational speed of 60 rpm is approximately equal to the mean value of the downhole rotational speed (63.9 rpm), confirming the reliability of the measurement. The maximum speed during stick-slip is 146 rpm, more than twice the indicated surface speed. The minimum speed is zero. Fig. 2 shows the spectrum of bit rotational speed. The stick-slip frequency is approximately 0.7 Hz. This frequency is lower than but close to the first torsional natural frequency of the drillstring calculated with a dynamic stiffness method.8 The numerical model shows that the stick-slip frequency is independent of the bit rotational speed.9 For a PDC bit run at the same rig, it was found that the frequency of stick-slip, once initiated, was also approximately 0.7 Hz.7 These facts indicate that, once initiated, stick-slip frequency is dependent on the bottomhole assembly (BHA) rather than bit type or bit-formation interaction. However, bit-formation interaction may play some role in the initiation of bit stick-slip vibration. It is interesting to note that even when the effective duration of bit rotation is reduced to approximately 70% of the total drilling time, the penetration rate of the bit in stick-slip is not reduced. In other words, the stick-slip vibration does not adversely affect the ROP. This phenomenon was also observed in laboratory and field tests for PDC bits.5,10 An analysis of WOB and TOB during bit stick-slip is shown in Figs. 3 and 4. There is considerably less variation in both WOB and TOB during the "stick" period than during the "slip" period. This is because during the "stick" period, the bit stops drilling while the WOB and TOB are still applied. During the "slip" period, the bit rotational speed changes from 0 to 146 rpm, WOB increases from 8,000 to approximately 21,000 lb, and TOB changes from 200 to 800 lb-ft. Such significant fluctuations of both WOB and TOB may be the major cause of insert breakage and may be partly responsible for premature bearing/seal failure.

Downhole vibrations is still a problem to solve because models have failed to be predictive and surface measurements do not reproduce downhole phenomena precisely. The paper describes the instrumentation of PDC bits with accelerometers, strain gauges and contact sensors. From these sensors it has been possible to produce direct measurements in lime domain and frequency domain of: instantaneous RPM, accelerations along the three axis, downhole WOB, downhole torque, power dissipated at the bit, eccentricity of hole, bit center trajectory, also indirect measurements of downhole estimated ROP, wear, specific energy. Military type micro electronics of the latest technology have been used to integrate evervthing in the bit shank. With these measurements dynamics can be analyzed : stick slip and whirl are clearly identified. Lab tests have been performed with promising results. Three campaigns have been done in the field to confirm the lab results with different bit profiles in France and UK. Some of the recorded results compare well to the results of existing BHA models. The paper discusses these results and outlines the future developments. P. 457

No abstract

A corehead was designed, manufactured and tested to reduce fluid invasion of the core. This is obtained by minimizing the exposure time of the core to the drilling fluid in increasing the rate of penetration (ROP). The design incorporates a medium heavyset polycrystalline diamond compact (PDC) cutting structure developed in accordance with cutting models and balancing methods used for drill bits. The highest ROP is achieved by a particular hydraulic design: flow ports shape and positioning to clean the cutting structure enhance the drilled cuttings removal while preventing drilling fluid in the throat of the corehead. Moreover, an internal lip works with a special inner barrel shoe to effectively seal off mud flow from the throat. All the design features have been subjected to laboratory tests, including measurement of pressure drop across the corehead and flow visualization studies. Flow visualization tests include high-speed filming of the flow and paint tracing to indicate the special flow pattern. In conjunction with lab tests, a numerical simulation was performed using fluid dynamics software to optimize hydraulic parameters. The low invasion core bit has been used in numerous applications. The performance achieved was significantly better than the average achieved over a period of years using various PDC coreheads. The rate of penetration was increased by a factor of 4.8 and bit life by 2.3 (often with reusable condition).