Drilling Tests of an Active Vibration Damper

Abstract

Abstract Drillstring vibration is a serious problem, particularly in deep and hard rock drilling; it can reduce the rate of penetration (ROP), shorten bit life, and damage expensive downhole components. Testing of an active drilling vibration damper (AVD) system at TerraTek Laboratory, under conditions designed to induce vibration, demonstrated that the use of the AVD reduced vibration, maintained more consistent weight-on-bit (WOB) and increased ROP. The AVD has a structure similar to that of a shock sub with the shock absorber filled with magnetorheological fluid (MRF), rather than hydraulic oil. Under the influence of a magnetic field, MRF instantaneously increases its viscosity. Using a series of coils to induce intense electromagnetic fields across the fluid gap, the damping coefficient can be changed in milliseconds by a factor of 7–10. A linear motion detector provides feedback to control the AVD in response to bit motion.1 In these tests, the AVD was used behind a tricone bit to drill through blocks of hard concrete, each of which had a 12" granite slab mounted within it at a 10º angle. By inducing an asymmetric load on the bit, the interfaces produced severe vibration during drilling the control holes. A total of 28 holes were drilled, including 11 control holes, at varying WOB and rotation rates. Analysis of the data confirmed the anecdotal observations made during drilling. The vibrations at the bit were reduced significantly; the variation of the measured WOB was significantly curtailed, and the ROP was increased. These tests demonstrated that the AVD is likely to provide significant time and cost savings, particularly in deep wells. These will arise, not only from the increased instantaneous ROP, but also from fewer trips for bit or equipment changes, and lower costs for replacing damaged MWD tools, motors or other expensive components. Introduction In deep drilling, ROP is a major determinant of the ultimate cost of the well, and vibration is the enemy of ROP. When the bit vibrates, it is not drilling at maximum efficiency, and sometimes not at all. Bit bounce can reduce bit life and the resultant drillstring vibration can increase the number of trips required as a result of premature failure of MWD tools, motors and other components. Reduced instantaneous ROP and more frequent trips can harm the financial viability of deep drilling projects, even at today's oil prices. While shock sub can help reduce these problems, on occasion, they can resonate and make them worse2. A typical, massive drillstring (See Figure 1) has a relatively low resonance frequency. At rotation rates below those that would excite it, the bit maintains contact with the well bottom, but may not be drilling optimally Above the resonance, vibration is reduced because the drillstring cannot react to irregularities in the well bottom, and essentially skips over them, drilling only some of the time. These effects limit the range of WOB/RPM combinations that can be run, and frequently exclude the most desirable from a bit performance point of view. The addition of the AVD has two effects on the drillstring response, as shown in Figure 2 This combination allows the choice of WOB and RPM to be optimized for the bit and formation, without consideration of drillstring resonance effects.