The major drawback to using bi-center bit technologies has always been the detrimental downhole vibrational effects. The excessive transverse energy generated and translated to the drill string by these bits, often caused more problems than the bits prevented.
Analysis of downhole mechanical data showed bit whirl as a major culprit in downhole vibration generation. Distinct changes in vibration intensities are seen when comparing conventional PDC and the DPI Bi-Center bits.
Numerous past examples of bottomhole assemblies coming out of the hole in pieces - and most unfortunately, pieces left in the hole - have been well documented.
Increased focus on engineering changes to these bits was initiated in past years to offset these detrimental effects and offer a better alternative for the drilling industry. One of the changes was an improved bi-center bit design from Diamond Products International (DPI). The SpeedReamertrademark bi-center drill bit design offers improved force balancing and has been shown to decrease the high torque and vibrational tendencies seen in the past.
One main advantage of a bi-center bit is the ability to provide a 15-20% increase in hole size over the "pass thru" or internal drift diameter of the casing. This is highly applicable in through-tubing drilling (TTD) operations. Bi-center drill bits are designed to drill and under-ream in one continuous operation making the new designs capable of directionally drilling from kick-off to the next well plan objective.
Steering in re-entry wells
Two successful slimhole, re-entry applications using bi-center drill bits required high build rates and precise directional control:
- The first well used a 6 in. by 7 in. 4 3/4 in. bi-center drill bit, drilling 493 ft to TD of the build-up section. The bit run included kicking off cement through the existing 7-in. casing with a steering tool and a 4 3/4-in., 4/5 lobe, positive displacement motor (PDM). The 3-degree bend motor built 8-12°/100 ft for 165 ft. After a trip to pick up a 1.56° bent sub, the assembly drilled another 328 ft at 15-18°/100 ft. The bit was pulled and 5-in. casing successfully run through the curve.
- The second well required running 4-in. casing to contain excessive water production prior to drilling the production zone. A 4 3/4 in. by 5 5/8-in. by 3 3/8 in. bi-center bit was used to drill below the existing 5-in. casing from 6,890 ft measured depth (MD) to 7,645 ft. The bit and steerable bottom hole assembly drilled 755 ft in 87 hours. The hole angle was successfully built from 2.5° to 77° at total depth. Both runs were milestones, marking the first time a bi-center has been used to successfully build a high angle wellbore in this area of northeast Texas. "These SpeedReamer bits demonstrated predictable reactive torque and the motor/steering tool with bit performed well, explained Pedco's Bret Cook. "There was no more torque or vibration than conventional bits under the same circumstances. Now, we are able to drill a larger hole and get production casing through the curve to TD."
PDC bit performance
Pilot bit center is in middle axis of the hole while drilling, but offset from middle of the hole while tripping. A diameter increase of 15-20% can be realized in some tool sizes.
Extended reach wells and more aggressive, multi-target well planning in recent years are increasing focus on fundamental drilling mechanics. Downhole measurements such as torque, weight, longitudinal and axial shock are now regularly used to interpret drilling efficiency and predict bit and bottom hole assembly fatigue.
Measurement-while-drilling (MWD) tools run above the SpeedReamertrademark have recorded limited vibrational effects and lower downhole torques than conventional PDC bits in smaller hole sizes. A recent example on the North Slope of Alaska featured a Sperry Sun Drilling Services 3 1/8-in. Drill-string Dynamics Sensor (DDStrademark), and a 4 1/8-in. bi-centered bit from DPI, used in a through-tubing drilling (TTD) application in glacial till and rubble zone lithologies.
In the TTD tight environment, vibration levels are typically severe, and the source poorly understood. DDS data from the wells has since been analyzed and a major source of drill string vibrational excitation identified as bit whirl. Bit whirl is caused by the instantaneous center of rotation moving around the face of the bit, increasing the lateral oscillation of the drillstring. This results in cutter damage and reduced bit performance, as well as bottom hole assembly and MWD tool deterioration.
The 4 1/8-in. bi-centered bit was used on 3 wells, accumulating 5,900 total ft drilled and 473 circulating hours. Minimal vibration was measured over the course of all bi-center bit runs with average and peak (x, y, and z) values below minimum expected levels.
Note the drilling sequence illustrated in the depth log, where a 4 1/8-in. bi-centered bit was changed out for a 3 3/4-in. PDC bit. The PDC bit was used for two bit runs with excessive downhole vibrations observed in both. The transverse vibrational energies observed in the X and Y axes in these two bit runs average an increase in intensity of 800%. This trend continues even after drilling through the rubble zone. The pronounced decrease in transverse vibrations decreases only after again picking up a bi-center bit.
The rubble zones, indicative of the region's geology, are not conducive to conventional PDC bit use, with decreased rates of penetration, poor hole cleaning, borehole spiraling, and catastrophic MWD failures resulting. The final run of the sequence used the bi-centered bit used in the initial run. Drill-string vibration was noticeably reduced and the drilling assembly finished the well without any further problems.
These examples demonstrate the ability of these new-design bi-center bits to drill in aggressive build sections and inconsistent geologic strata. Flexibility in design and manufacturing processes yields tremendous "fit-for-purpose" applications. This should prove beneficial in overcoming past perceptions and building confidence in newer versions of this technology.
The author would like to acknowledge assistance from R. Jackson and T. Flynn of Sperry Sun Drilling Services, B. Cook of the Petroleum Development Corporation (Pedco), and Arco Alaska.