The bi-center bit shown in this photograph uses 19 mm polycrystalline diamond compact cutters with 48 face cutters, 7 gauge cutters, 9 gauge trimmers, 6 blades, and 6 nozzles on the pilot bit. The reamer has 38 face cutters, 8 gauge cutters, 8 trimmers, 4 long and 2 short blades, and 4 nozzles. Its overall length is 41.92 in., and it weighs 1,400 lb (Fig. 1).
Bi-center bits used to drill and underream in one continuous operation can now provide steerability in large hole diameters during directional surface drilling operations.
The bi-center bit shown in Fig. 1 is currently the world's largest. It is 16 in. x 20 in. x 23 in. and weighs 1,400 lb. This bit drilled the conductor-hole sections for four wells in Malaysia, replacing a typical 171/2-in. (pilot bit) and 24-in. (underreamer) tandem assembly.
The tandem assembly failed to provide directional control with required build rates of 3°/100 ft in such a large hole size, so Diamond Products International designed and manufactured the bit to overcome directional problems while ensuring successful casing and cement jobs.
The bit features force balancing between the pilot bit and reamer section so that a more equal distribution of cutter vs. well bore contact would reduce sideloading. In addition, the bi-center bit incorporates stabilizing elements such as dome-shape cutters and cutters with 45° edge chamfers that provide low-torque response.
The split-blade reamer design produces a stable base for fast, smooth drilling. Advantages include:
- The ability to drill a larger hole than the set casing normally allows (Fig. 2 [40,918 bytes])
- Directional control from vertical to horizontal
- Penetration rates comparable to standard polycrystalline diamond compact bits
- Reduced drilling costs
- Improved cement jobs
- Ability to run one size larger diameter casing
- Easier casing runs through curve sections
- Larger well-bore diameter for multilaterals
- Reduced risk of pipe sticking in high-angle wells, swelling formations, flowing formations, and depleted sands
- No risk of mechanical underreamer parts lost downhole.
The bi-center bit incorporates reverse bullets on all full blades of the reamer section to maintain hole-size diameter integrity.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.
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