5.1 TRI CONE ROLLER (TCR) BIT
The performance of the TCR bit is dependent on many factors such as selection of proper bit, operating parameters and hydraulics. Consider the following points while using TCR bits.
1. Select the bit on the basis of lithology/formation to be drilled.
2. Select the tooth bit with tungsten carbide inserts on gauge if sand streaks are expected in the formation.
3. Select a tungsten carbide insert bit with minimum or no offset when drilling hard and abrasive lime stone, hard dolomite, chert, pyrite, quartz and basalt etc.
4. Select a bit with long widely spaced teeth where balling tendency of the formation is expected.
5. Check the bit for any physical damage.
6. Record the bit serial No., type, diameter before lowering into the well. 7. Use lifter for handling TCR bit to avoid damage of threads.
8. Use proper bit breaker for tightening & breaking the bit.
9. Do not consider a new TCR bit as oversized on checking with the ring gauge as TCR bits can have positive tolerances.
10. Do not use rotary table for initial tightening of the bit.
11. Ensure free rotation of rollers in case of standard bearing bit. In case of sealed bearing, do not try to make rollers free.
12. Clean reservoir cap equalization ports of sealed bearing bits before running in a hole. 13. Use proper type and size of substitute with bit.
14. Tighten the bit at recommended torque.
15. Lower the bit slowly into ledge, dogleg and at liner hanger top. 16. Reaming with TCR bit should be done with low WOB and low RPM.
17. While approaching well bottom, rotate the bit and circulate mud to clear out any fill and to avoid plugging of the nozzles.
18. Break the bit at bottom with low WOB & slow RPM for 1-2 m to establish bottom hole pattern.
19. Conduct drill off test to ascertain optimum drilling parameters.
20. WOB can be increased up to the recommended value or until the desired ROP is achieved. 21. Gradual breaking of sealed bearing bits will enhance the life of the bit.
22. Use of junk sub above the insert bit will help in avoiding the damage to the bit due to presence of broken inserts and previously left metallic junk in the well.
23. Use of junk sub above last mill tooth bit run prior to drilling with an insert bit is recommended. 24. After pulling out bit, wash the teeth, cones & around nozzles area to grade the bit judiciously. 25. Use 2/3rd rules to find wear of tricone roller bit with a ring gauge.
26. Relate dull bit grading to operating practices, formation characteristics and bit design to improve the next bit selection.
27. Do not try to push the bit through tight spots. 28. Do not load the bit abruptly.
29. Never put excessive WOB and high RPM while reaming. 30. Do not touch well bottom with a sudden jerk/impact.
5.2 POLYCRYSTALLINE DIAMOND COMPACT (PDC) BIT
The performance of the PDC bit is dependent on many factors: Selection of proper bit, operating parameters and hydraulics PDC bit running procedures are quite different from roller cone bits or even natural diamond bits. Consider the following while using PDC bits.
5.2.1 Hole Preparation
1. Check the condition of the pulled out bit for a) Physical damage
b) Cutters condition and c) Gauge.
2. If inserts are found severely damaged and bit is under gauge then clear & condition the hole with roller cone bit and junk sub/junk basket.
3. Prior to running in with a new PDC bit run junk sub with last bit to clear bottom hole for any metallic junk.
5. It is advisable to use TCR bit with junk basket in case of metallic junk or severely under gauged hole is observed prior to running of PDC bit.
6. Drill cement and casing shoe with used TCR bit, then lower PDC bit to drill further.
5.2.2 Making Up PDC Bit
1. Take out PDC bit from its box and place it on a piece of plank carefully and do not roll or place the bit directly on steel surface to avoid damage to cutters /inserts.
2. Check PDC bit for any damage of cutters, inserts and nozzles.
3. Record the bit make, serial No., bit type and diameter before running into the well. 4. Ensure that all nozzles of bit are of required size & properly tightened.
5. Clean bit pin and apply recommended thread dope compound. 6. Use the bit breaker of the same company to which PDC bit belongs.
7. Place the bit with breaker in the rotary table and make up the drill collars string to the specified torque as per API.
5.2.3 Tripping In
1. Care must be taken while running in PDC bit through surface equipment as PDC bit being one piece and is not flexible as roller cone bit.
2. Pass through tight spots very slowly as striking ledges can damage gauge cutters.
3. In case of tight spots, pick up Kelly and run pump at slow rate as compared to drilling. Keep rotary speed at about 60 RPM and proceed slowly through a tight spot with maximum WOB 1.5 -2 T. Avoid high torque while reaming.
4. Avoid PDC bit for reaming long interval in case previous bit was observed significantly under gauged.
5. While tagging bottom with PDC bit, keep on circulating at required pump SPM & RPM to avoid plugging of the bit.
6. Keep constant watch on rotary torque & weight indicator while tagging bottom. If sudden increase in torque is observed at well bottom due to aggressive cutting structure of PDC bits, Pick up the bit 6’’ to 12’’ off bottom. Circulate out the cuttings before resuming drilling.
5.2.4 Breaking- In
1. New PDC bit should be slowly set on well bottom at a slow rotary speed (50-60 RPM) & WOB not more than 2 T to establish bottom hole pattern.
2. If bit does not perform well after few minutes then increase the W. O. B. in steps till it performs well.
3. Increase the WOB to the recommended value to achieve the desired ROP. 4. Optimum WOB necessary for PDC bit is 2/3rd of that required for a roller cone bit.
5.2.5 Drilling Procedure
Rotary torque is effective parameter to know the activity at the bit face at bottom. In soft formations, there may be indications of rotary torque even if the bit is off bottom. Steady torque is observed while drilling through shale & heterogeneous formations. Varying surface torque is observed in layered formations. Following steps are to taken while drilling.
1. Reduce the WOB if torque increases considerably while drilling through sands/fractured formation.
2. In order to avoid the bit slip phenomena in plastic/soft formations reduce the WOB and increase rotary RPM preferably.
3. When there is reduction in ROP, more WOB should be applied in increments of one ton to take care of wearing of PDC cutters.
4. Carry out drill off test to achieve optimum ROP.
5. WOB should be increased before increasing the rotary speed in order to attain maximum depth of cut by PDC cutter. This will help in preventing bit from becoming unstable, thereby stopping backward whirling motion.
6. In softer formations increase in rotary speed results in better ROP but reverse may occur in harder formations as PDC cutters are unable to ‘dig- in’ beyond a certain threshold RPM limit.
Making Connections
1. Maintain normal circulation rate when Kelly is raised.
2. Bit should be washed back to bottom slowly at normal circulation rate.