Penetration Drop and Hole Deviation in Layered Rock
You are drilling a 1.2 m diameter, 26 m deep shaft with a crawler rotary rig using a soil auger and bentonite slurry. The upper 8 m is loose sand, 8–18 m is weathered sandstone, and below 18 m is hard limestone. At 19 m, penetration drops from 0.7 m/min to 0.12 m/min, the mast begins to shudder, and the returns contain angular gravel and fine rock flour but few limestone chips. The hole is at 1.3% deviation; the specification is 1.0%. The bucket teeth are worn 20 mm. The operator proposes increasing crowd force and rotary torque and continuing. The shift target is two holes in a 10-hour shift; each hole requires 30 min for setup, 45 min for slurry and cleaning, and 20 min to move.
1. Diagnose the likely cause of the penetration drop and shudder. Is the operator's proposal reasonable?
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The change at 19 m, just below the weathered sandstone/hard limestone contact, with angular gravel and rock flour but few limestone chips, points to a hard or fractured layer, a boulder/cobble zone, or a dull or partially blocked cutting tool rather than simply 'hard rock.' The shudder suggests bit chatter, a loose mast, or an unstable tool cutting on one side. The 1.3% deviation against a 1.0% limit is already a warning: increasing crowd and torque will likely increase side load and make deviation worse.
The operator's proposal is not reasonable as a first response. More crowd on a dull bucket in a hard layer can stall the tool, damage the kelly or mast, overload the winch, and steer the hole further off line. More torque can also worsen chatter and cause the bucket to walk. The correct response is to stop and diagnose: check the cutting tool and teeth, check the mast and kelly for looseness, inspect the returns, verify slurry properties and cleaning, and survey the hole. Only then should parameters be adjusted.
2. Recommend operational changes and measurements to correct the deviation and improve penetration safely.
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Recommended actions:
- Stop drilling and survey. Confirm the deviation with a reliable survey method and record the depth and direction. Do not continue until the correction method is chosen.
- Inspect the tool. Replace or build up the worn teeth; check the bucket, pilot, and openings for packing. If the layer is too hard for a soil auger, switch to a roller bit, coring bucket, or other suitable tool.
- Reduce crowd and control rotation. Use light, steady crowd and moderate rotation to re-establish gauge rather than forcing the tool. If correction is needed, use a controlled reaming or steering pass, not side-loading the mast.
- Improve hole cleaning. Check slurry density, viscosity, sand content, and flow. Rock flour and gravel must be removed; poor cleaning can cause bit balling and deviation.
- Stabilize the mast. Re-check level and plumb, and inspect the kelly and guide for play.
- Monitor. Record penetration rate, torque, crowd, vibration, slurry returns, and deviation at each change. Stop if deviation increases or shudder worsens.
If the deviation cannot be corrected within specification, the hole may need to be backfilled and redrilled, or the design changed after engineering review.
3. Using the given rates and non-drilling times, determine whether two holes can be completed in the 10-hour shift at the current 0.12 m/min rate. What average rate is required?
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Non-drilling time for two holes is:
- Setup: 2 × 30 min = 60 min
- Slurry and cleaning: 2 × 45 min = 90 min
- Move: 2 × 20 min = 40 min
- Total non-drilling = 190 min = 3.17 h
Available drilling time = 10 h − 3.17 h = 6.83 h.
At 0.12 m/min, drilling 52 m takes 52 ÷ 0.12 = 433 min = 7.22 h. Total time = 7.22 + 3.17 = 10.39 h, so the shift misses the target by about 23 min before any downtime.
The required average rate is 52 m ÷ 6.83 h = 7.61 m/h = 0.127 m/min. That is only slightly above the current rate, so the target is close but not met. Improve the rate through tool condition, slurry cleaning, and reduced vibration, or reduce non-drilling time. Do not chase the rate by over-crowding the tool, which risks deviation and equipment damage.