Introduction

Pebble geology is widely distributed across our country, commonly found in riverbeds and alluvial plains. It can be classified into three types: sandy pebble, muddy pebble, and pure pebble formations. These formations share common challenges in construction, including severe drill tool wear, collapse risks, slurry leakage, and difficult drilling conditions.

Drilling Methods for Pebble Formations

1. Shallow Pebble Layers (Near Riverbanks & Floodplains)

• Common Issues: Difficulty in casing installation, frequent collapse at the borehole opening.
• Professional Solution (Bauer Rigs): Use a casing oscillator to drive the casing down while drilling. This method effectively prevents slurry leakage and collapse in the upper pebble layers.
• Domestic Rig Solution (Without Casing Oscillator):
  • Ensure proper casing alignment and reinforce the borehole opening.
  • Lay steel plates beneath the rig to prevent subsidence and rig tilting.

2. Slurry Leakage Solutions

• Problem: High-density slurry cannot stabilize pure pebble layers due to the pebbles’ higher specific gravity.
• Solution:
  • Pour dry yellow clay into the borehole (3-4 buckets via excavator).
  • Use a reverse rotation drill bit (with flushing holes) to press the clay against the borehole walls.
  • Drill forward, lift the bit 2-3m, and repeat until passing the pebble layer.
  • Switch to a smaller drill bit (e.g., 1.25m for a 1.5m designed hole) to reduce wall erosion and collapse risks.
  • Ream to final diameter after reaching the desired depth.
• Note: This method is ineffective for pebble layers deeper than 30m—specialized techniques are required.

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Rotary Drilling Rig Construction Techniques (Part 2): Drill Bucket Selection

1. The Role of Drill Tools

• Drill tools must match geological conditions to maximize efficiency and minimize wear.
• In China, sand buckets (捞沙斗) are the most commonly used tool (80% of rigs).

2. Drill Bucket Resistance Analysis

Drilling resistance comes from three sources:

A. Front Resistance (Cutting Force)

• Cause: Teeth encountering geological formations.
• Reduction Methods:
  • Use varying tooth lengths to distribute the load.
  • Ensure proper tooth spacing for full 360° coverage.
  • Keep the center tip width <15cm and shorter than the inner teeth.

B. Side Resistance (Friction Against Borehole Wall)

• Reduction Methods:
  • Use side teeth 10cm wider than the bucket diameter.
  • Add relief slots & side cutters to reduce friction.

C. Internal Resistance (Soil Friction Inside Bucket)

• Solution: Maximize the bucket inlet size to reduce clogging.

3. Borehole Diameter vs. Resistance

• Smaller diameter = Less resistance (shorter torque arm).
• Larger diameter = Higher contact area = Increased resistance.

4. Jamming Prevention

• Causes:
  • The conical bucket shape traps falling debris.
  • Worn bucket components leading to mechanical failure.
• Solution: Avoid forced drilling/lifting—use proper techniques.

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Rotary Drilling Rig Construction Techniques (Part 3): Mud Functions

Key Mud Functions:

1. Pressure Stabilization – Maintains borehole integrity (pressure increases by 98 kPa per 10m depth).
2. Wall Protection – Forms a viscous layer to prevent collapse.
3. Suspension – Keeps cuttings from settling too quickly.
4. Lubrication – Reduces drill resistance.
5. Buoyancy lowers hoisting resistance.
6. Cooling – Prevents overheating in rock drilling.

Drilling Fluids vs. Traditional Mud

• Drilling Fluids: Quick-mix, water-like viscosity (no wall protection, prone to leakage).
• Bentonite Mud: Better suspension & lubrication, but requires proper mixing.

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Rotary Drilling Rig Construction Techniques (Part 4): Operational Best Practices

1. Operator Fundamentals (Look, Listen, Feel)

• Look: Monitor rig parameters, soil samples, and wear points (wire rope, drill pipe, casing).
• Listen: Detect abnormal sounds (engine strain, gearbox load changes, drill pipe collisions).
• Feel: Sense vibrations to adjust torque/pressure based on geology.

2. Geology-Specific Drilling Techniques

Geology Type	Drilling Approach	Key Adjustments
Soft Clay	“Overcome Softness with Hardness”	High torque, heavy pressure
Sandy Layers	“Balance Hard & Soft”	Moderate pressure, avoid overloading
Pebble Layers	“Overcome Hardness with Softness”	Reduce force to minimize vibration
Hard Sandstone	“Hard vs. Hard”	Maximum torque & pressure

3. Operational Summary

• Avoid sudden movements to reduce mechanical stress.
• Adjust flow/pressure based on geology.
• Master force dissipation techniques for efficiency and safety.