Drilling Methods for Rotary Drilling Rigs in Hard Clay and Mudstone Layers

Hard clay and mudstone layers are characterized by high strength and compactness, making drilling significantly more challenging compared to softer soil layers. These formations require stricter demands on the selection of drill bits, operational parameters, and drilling techniques. Improper handling may lead to low drilling efficiency, severe wear of drilling tools, or even equipment jamming and borehole deviation. To ensure drilling efficiency and borehole quality in such formations, targeted methods and measures must be employed. Below is a detailed explanation of the methods and operational techniques:

1. Drill Tool Selection and Equipment Requirements

1. Choosing Suitable Drill Bits:

• For hard clay layers, short auger drill bits or double-bottom bucket drill bits are recommended for enhanced efficiency and debris removal.

• For mudstone layers, reinforced cutting-tooth drill bits or rock drill bits with carbide teeth should be used to handle the high strength and abrasiveness of the formation.

2. Power Head Requirements:

• High hardness in hard clay and mudstone requires the power head to have high torque output to maintain stable drilling under high resistance.

• Shock absorption performance should also be ensured to minimize the impact of hard formations on the equipment.

3. Auxiliary Devices:

• Vibration or impact devices can be added to break through mudstone layers and improve drilling efficiency.

2. Drilling Techniques

1. Optimizing Operational Parameters:

• Adjusting Downward Pressure: Increase downward pressure to overcome resistance in hard clay and mudstone, but ensure it remains within the drill bit's tolerance to avoid overload.

• Controlling Drilling Speed: Reduce drilling speed to improve cutting precision and protect the drilling tools, especially in mudstone layers where high friction can cause overheating.

• Torque Settings: Adjust torque according to formation hardness; slightly lower torque is suitable for hard clay, while higher torque is needed for mudstone.

2. Layered Drilling:

• For hard clay, adopt a layered drilling method, starting shallow and gradually deepening to reduce formation resistance.

• For mudstone, use powerful drill bits to first break the upper structure, clear debris, and then continue deeper.

3. Mud Management:

• In hard clay and mudstone, the lubricating and sand-carrying properties of mud are critical.

• Mud viscosity should be moderate to prevent excessive resistance from high viscosity or incomplete debris removal from low viscosity.

3. Special Treatments for Specific Formations

1. Hard Clay Treatment:

• Auger Drilling Method: Use short auger drill bits for efficient cutting and debris removal in hard clay layers.

• Mud-Assisted Lubrication: Hard clay tends to stick to the drilling tools, so inject high-viscosity mud to lubricate the tools and reduce resistance.

2. Mudstone Treatment:

• Impact Drilling Method: In mudstone layers, use impact drilling devices to fracture the rock before rotary drilling.

• Hydraulic Cleaning: Use high-pressure water jets to clean borehole walls and bottoms, preventing debris accumulation and reducing mudstone adhesion.

4. Borehole Cleaning and Wall Stabilization

1. Timely Debris Cleaning:

• In hard clay layers, debris tends to form lumps that hinder drilling; frequent mud circulation or reverse circulation is needed to clean the borehole.

• In mudstone layers, thorough debris removal is essential to prevent tool jamming.

2. Casing Usage:

• For mudstone layers mixed with soft soils, steel casings should be installed early to stabilize borehole walls and prevent collapse or diameter reduction.

5. Common Issues and Solutions

1. Tool Jamming:

• If tools get jammed in mudstone or hard clay, stop drilling, inject mud, and reverse the drill rod lightly to release the tools.

• If the issue persists, consider replacing the drill bit or re-drilling.

2. Equipment Overheating:

• The high friction in hard clay and mudstone may cause equipment overheating. Allow intervals for cooling or reduce drilling speed.

3. Wall Collapse:

• For unstable borehole walls, use high-viscosity mud along with steel casings and reduce drilling speed to minimize vibration impact.

Conclusion

Drilling in hard clay and mudstone layers requires comprehensive optimization in equipment selection, operational parameter settings, and technical operations. Selecting appropriate drill bits, controlling drilling speed and downward pressure, employing layered drilling, and managing mud effectively can significantly improve efficiency and quality. Real-time monitoring and quick troubleshooting further reduce risks and ensure smooth project execution.

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