Drilling Methods for Rotary Drilling Rigs in Boulder And Cobble Layers

Boulder and cobble layers present a challenging geological condition characterized by high hardness, uneven distribution, and a mixture of soft soil or other strata. Such conditions impose strict requirements on equipment and techniques. Without proper handling, issues like drill bit damage, reduced efficiency, or even operational failure can occur. To safely and efficiently complete drilling tasks in boulder and cobble layers, tailored methods and strategies must be adopted. Below are specific procedures and considerations for such operations:

1. Equipment and Tool Selection

1. Choosing Suitable Drill Bits:

• For boulder layers, reinforced rock drill bits or drill bits with hard alloy teeth are recommended to enhance wear resistance and cutting capability.

• For hard cobbles, tapered or cutting tooth drill bits can effectively break the cobbles while maintaining efficiency.

2. Power Head Requirements:

• The power head must provide high torque output to handle the hardness of boulders and cobbles.

• Stability is crucial to minimize vibration and reduce wear on drill rods and bits.

3. Equipping Impact Devices:

• In areas with numerous cobbles, impact devices can be added to the rig. These devices use vibration to break cobbles and improve drilling efficiency.

2. Drilling Techniques

1. Accurate Positioning and Casing Usage:

• Before drilling, perform geological surveys to locate boulders or cobbles precisely and maintain accurate positioning during drilling.

• For layers with mixed soft soil and boulders, pre-embed steel casings to stabilize the borehole walls and prevent collapse.

2. Segmental Drilling Method:

• Layer-by-Layer Drilling: Drill through the softer overlying soil first, then use specialized drill bits to break through boulders or cobbles.

• Gradual Hole Enlargement: Drill in segments and gradually enlarge the hole to reduce the impact of large cobbles on the equipment.

3. Adjusting Drilling Parameters:

• Reducing Drilling Speed: Lower drilling speed when penetrating boulders or cobbles to reduce equipment stress and improve precision.

• Controlling Pressure and Torque: Apply appropriate downward pressure and torque to avoid excessive force, which may damage the drill bit or cause jamming.

3. Special Treatments for Difficult Strata

1. Handling Cobbles:

• Using Impact Breaking: Employ impact drilling or hydraulic hammers to break cobbles before proceeding with rotary drilling.

• Adopting Cutting Tooth Drill Bits: For large cobbles, use cutting tooth drill bits to break them into smaller fragments for easier removal.

2. Managing Boulders:

• Mud-Assisted Cleaning: Use high-viscosity mud to enhance wall stability and suspend boulder fragments for easier cleaning.

• Reverse Circulation Cleaning: Employ reverse circulation drilling to remove boulder fragments efficiently.

4. Borehole Cleaning

1. Regular Cleaning of Debris:

• In layers with abundant boulders and cobbles, fragments and debris can accumulate at the borehole bottom. Regularly clean the hole using mud circulation or reverse circulation systems to maintain cleanliness.

2. Preventing Tool Jamming:

• If tools become jammed, stop drilling, inject more mud, and lightly reverse the drill rod to clear fragments before resuming operations.

5. Precautions

1. Real-Time Monitoring:

• Use monitoring equipment to track drilling parameters such as torque, pressure, and speed, ensuring stable operations. Adjust methods immediately if abnormalities occur, such as by changing drill bits or modifying mud properties.

2. Enhanced Mud Management:

• For boulder layers, mud should have excellent lubricating properties and sand-carrying capacity to reduce friction and aid in fragment removal.

• Adjust mud viscosity and density based on strata conditions. Typically, mud density should range between 1.2 and 1.4.

3. Avoid Continuous Drilling:

• Continuous drilling in boulder-rich areas can overheat or damage tools. Allow intervals to protect the equipment.

Conclusion

Drilling in boulder and cobble layers requires adaptive strategies based on geological conditions and equipment performance. Selecting the right tools, optimizing drilling parameters, employing segmental drilling, and maintaining borehole cleanliness are essential for safe and efficient operations. Additionally, real-time monitoring and thorough equipment maintenance further reduce risks and ensure project quality.

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