Geological Drill Pipe Selection Guide

Geological Drill Pipe Selection Guide: How to Choose the Most Suitable Drill Pipe Based on Working Conditions?

In geological exploration and drilling engineering, the geological drill pipe is a key component connecting the drilling rig and the drill bit, and its performance directly affects drilling efficiency, core quality, and construction safety. Correct drill pipe selection requires a systematic consideration of key factors such as materials, specifications, connections, and maintenance. This article aims to provide a concise and practical selection guide.

1. Drill Pipe Material Selection

The drill pipe material directly affects its strength, rigidity, toughness, and wear resistance. Currently, the commonly used geological drill pipe materials are carbon steel, alloy steel, and composite materials.

Carbon steel is the most common drill pipe material, with advantages such as high strength and low price, but it also has disadvantages such as being prone to rust and wear. Alloy steel has relatively better material properties, but its price is also relatively high. Composite material drill pipes are a new type of drill pipe material that has emerged in recent years, with advantages such as light weight, high strength, and good wear resistance, but its price is also relatively high. 

Specifically:

a. Carbon steel/low-alloy steel: Carbon steel boasts high strength, low cost, and mature manufacturing processes, but its corrosion and wear resistance are generally poor, and it is prone to rusting in humid or corrosive environments. Suitable for conventional formations, shallow to medium-deep holes, and cost-sensitive projects.

b. High-strength alloy steel: Offers better overall mechanical properties (e.g., superior strength and toughness) and durability, but has a higher procurement cost. Suitable for deep-hole drilling, hard rock drilling, and projects with high requirements for drill pipe lifespan.

c. Composite materials (e.g., aluminum alloys): Composite materials are lightweight, facilitating relocation, reducing drilling rig load, and exhibiting excellent corrosion resistance. However, initial investment is high, and bending stiffness is typically lower than steel rods. Suitable for areas with difficult relocation, such as mountains and swamps; underwater drilling; and scenarios with stringent requirements for lightweight equipment.

Material selection recommendations: 

For most onshore solid mineral exploration, high-strength alloy steel is the mainstream choice due to its excellent cost-effectiveness. When relocation efficiency is extremely important or there are special requirements for corrosion resistance, the long-term value of composite material drill pipes can be evaluated.

2. Drill Pipe Size Selection

The size of the drill pipe needs to be comprehensively considered based on the geological environment and drilling requirements.

Drill Pipe Diameter: 

Following the "drill pipe-hole-casing pipe" gradation relationship.

Small diameter drill pipes (such as the AQ/BQ series, outer diameter approximately 75mm) are suitable for shallow hole sampling; standard specifications (such as the NQ/HQ series, outer diameter approximately 96mm) are the mainstay for most core drilling; large diameter (such as the PQ series) are used for deep holes or projects requiring large-diameter coring. For hard rock formations, larger diameter, thicker-walled drill pipes with better rigidity are preferred to enhance bending resistance and ensure drilling stability.

Drill Pipe Length:

Drill pipe length refers to the total length of the drill pipe, including the actual length of the drill pipe and the length of the connector. The selection of drill pipe length should be comprehensively considered based on factors such as the geological environment and drilling depth. The standard single length of drill pipe is usually 3 meters, 4.5 meters, or 6 meters. The selection must match the height of the drilling rig's support. The total length is determined by the designed hole depth. For deep hole operations, the tensile strength and fatigue resistance of the drill pipe should be given priority.

Generally, the deeper the borehole, the longer the drill pipe needs to be. Geological conditions also influence the choice of drill pipe length; for example, in harder rock environments, longer drill pipes are required.

3. Drill Pipe Thread Selection

The thread is the weakest point of the drill string, making its selection crucial. Drill pipe threads refer to the connecting parts of the drill pipe, including upper and lower threads. The selection of drill pipe threads should be matched to the drilling rig model and drill pipe model.

Simultaneously, the thread pitch and thread profile also need to be appropriately selected. The pitch selection should be based on a comprehensive consideration of the formation properties and drilling technology, while the thread profile needs to be matched to the drilling rig model and drill pipe model.

Selection Recommendations:
Standardization: Prioritize API standard drill pipe threads or geological drill pipe-specific threads to ensure interchangeability.
Type Matching: Must be fully compatible with the drill rig holder and drill pipe specifications.

Process Considerations: If wireline coring is used, double-walled drill pipes and their dedicated internal and external thread connection system are required.

4. Process Compatibility

Wireline Coring: Double-walled wireline coring drill pipes with a unique internal tube assembly design are essential.
Impact Drilling: Thickened or specially reinforced drill pipes are required to withstand high-frequency impact loads.
Directional Drilling: Drill pipes with high bending strength and better wear resistance are required, and non-magnetic drill collars may be necessary.

Drill Pipe Storage and Maintenance:

Proper maintenance can significantly extend the lifespan of drill pipes and ensure operational safety. The following points should be noted when storing and maintaining drill pipes:

1. Storage: Drill pipes should be stored vertically or horizontally in a dry place to prevent bending and deformation. Threaded parts must be fitted with protective sleeves.
2. Cleaning and Rust Prevention: Clean promptly after use, dry thoroughly, and apply rust-preventive grease if necessary.
3. Thread Maintenance: Clean the threads before each connection and apply a dedicated thread grease evenly to reduce wear, ensure a seal, and facilitate disassembly.
4. Regular Inspection: Establish a regular inspection system, focusing on thread wear, pipe bending, and surface cracks. Severely damaged drill pipes should be scrapped immediately.

Conclusion: 

Choosing suitable drill pipes is crucial for geological drilling. Factors such as the material, diameter, length, thread type, and compatibility with drilling techniques need to be comprehensively considered. Attention to detail in drill pipe storage and maintenance is also necessary to ensure their service life and performance. It is recommended to communicate closely with experienced drilling engineers or reputable suppliers at the initial project design stage to determine the optimal drill pipe combination based on specific hole depth, lithology, coring requirements, and budget.

Read more: Types of Oil Drill Pipe or API 5DP Drill Pipe Specification