How to Select Drill Pipe Specifications Based on Application Scenarios?

Choosing drill pipe specifications involves matching the most suitable materials, dimensions, and connection methods to the specific drilling purpose and geological conditions.

1) Conventional Oil and Gas Drilling

This is the most typical application scenario, usually referring to onshore vertical wells or conventional directional wells with depths between 3000 and 5000 meters.

Selection Points:

Outer diameter is primarily 5 inches, the mainstream size for deep wells globally, although 4-1/2 inches are also common. A diameter that is too large increases drill string weight, while a diameter that is too small results in insufficient torsional strength.

Steel grade is generally G-105. If the well depth exceeds 4500 meters, or if greater torque is required, upgrade to S-135.

Wall thickness is initially selected based on standard weight, then checked using strength calculations. If the safety factor is insufficient, use a heavier wall thickness.

Threads use the API standard NC series, such as NC50 threads for 5-inch drill pipe. This type of thread has good versatility, low cost, and is perfectly adequate for conventional wells.

Single length is R2, approximately 9.5 meters per thread, for the most convenient field operation.

2) Deep and Ultra-Deep Wells

Deep wells, exceeding 5000 meters and even reaching depths of 8000 meters or more, have enormous drill string weights, placing extremely high demands on the tensile strength and fatigue resistance of the drill pipe.

Selection Points:

The outer diameter remains primarily 5 inches, with 5-1/2 inches or 5-7/8 inches used where feasible to achieve higher torsional strength.

The steel grade must be S-135, with V-150 used for particularly heavy-duty applications. G-105 is adequate below 5000 meters, but insufficient for deeper depths.

The wall thickness cannot be the standard weight; a heavier wall thickness must be selected, for example, upgrading from 19.5 lbs per ft to 25.6 lbs per ft for 5-inch wells. The trade-off is a smaller inner diameter, but increased strength.

The thread cannot be skimped on; special threads such as VAM, FOX, and BGT series must be used. API threads are prone to leakage or damage under the high torque and bending stress of ultra-deep wells.

The length is still chosen as R2, but some wells may use R3 (approximately 13 meters) to improve efficiency and reduce the number of joints.

3) Directional and Horizontal Wells

Shale gas and tight gas development extensively utilizes horizontal wells. The drill string must rotate in curved sections with large doglegs, placing high demands on the fatigue life and bending resistance of the drill pipe.

Selection Points:

The outer diameter is primarily selected between 4-1/2 inches and 5 inches. Too large a diameter makes it difficult to navigate curved sections, while too small a diameter cannot withstand torque.

The steel grade is S-135 because the horizontal section requires prolonged rotation, demanding high material fatigue performance.

The wall thickness is determined based on calculations of the dogleg and friction of the build-up section, typically at standard weight or slightly heavier.

Special threads must be selected; this is a strict requirement. For shale gas fracturing and long horizontal section rotation, API threads are insufficient for sealing, easily leading to leaks or seizing. Threads with metal seals and torque shoulders, such as VAM, FOX, and BGT, should be used.

Length should be R2; excessively long single drill pipes are difficult to run into curved sections.

4) Geological Exploration Drilling

Solid mineral exploration, hydrogeological, and engineering geological exploration involve small drilling rigs, shallow holes, and frequent relocation, placing completely different requirements on drill pipes compared to oil drilling.

Selection Points:

Use the letter series for geological drill pipes for the outer diameter. NQ (approximately 75 mm) is the main type; HQ (approximately 96 mm) is used for deeper holes or when large-diameter cores are required; BQ (approximately 60 mm) can be used for shallow holes.

Use high-strength alloy steel for the material, ensuring both strength and lightness.

Choose wall thickness according to the standard series; the wall thickness of geological drill pipes is designed to match the corresponding hole depth.

Two types of threads are available: tapered threads for diameters of 50 mm and above, providing high torque transmission and easy assembly/disassembly; rectangular threads (commonly known as flat threads) are used for smaller diameters from 34 mm to 50 mm.

Lengths are typically 3 meters, 4.5 meters, or 6 meters, suitable for XY series drilling rigs, allowing them to fit inside the drilling tower for easy transport.

5) Engineering Drilling

Drilling for rotary pile foundations, anchor bolt drilling, and foundation pit support requires high-powered drilling rigs and drill rods that need to transmit enormous torque.

Selection Points: Common outer diameters are 76mm, 89mm, 108mm, and 133mm, which must match the diameter of the rotary drill bit.

The cross-sectional shape must be hexagonal drill rods. The hexagonal structure transmits torque efficiently and avoids slippage like round drill rods.

The wall thickness must be increased because of the significant impact and vibration; thin-walled pipes are prone to deformation or breakage.

Tapered threads are used for quick assembly and disassembly, suitable for frequent on-site handling.

The length is generally 12 to 12.5 meters, matching the working height of the rotary drilling rig.

6) Acidic Environments (Containing Hydrogen Sulfide)

Regardless of the scenario, if the well contains hydrogen sulfide, the selection logic must be completely changed—safety is paramount, and conventional methods cannot be used.

Selection points:

The steel grade cannot be ordinary G-105 or S-135; it must be replaced with a sulfur-resistant steel grade, such as S-135G or G-105S.

Other parameters, such as outer diameter, wall thickness, and thread type, are the same as conventional selection, but must be based on the premise of using a sulfur-resistant steel grade.

Ordinary steel grades must never be used in wells containing hydrogen sulfide, even if the strength calculations show it is sufficient; hydrogen sulfide stress corrosion cracking is catastrophic.

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