Screw drilling tools, also known as progressive cavity type positive displacement bottom hole power tools, are commonly referred to as screw drills. These tools deliver drilling mud or water through the central hole of the drill string as a power medium, converting the pressure energy of the fluid into mechanical energy, which drives the drill bit to rotate. The screw drill can directly drive the core barrel and drill bit connected to it, while the entire drill string only serves as a channel for transmitting the working medium and supporting the drill bit’s reaction torque, thus preventing the entire drill string from rotating.
Compared with conventional drilling, screw drilling tools significantly reduce drill string wear and increase drilling speed. These tools are primarily used for directional drilling and play an important role in drilling applications. Originally used for vertical wells, screw drills are now widely used for various directional wells and special engineering wells (e.g., mine freezing wells).
Currently, the maximum drilling depth of screw drills has reached 9,023 meters, with a minimum diameter of 44.5 mm and a maximum diameter of 304.8 mm. When using screw drills for core drilling, core barrels are usually added between the drive shaft and the drill bit.
Components of Screw Drilling Tools
Screw drilling tools consist of several main components, including bypass valves, screw motors (rotor and stator), universal joints, bearings, and drive shafts. The core component is the screw motor. In the screw motor, the stator has one more tooth than the rotor, i.e., Z1=Z2+1, where Z1 is the number of stator teeth, and Z2 is the number of rotor teeth. This tooth ratio is called the transmission ratio, and it can be designed in different ratios, such as 1:2, 2:3, …, 9:10, depending on the application. Screw drills can be designed with either high-speed motors with a small tooth ratio or low-speed, high-torque motors.
The output torque of the screw drill is proportional to the pressure drop through the motor, while the output speed is proportional to the flow rate of the working medium through the screw motor. As the tooth ratio increases, efficiency gradually decreases. When using screw drills, they must be paired with conventional drilling equipment, such as drilling rigs, mud pumps, drill pipes, and derricks, as well as directional instruments and deflection tools for precise directional drilling.
The lifespan of screw drills is typically 150 to 200 hours. The weak points mainly lie in the stator’s ability to withstand high temperatures and the durability of the bearings. Researchers are currently working on developing more wear-resistant and high-temperature-resistant materials to improve the lifespan of screw drills.
Operating Principle of Screw Drilling Tools
Screw drilling tools are downhole power tools that convert the pressure energy of drilling fluid into mechanical energy. When drilling mud pumped by a mud pump flows through the bypass valve into the screw motor, a pressure difference is created, which causes the rotor to rotate around the stator’s axis. The speed and torque are then transmitted to the drill bit through a universal joint and drive shaft, completing the drilling operation.
Usage of Screw Drilling Tools
Before using screw drilling tools, a surface inspection is required. The specific steps are as follows:
- Pre-drilling Inspection:
The drilling tool should be lifted using a short section and installed in the turntable clamp. Ensure the bypass valve is above the turntable. The valve’s flexibility should be checked repeatedly to ensure the valve core can return to its original position under spring action and is not obstructed. - Lowering the Tool:
During lowering, it is essential to control the speed strictly to avoid causing the motor to rotate backward, which could result in a pressure drop and tool overload. Drilling should not proceed too quickly, as the wellbore may still be uncleaned, leading to mud packing on the drill bit. - During Drilling:
Increasing the drilling pressure can increase the torque. Uniform drilling can help maintain smoothness in the wellbore and improve directional accuracy. - Pulling the Tool and Inspection:
When pulling the tool, flush the bypass valve with clean water to ensure the valve core can close freely, and use mineral oil to lubricate the motor. If the bearing gap exceeds the maximum allowable tolerance, the bearing should be replaced promptly.
Advantages and Disadvantages of Screw Drilling Tools
Advantages:
- High Rock Breaking Efficiency:
Screw drills can break rock both through impact and shear forces caused by sliding, making them highly efficient. - Versatility:
Screw drills are suitable for various geological formations, with particularly good adaptability to unstable formations. - Self-Cleaning Capability:
The drill bit has strong self-cleaning abilities and is less prone to mud packing. - Lower Drill Bit Cost:
Compared to other types of drilling tools, screw drill bits are relatively inexpensive.
Disadvantages:
- Shorter Drill Bit Lifespan:
Due to the limited lifespan of bearings and tooth wear resistance, screw drill bits have a relatively short lifespan. - Potential for Tooth Loss:
In weak points such as the bearing seals and locking areas, the teeth of the drill bit may detach, potentially leading to drilling accidents. - Not Suitable for High Speed:
Due to the short lifespan of bearings at high speeds, screw drills are not suitable for high-speed operations and are typically used below 200 RPM. - Prone to Damage in High-Temperature Environments:
The seals and lubrication systems of the drill bit are vulnerable to damage at high temperatures, making screw drills unsuitable for high-temperature formations. - Limited Suitability for Small Boreholes:
Screw drills have a shorter lifespan in small borehole operations, making them less suitable for small-diameter drilling. - Requires Higher Drilling Pressure:
Screw drills require higher drilling pressure and are not suitable for formations prone to deflection.
Conclusion
Screw drilling tools, with their high rock-breaking efficiency, excellent adaptability, and self-cleaning capabilities, have become essential tools for directional drilling and special engineering applications. However, they do have some limitations, such as shorter lifespans and the need for higher drilling pressures. With ongoing advancements in technology, the design and materials of screw drills continue to be optimized, promising a broad future for their application in the drilling industry.
Post time: Dec-06-2024
