Drill Bit: An Important Extraction Tool in Oil Drilling Engineering

The drill bit plays a crucial role in oil drilling, being used to crush rocks to form boreholes. Its performance has a significant impact on drilling quality, efficiency, and cost. The main types of drill bits include roller cone bits and PDC bits, which can further be categorized by function into full drilling bits, coring bits, and special process bits.

Roller Cone Bit

Since the first roller cone bit was introduced in 1909, it has been widely used worldwide, with the three-cone bit being the most common in rotary drilling operations. It can adapt to various types of formations, and by correctly selecting the suitable structure of the roller cone bit according to the drilled formation properties, satisfactory drilling speed and penetration can be achieved.

Basic Structure

It consists of the bit body, cones, bearings and their lubrication sealing devices, and nozzles. The bit body is assembled and welded from three sections, with a connection thread on the upper part. The cones are conical metal bodies made of the cone body and teeth, which are of two types: milled teeth and inserted teeth.

Working Principle

When the roller cone bit works at the bottom of the well, the whole bit rotates around the axis, which is called revolution, and the three cones roll on the bottom of the well along their axes, which is called rotation. The weight on the bit (WOB) acts on the rock through the teeth, causing it to break. During the rolling process, the cones contact the well bottom alternately with single or double teeth, producing longitudinal vibrations that compress and stretch the drill string and transform elastic deformation energy into impact force to fracture the rock. Additionally, the roller cone bit exerts a shearing force on the bottom rock, similar to a scraper bit, that scrapes the bottom of the well. This shearing force mainly arises from the cone's crowning, secondary cone, and offset arrangement.

Classification and Selection

There are many manufacturers of roller cone bits, and the International Association of Drilling Contractors (IADC) has established a unified classification standard and numbering method. Each type of bit is represented by a three-digit number: the first digit indicates the type of teeth and suitable formation, the second digit represents the formation hardness grade, and the third digit signifies the bit's structural features.

PDC Bit

PDC bits, also known as polycrystalline diamond compact bits, have been extensively used in oil drilling since General Electric introduced the first PDC bit in 1973 with PDC cutters. They are favored for their fast drilling speed, long lifespan, and high footage.

Structural Features

PDC bits consist of the bit body, PDC cutters, and nozzles, and are divided into two main series: steel body and matrix body. Steel body PDC bits are easy to manufacture but are less resistant to erosion and have difficulty securing the cutters, making them less commonly used. Matrix body PDC bits have a steel upper body and a tungsten carbide wear-resistant matrix lower body, made using powder metallurgy sintering processes. They have a long lifespan and high footage, making them widely used.

Working Principle

They break rock through cutting, where the self-sharpening cutters easily penetrate the formation under WOB and shear the rock as they move forward under torque, with multiple cutters working simultaneously for high rock-breaking efficiency and fast drilling speed.

Proper Usage

They perform best in large, homogeneous soft to medium-hard formations and are not suitable for gravel layers or alternating soft-hard formations. They should be drilled with low WOB, high rotation speed, and high fluid flow rate. The well bottom should be clean and free of metal debris before running the bit in hole. When the bit is first run, it should be operated with low WOB and low rotation speed until the well bottom is formed, then normal drilling can resume. They are suitable for high-speed turbine drilling.