0 Drilling Bit Types and Drilling Bit Selections

Currently in the drilling industry, there are two main categories of drilling bits: rolling cutter and fixed cutter. Plus, bit sizes vary from 3-7/8 inch to 36 inches.

1. Rolling Cutter Drilling Bits

Rolling cutter bits, which some may also call roller cone bits or tri-cone bits, have three cones. Each cone can be rotated individually when the drill string rotates the body of the bit. The cones have roller bearings fitted at the time of assembly. The rolling cutting bits can be used to drill any formations if the proper cutter, bearing, and nozzle are selected.

There are two types of them which are milled-tooth and tungsten carbide inserts. These bits are classified by how the teeth are manufactured:

1.1 Milled-Tooth

Milled-tooth bits have steel tooth cutters, which are fabricated as parts of the cone body. The bits cut or gouge formations out when they are being rotated. The teeth vary in size and shape, depending on the formation. The following guidelines demonstrate you type of teeth that you need to select to match with the formations.

Soft formation: The teeth should be long, slender and widely spaced. These teeth will produce freshly broken cuttings from soft formations.

Hard formation: The teeth should be short and closely spaced. These teeth will produce smaller, more rounded, crushed, and ground cuttings from hard formations.

1.2. Tungsten Carbide Insert (TCI) or Insert

Tungsten Carbide Insert (TCI) or Insert bits generally have tungsten carbide inserts (teeth) that are pressed into the bit cones. The inserts have several shapes such as long-extension shapes, round shaped inserts, etc.

Teeth of the bits are different depending on the formation as follows:

Soft formation: Long-extension, chisel shape inserts

Hard formation: Short-extension, rounded inserts

2. Fixed Cutter Drilling Bits

Fixed cutter bits consist of bit bodies and cutting elements integrated with the bit bodies. The fixed cutter bits are designed to excavate holes by shearing formations rather than cutting or gouging formations, such as rolling cutter bits. These bits do not have moving parts such as cones or bearings. The components of bits are composed of bit bodies fabricated from steel or tungsten carbide alloy and fixed blades integrated with abrasion-resistant cutters. The cutters in the bits that are available on the market are Polycrystalline Diamond Cutters (PDC) and natural or synthetic diamond cutters.

Nowadays, with the improvement that have been made in fixed cutter bit technology, the PDC bits can drill almost any kind of formations from soft to hard formation. However, if you plan to drill very hard formations, you might consider using diamond bits instead.

Rachain J. is working in drilling operations and he would like to share basic knowledge about drilling formulas and drilling operations. He and this team member create very useful oilfield related websites which are drilling formulas and oilfiled dictionary.