Rollercone vs Fixed Cutter Drill Bits

This article talks about the key differences between Rollercone and Fixed Cutter Drill Bits. 

Generally speaking, there are two different categories of drill bits, Rollercones and Fixed cutter bits.

  • Rollercone bits have rollers rotating around bearings. Rollercone bits use steel teeth or tungsten carbide inserts as cutting elements.
  • Fixed cutter bits use diamond cutting elements to fragment the rock. Most types of fixed cutter bits are polycrystalline diamond compact (PDC), natural diamond (mills), and diamond impregnated bits.

Types of Drill Bits - Rollercone vs Fixed Cutter Bits - Trendon Bit Service

A fixed cutter bit’s life depends mainly on the wear of the diamond cutters while rollercone bits life is generally limited by bearing life rather than cutter wear.

When properly selected for the formation to be drilled, a fixed cutter bit usually will have a longer life than a rollercone bit, and in general PDC bits drill faster.

PDC bits remove the rock mainly by shearing, whereas natural diamond and impreg bits drill mainly by plowing and grinding the rock. A rollercone bit removes rock by crushing, gouging and scraping.

Rollercone bits are made up of three legs welded together.

Each leg holds a rolling cone fitted with teeth or inserts. Teeth are milled directly in the cone where tungsten carbide inserts are pushed in holes that are drilled in the cone.

Rollercones that have teeth milled into them are called tooth bits. These steel teeth are brazed over with a protective hard facing which substantially increases the life of the bit. Steel tooth bits are mainly used for fast soft rock applications like surface bits or shallow wells. When the cutters are made from tungsten carbide inserts, the bit is called a TCI (tungsten carbide insert) bit. TCI bits are more durable but are not as aggressive as a steel tooth bit. TCIs are used in harder more abrasive applications.

Steel Tooth vs. TCI

Since each rollercone contains bearings that allow the rollercone to rotate, lubrication or grease is required to keep the bearings moving smoothly. A pressure regulating system and grease reservoir is required to keep the grease in the bit and to ensure the seals are not squeezed out of their housing when the bit is subjected to extremely high pressures down hole.

When drilling, the weight on the bit and rotation from the drill pipe and or downhole motor makes the cones rotate and interact with the rock. The inserts or teeth penetrate the rock which fractures and break is up. The drilling fluid from the bit cleans and removes these fragments from the bottom of the hole so that new formation is revealed and the process continues uninterrupted.

Fixed cutter drill bits use natural or synthetic diamond cutters to fragment the rock. They are sometimes called a shear bit because it removes the rock by shearing it instead of gouging and crushing the rock like a rollercone bit. A diamond bit has three main parts: the cutters, the body, and the shank. Instead of having three independently moving cones, a diamond bit has a stationary head that rotates as one piece with the drill string. Because of the increased hardness and lower impact strength of diamond cutters compared with steel teeth or tungsten carbide inserts, the design of a diamond bit is quite different from a roller cone bit. Fixed cutter drill bits use various diamond cutters (PDC, thermally stable polycrystalline TSP, natural diamond, impregnated) to fragment the rock.

Steel and Matrix Drill Bits

Steel and matrix are the commonly used materials for drilling bit bodies.

Matrix bit bodies are manufactured using a cast mold by bonding tungsten carbide powder with an alloy binder. The mold is placed in a furnace to allow the binder to melt and infiltrate the matrix powder. As the binder infiltrates the matrix powder, a solid metal casting is formed. The two types of matrix body bits incorporate PDC cutters for cutting elements or incorporate natural diamonds mechanically or impregnated. The matrix bit body remains the predominant body type used in the industry. 

Matrix PDC bits utilize PDC cutters that are brazed in place. Depending on the magnitude of wear or damage to the matrix body, the PDC cutters and be removed and replaced with new cutters allowing the PDC bit to be reused multiple times.

PDC Cutters, Trendon Bit Service, Drilling Corner, Oil and Gas Drilling, Geothermal Drilling

Steel bit bodies are made of machined steel that is precisely milled. The PDC cutters (click here to read our article on how PDC Cutters are made) are brazed in each pocket. On steel bits, hardfacing reinforces the critical areas subject to erosion such as gauge areas, the front and back blade areas and nozzle exit areas. PDC bits make up more than 90% of all drill bits used in the oil and gas industry. This number is only getting larger.

Natural Diamond Bits

On a natural diamond bit, the cutters have natural industrial diamonds arranged in rows. On the bit, the diamonds are embedded on the surface of the bit which touches the bottom of the hole. The body is the main section and holds the diamonds. The shank is a steel base for the body that gives structural strength and provides a place for the threads to make up the bit on the drill string

Natural diamond bits were used from the early days in the petroleum industry, and some are still used for specific applications. Designers were always trying to solve natural diamond bit hydraulic problems caused by overheating and degradation of the diamond. These days natural diamond bits have nearly disappeared due to the limitations of short life and cost.

Impregnated Diamond Bits

Impregnated diamond bits are similar in theory to a natural diamond bit however they utilize smaller synthetic and natural diamond particles that are infiltrated in the supporting matrix throughout the blade (not just on the surface) . As the diamonds wear and fall out the matrix is worn and new sharp diamonds are exposed. In order generate and economical rate of rock removal a high speed motor or turbine is required to spin the bit any where from 1000 to 4000 RPM.

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