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• Maintain a full gauge hole

Figure 40 An integral blade stabilizer.

The use of a reaming stabilizer provides the cleanest and straightest hole, as the reamer centralizes the bit and the drill string in the hole. Apart from the problems involved when attempting to log, run casing, cement, complete and test, bit life is drastically reduced and rate of penetration is slowed, as rings of uncut bottom develop due to drilling off-center.

For best results, stabilizers should be between 3 mm and 6 mm (¹/8" and ¼") less in diameter than the drill bit. The first stabilizer should be positioned as close as possible behind the drill bit with the next stabilizer following the first or second rigid drill collar.

A reamer is a sub that is run between the bit and drill collars to provide gauge hole while drilling in hard formations.

The bit sub is a short sub with a box on both ends. This allows the bit to be joined to the drill string.

7.5.6 Bits

The drill bit is probably the most critical item of a rotary rig operation. It is the most refined of the rotary-rig tools, available in many styles, and is more highly specialized for every condition of drilling than any other tool on the rig. To select the proper bit, some information must be known about the nature of the rocks to be drilled. There are four main types of bits used for rotary drilling. There are several variations within these types, primarily based on the cutting structure used for drilling the rock.

There are four main types of bits:

Drag bits

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Roller cone or rock bits (steel-tooth and insert bits)

Polycrystalline diamond compact (PDC) bits

Most bits have nozzles or jets that direct a high velocity stream of mud to the sides and bottom of the bit.

Two other drilling tools used during rig operations are:

Reamers Hole Openers 7.5.6.1 Drag Bits

The earliest rotary drill bit designs were the drag and fishtail bits, which sheared and scraped soft formation. These bits are manufactured from high-grade, heat-treated steel, either forged or cast according to size. Blades are wing-shaped with thinner cutting edges. Today the cutting edges are faced with tungsten carbide inserts that can be sharpened. The bit body has a central hole. Fluid flushes cuttings away from the cutting edges keeping them clean and cool.

Drag bits are available in either three-way or four-way design with wings shapes of regular step-type for softer formations and heavy duty for harder consolidated formations.

Figure 41 4-wing drag bits.

Unfortunately, these bits could not be used to drill harder formations. They were made obsolete by the introduction of roller bits. They could only be used in soft formations, and are rarely used today.

7.5.6.2 Roller Cone Bits

Roller cone bits, commonly called tri-cone bits, are the most common bits used today. The bit has three cones that are free to turn as the bit rotates, a forged steel body and a cone axle or pin that forms part of the body. The cones have roller bearings fitted at the time of assembly. The shape of the cutting surfaces and the design of the teeth are varied to drill in a variety of formations. Some have inter-fitting or self-cleaning teeth. All cutting surfaces are flushed or cleaned by the circulating fluid.

There are two types of rock bits, depending on how the teeth were manufactured:

Milled-tooth bits

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7.5.6.2.1 Milled-Tooth Bits

Milled-tooth bits have steel teeth that have been milled on the cones. The teeth of the bit are responsible for cutting or gouging out the formation as the bit is rotated. The teeth vary in size and shape, depending on the formation they are expected to drill.

The teeth may be long, slender and widely spaced for drilling “soft” formations or short and broad for drilling hard formations. For longer life, the steel teeth may be hard-faced with abrasion-resistant material.

Figure 42 A milled-tooth bit.

Long slender tooth-bits will produce large, freshly broken cuttings from soft formations.

Broad tooth-bits and insert bits will produce smaller, more rounded, crushed, and ground cuttings from hard formations.

The rock bit has numerous bearings and seals, and therefore their cutting ability and cone rotation deteriorate in a short time, causing a decline in the penetration rate and the quality of the cuttings.

7.5.6.2.2 Insert bits

These bits generally have tungsten carbide inserts (teeth) are pressed into the cones instead of steel teeth milled into the cones. Because of this, these type of bits are called Tungsten Carbide Insert (TCI) bits. These bits are specially designed for use in hard abrasive formations, such as limestones, dolomites, granites and hard, sandy shales normally too hard for steel tooth bits. They last longer when drilling hard formations. The inserts can come in a variety of shapes, from long chisel shapes for firm formations to short round buttons for hard, brittle formations. The diamond gage chisel insert is the latest development in insert bit technology.

Figure 43 Insert bits.

Figure 44 Different types of inserts.

7.5.6.3 Diamond Bit

Diamond bits are composed of three parts:

Diamonds

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Matrix Shank

Diamond bits consist of a solid powdered tungsten carbide composite matrix body, which is bonded to the steel shank, with central ports for mud circulation. They do not have cones and teeth; instead they have several boart-grade diamonds. The diamonds are held in place by the matrix and are embedded into the bottom and sides of the bit. The cutting action of the bit depends upon grinding and scraping as the bit rotates on bottom. This produces fine cuttings and rock flour, which, due to frictional heating, often show signs of thermal degradation.

Diamond bits are usually slightly smaller than tri-cone bits to prevent diamond damage while being run into the borehole. The design of diamond bits varies greatly in the shape of the head, the size and setting of the diamonds, and the watercourses for cooling.

The advantages of diamond bits are, because of the simplicity of its design and the hardness of its cutting structure, its long drilling life and they can drill almost any formation.

Its disadvantages are:

High cost Slow ROP

Unresponsive to changes in lithology Poor quality of drill cuttings

Figure 45 A diamond bit.

7.5.6.4 Polycrystalline Bits

Polycrystalline (PDC) drill bits are sometimes called Stratapax or TSP (Thermally Stable Products). The PDC bit has blades studded with large, hard, abrasion-resistant drilling blanks manufactured from a bonded mass of microcrystalline synthetic diamonds. For harder formations, the PDC bit may look like a diamond bit, but the small diamonds are replaced with larger PDC blanks.

Figure 46 A PDC bit.

PDC bits produce large, fresh, sheared cuttings with little surface damage or mechanical deformation. They are relatively cheap and drill very quickly in soft to moderately hard formations. In very hard formations or when drilling with unsuitable drilling parameters PDC bits can produce churned and ground cuttings (almost as bad as the diamond bits).

The cutting structure of these bits is composed of man-made diamond dust/crystals bonded to a tungsten carbide stud. These studs are then either pressed or molded into the bit body. Because of the crystal structure in the cutter, it is self-sharpening (exposing new crystals while others are broken off). They are used in soft to medium-hard formations.

7.5.6.5 Under-reamers and Hole Openers

Hole Openers and under-reamers are tools that are run immediately above a bit to enlarge the hole size. Rotating cones built out from the central stem does hole opening. Both types perform similar functions, but the cones of an under-reamer are built into retractable arms that are held out while drilling by the pressure of the drilling fluid being circulated down through the center of the tool. In this way, it may be run in

or pulled out through a smaller diameter of the borehole before the arms are extended and underreaming operations begin.

Figure 47 Hole openers.