Overview of drill

Drill bit is a tool used to break rock to form borehole in oil drilling. Its performance will directly affect drilling quality, drilling efficiency and drilling cost.

1. Drill bit classification

According to the type, it can be dividing into four kinds: scraper bit, cone bit, diamond bit and PDC bit.

According to the function, it is dividing into full drilling bit, core bit and special process bit (such as reaming bit, directional oblique bit, etc.).

2. Scraper drill

The scraper bit is one of the earliest types of drill used in rotary drilling. It has been using since the rotary drilling method was introducing in the 19th century and is still usibing in some fields today. The drill bit is using in soft and viscosoft formations and has a high ROP and bit penetration rate. The biggest advantage of scraper bit is simple structure, convenient manufacturing, low cost, each oilfield can design and manufacture.

1) Structure of scraper bit

The scraper bit is composing of four parts: the bit body, the scraper blade, the water parting cap and the nozzle. The bit body is the body of the scraper bit welding blade and the water parting cap. It is made of medium carbon steel. The lower end is welding with the scraper blade and the splitter cap, and the upper end is connecting with the wire buckle and drill string. Scraper blade, also known as the blade, is the main working part of the scraper drill.

2) Working principle of scraper bit

A scraper bit breaks up rock in a cutting manner. When a scraper bit works in a soft plastic formation, the cutting process is similar to that of a tool cutting soft metal. The blade eats into the formation under weight on bit, while the rock in front of the blade continues to flow in a plastic way under torsional forces, peeling away layers of rock at the bottom of the hole.

When the scraper bit drills into the brittle formation, the rock crushing process is divided into three stages: collision, crushing, small shear and large shear.

Collision: After the rock in front of the edge is broken, the torsional resistance of the rock to the blade is reducing, and the blade pushes forward and collides with the rock in front of the edge;

Crushing and small shear: the blade crushes the rock in front under the action of torsional force, so that it produces small shear crushing;

Increase of torsional force: the blade continues to squeeze the rock in front. When the torsional force increases to the limit value, large shear breakage occurs along the shear plane of the rock, and then the torsional force suddenly decreases.

The three processes of collision, crushing, small shear and large shear are repeating, forming the whole process of the scraper bit crushing brittle rock.

3) Proper use of scraper bit

Squeegee bit is suitable for soft and soft formations. During drilling, it is necessary to properly control the weight on bit and speed, and pay attention to prevent oblique, prevent breakage and prevent blade fracture.

Due to the faster drilling rate and large amount of cuttings of the scraper bit in soft strata, it is advisable to use large displacement drilling to fully clean the bottom hole and cool the bit.

When the scraper bit is drilling, the speed of the outside line of the blade is higher, and the wear speed is faster, so the bit is easy to be worn into a cone. At this time, special attention should be paid to preventing inclination and reducing the diameter of the well.

3. Cone drill

Since the introduction of the first cone bit in 1909, the cone bit has been the most widely using in the world. Tricone bit is the most common bit used in rotary drilling.

This type of bit has different tooth designs and bearing junction types, so it can adapt to a variety of formation types. In drilling operations, the right structure of the cone bit according to the properties of the formation can be using to obtain satisfactory drilling speed and bit footage.

1) Basic structure of tricone bit

Bit body: three pieces of teeth are assembling and welded together, and the upper part is connecting with a wire buckle.

Cone: A conical metal body consisting of a cone body and teeth. There are two types of teeth: milling teeth and insert teeth

Bearing and its oil storage sealing device nozzle

2) Working principle of cone bit

When a cone bit works in the bottom hole, the whole bit rotates around the bit axis, which is calling revolution, while the three cone bits roll in the bottom hole according to their own axis, which is calling rotation. The weight on bit exerted by the bit acts on the rock through the teeth, breaking the rock (crushing). In the rolling process, the cone alternately contacts the bottom hole with single teeth and double teeth, and the position of the center of the cone rises and falls, so that the bit produces longitudinal vibration. This longitudinal vibration causes the drill string to compress and stretch continuously. The lower drill string converts this periodic elastic deformation through the teeth into an impact force on the formation to break the rock. This kind of impact and crushing is the main way that cone bit breaks rock.

Besides impacting and crushing the rock at the bottom of well.

The cone bit also produces shear effect on the rock at the bottom of well. When the roller rolls at the bottom of the well, it also produces the sliding of the teeth to the bottom of the well. The sliding of the teeth forms the shearing effect on the rock at the bottom of the well, and scrapes the bottom of the well like a scraper bit. The sliding of teeth is mainly causing by the overtop, secondary cone and axis displacement arrangement of the cone.

The overtop arrangement of the cone cone leads to the tangent direction of sliding, and the axis displacement arrangement of the cone leads to the axial direction of sliding. Generally, the bit drilled in soft to medium hard formations has overtop, multiple cone and shaft shifting. Drill bits that drill hard to hard formations are designing with overtops and multiple cones; Drill bits that drill very hard and abrasive formations often use a single cone cone without overtopping or shifting the shaft.

3) Classification and selection of cone bit

Cone bit manufacturers offer a variety of types and configurations. In order to facilitate the selection and use of roller cone bits, the International Institute of Drilling Contractors (IADC) has developed a universal standard for classifying and numbering roller cone bits.

ø What do three digits of drill stand for?

According to the IADC, each type of bit is representing by a three-digit number with the following meanings:

In other words, The first digit indicates tooth type and suitable formation: 1-milling tooth, soft formation. 2- Milling teeth, medium to medium hard formation. 3- Milling teeth, hard, grinding or semi-abrasive formation. 4- Standby. 5- Inserts, soft to medium formation. 6- Inserts, medium hard formation. 7- Inserts, hard, abrasive or semi-abrasive formations; 8- Inserts, very hard, highly abrasive formation.

The second number indicates that the formation is subdividing into four hardness levels: 1,2,3,4.

The third digit indicates the structural characteristics of the bit: 1- unsealed rolling bearing; 2-T outer row tooth diameter protection. 3- Insert tooth on the gauge diameter; 4- sealed rolling bearing. 5- sealed rolling bearing with insert tooth on the gauge diameter. 6-sealed sliding bearing. 7- sealed plain bearing with insert tooth on the gauge diameter; 8- directional inclined bit; 9- Other.

4. Diamond drill bit

A diamond bit is a bit that relies on diamond particles embedded in the bore body to break up rock. Diamond is the material with the highest hardness and wear resistance known to mankind. So diamond bits are using in hard, highly abrasive formations and can achieve relatively high bit footage. Although diamonds are expensive, they are competitive in today’s oil drilling because of their wear and high per-bit penetration.

Today, diamond bits are widely using in common rotary drilling as well as turbo-drilling and coring operations. With heat-stabilizing polycrystalline diamond teeth (TSPS) being particularly popular.

1) Structure characteristics of diamond bit

Diamond bit is an integral bit, the whole bit has no moving parts. Mainly includes the bit body, crown, hydraulic structure (including water hole or nozzle, water tank also known as flow channel, chip discharge groove). Diameter protection, cutting edge (teeth) five parts. Diamond bit crown is the bit cutting rock working part, its surface (working face) is equipping with diamond material cutting teeth. The hydraulic structure is arranging. And the side of the diameter retaining part (mounting diameter retaining teeth) is connecting with the bit body. Made of tungsten carbide carcass or steel material.

The bit body is a steel material body, the upper part is connecting to the drill string by a wire buckle, and the lower part is connecting to the crown carcass (the steel crown is integrating with the bit body).

2) Working principle of diamond bit

When a diamond bit is drilling, multiple diamonds on the bit surface act on the rock simultaneously. The effect of diamond on rock fragmentation shows different characteristics in different rock properties.

In a plastic formation (or a formation in which rock is plastic under stress). Diamonds are fed into the formation under weight on bit and subjected to the torque of the bit to break or flow the rock in front of them. The rock breaking process is similar to “plowing”, so it is calling “ploughing”.

When drilling in a brittle formation, the role of diamond crushing rock is mainly “crushing”. The stress generated under the action of bit weight and torque can make the rock under the edge crack along the maximum shear stress surface. Forming a fractured groove at the back of the diamond movement. In this case, the volume of rock crushing is much larger than that after the diamond is eaten. The rock crushing has the property of volume crushing and the rock breaking efficiency is higher.

In hard strata (such as chert, siliceous dolomite, etc.). Impregnating diamond bit is usually using to drill with fine grained diamond encrusted inside the carcass. The rock breaking process is similar to that of grinding wheel.

Each diamond on the drill is a small blade tooth. And the drill can be regarding as a tool with numerous blades. When the drill drill into the rock. The sharp edges of the diamond teeth exposing on the drill are using to break the rock by micro cutting, scoring, etc. Grinding rock breaking belongs to surface crushing, rock breaking efficiency is low.

3) Correct use of diamond bit

Diamond bits are suitable for drilling medium to hard, abrasive formations and turbo-drilling, deep and ultra-deep Wells, and coring operations. Before using the diamond bit, the bottom of the well should be cleaning to ensure that no metal falls.

When the bit is just running to the bottom of the hole. It should first run with low weight on bit and low speed. The drill is then drilled with relatively low weight on bit (compared to a cone bit). Higher RPM, and larger displacement. Reaming should be avoiding as much as possible.

If reaming is necessary, use low weight on bit. Low speed, and even operation to prevent diamond breakage and excessive wear in the bit gauge portion.

5. Polycrystalline Diamond Compact Bit

PDC bit is short for polycrystalline diamond composite bit. Also known as polycrystalline diamond cutting block bit or composite chip bit. Since the introduction of PDC cutting block by General Electric in 1973. PDC bit has been widely using in oil drilling with its advantages of fast drilling speing. Long life and high penetration. Almost every bit manufacturer has adopted this technology and started producing their own PDC bit families.

1) Structural characteristics of PDC bit

PDC bit is composing of bit body, PDC cutter and nozzle and other parts. According to the different structure and manufacturing technology is dividing into steel body and tire body two series.

The whole bit body of rigid PDC bit is made of medium carbon steel material. And machining by mechanical manufacturing technology. Drill a hole on the bit working face. And fix the PDC cutters on the bit crown by pressing in tight fit.

However, The bit crown is treating by surface hardening process (spraying tungsten carbide wear layer, carburizing, etc.) to enhance its erosion resistance. The main advantage of this bit is its simple manufacturing process. The disadvantage is that the bit body is not resistant to erosion. The cutting teeth are difficult to be fixing, and the current application is less.

The upper part of PDC bit body is steel body, and the lower part is tungsten carbide wear-resistant alloy body. Which is manufacturing by powder metallurgy sintering process. The PDC cutters were welding to the reserved groove of the carcass with a low temperature solder. Tungsten carbide matrix hardness, erosion resistance, so the matrix PDC bit long life, high footage, is widely using at present.

2) Working principle of PDC bit

PDC bits break up rock by cutting. Self-sharpening cutters can easily cut into the formation due. To weight on bit and move forward to shear rock due to torque. Multiple PDC cutters work at the same time. The rock at the bottom of the hole has more free surface. And the rock is easy to be broken under shear action, so the rock breaking efficiency is high and the drilling speed is fast.

3) Correct use of PDC bit

PDC bits work best in large sections of uniformly soft to medium hard formations. Not suitable for drilling gravel beds and hard and soft cross formation.

Using low weight of bit, high speed, large displacement drilling, the use of the bit effect is good.

In addition,The bottom of the well should be cleaning before the drill bit is run to ensure that no metal falls.

When the bit is first run in the hole. It should use low bit weight and low speed and resume normal drilling when the bottom hole is formed.

PDC bits are integral bits with no moving parts and are ideal for high speed turbine drilling.