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Knowledge base of drill assembly

Table of Contents

The drill string concept

Drill string is a general term for all downhole drilling tools from kelly to drill bit, consisting of kelly, drill pipe, drill collar, stabilizer joint and other accessories. The function is to start the drill bit, transfer the mechanical energy needed to break the rock to the drill bit, apply bit weight to the bottom of the hole, transport well washing fluid to the well and carry out other downhole operations. Once the drill string has an accident, it will bring some economic losses. Therefore, good management, good use and reasonable selection of drill assembly are particularly important in the drilling process.

Function of drill string
1) Provide drilling fluid flow channels;

2) Provide weight on bit;

3) Transfer torque;

4) Pull off the drill;

5) Measuring well depth.

6) Observe and understand downhole conditions (drilling conditions, borehole conditions, formation conditions);

7) Other special operations (coring, cement squeezing, fishing, etc.);

8) Drill Stem Testing (drill-stem testing), also known as midway testing.

Drill string working status
In the drilling process, the drill string works alternately in the two steps of tripping and normal drilling. When tripping, the drill string is in tension state. While in the drilling state is more complex, in tension, pressure, torsion and other states. In the rotary drilling, the working state and force of the drill string are especially complicated, and the drill string is like a slender rotating shaft. Under the axial pressure caused by part of the dead weight, the lower drill string is unstable and bending. Due to the limitation of borehole, it can bend several times. Because of the centrifugal force produced by rotation, the upper drill string does not keep a straight state, and coupled with the action of torsion, the whole drill string takes on a complex rotating motion in the form of an approximate spiral curve.

Common drill tool
1, kelly (kelly)
The kelly pipe is located at the top of the drill string, and its main role is to transfer the torque and withstand the total amount of the drill string. The end face of the drive part of the kelly pipe is divided into square and hexagonal, the most used in oil drilling is square, and the water eye is hexagonal. Because the wall thickness is about three times that of the drill pipe, and is made of high-strength alloy steel, it has high tensile strength and torsional strength.

  1. drill pipe
    The drill pipe is the basic component of the drill string, which is mainly used to transfer torsion and transport drilling fluid. The pipe body and joint of the drill pipe are connected together by butt welding method. In order to increase the strength of the joint, the butt welded part at both ends of the tube body is thickened, and the thickening form has three kinds of internal thickening, external thickening and internal and external thickening.
    The inner thickening reduces the inner diameter of both ends of the pipe body to increase the thickness of the pipe wall. The outer diameter of the drill pipe is the same, and the outer diameter of the joint is not too large. When rotating in the well, the contact between the joint and the well wall is small, and the wear is also small.

The inner diameter of the thickened drill pipe is the same, but the outer diameter of both ends of the pipe body is increased, and the outer diameter of the drill pipe joint is larger than that of the same size drill pipe joint, which increases the contact friction between the joint and the wall when rotating in the well, and is easy to wear. Because the inner diameter of the drill pipe is the same as the inner diameter of the pipe, the flow resistance of the circulating drilling fluid is less.
The internal and external thickening of drill pipe is to reduce the inner diameter of both ends of the pipe body and increase the outer diameter to increase the wall thickness of both ends of the pipe body. The drill pipe of this structure combines the advantages of the above two kinds of drill pipe.

3, extra-weight drill pipe (extra-weight drill pipe)
The heavy drill pipe is characterized by more than 2 times the wall thickness than the ordinary drill pipe, the joint body is longer than the ordinary drill pipe, and the middle of the pipe body has a special wear roller. The heavy drill pipe is mainly used in the following aspects:
1) It is used for the transition zone of drill collar and drill pipe to alleviate the change of bending stress of both to reduce the damage of drill pipe;

2) Replace drill collar in small hole drilling, easy to operate;
3) Replace most of the drill collar in the directional well to reduce the occurrence of torsion and sticking, so as to reduce the cost and help maintain the orientation of the directional well. Due to the large contact area between the drill collar and the tight hole, there is a lot of friction between the drill collar and the wall when rotating, so that the hole has the tendency of deflection. When the heavy drill pipe is used instead of the drill collar, this possibility can be reduced, and it is conducive to maintaining the orientation of the well.

  1. drill collar
    The drill collar is one of the main components of the drill string, its role is to provide bit weight, while making the BHA have a greater stiffness, so as to make the bit work stable, and help to overcome the hole inclination problem. Drill collar classification:

1) Cylindrical type: a drill collar made of ordinary alloy steel and round inside and outside the cross section of the tube body;

2) Spiral type: a drill collar made of ordinary alloy steel with a spiral groove on the surface outside the tube;
3) Non-magnetic type: made of stainless alloy steel with very low permeability, the pipe body cross-section inside and outside diameter are round drill collar. The types of drill collars are also square drill collars and non-magnetic spiral drill collars.

  1. spiral dirll collar
    The function of the spiral drill collar is to reduce the occurrence of sticking. Three right-handed spiral grooves are processed on the outer cylinder of the round drill collar, and a segment of 305-560mm is left in the outer thread joint part and 475-610mm is left in the inner thread end joint part. The weight of the spiral drill collar is 4% less than that of the same size round drill collar. Spiral drill collar is not included in the API standard, but there are many manufacturers in foreign countries, and domestic drill collar has been officially put into production, and its specifications and dimensions are the same as foreign products.

6, drilling tool stabilizer (stablizer)
1) Stabilizer commonly known as centralizer, a certain number of stabilizers are placed in the suitable position of the drill string to form the lower drilling assembly of the drill string, which can prevent the inclination of the well when drilling a straight well and control the trajectory of the hole when drilling a directional well.

2) The use of stabilizer can play a role in improving the stability of the bit, so as to extend the service life of the bit, which is particularly important for diamond bits.

Commonly used stabilizers are divided into three roller stabilizers, replaceable sleeve stabilizers, integral spiral stabilizers, and integral straight edge stabilizers.

7, non-magnetic drill collar (non-magnetic drill collar)
The structure of non-magnetic drill collars is the same as that of ordinary drill collars, but the chemical composition and mechanical properties are different from ordinary drill collars. Non-magnetic drill collar is a kind of drilling tool with special function in directional well engineering. When the magnetic clinometer is used to measure the inclination Angle and azimuth Angle of the hole in a directional well, in order to prevent the compass from being affected by the magnetic field of the drill string, the drill string must be equipped with a certain length of non-magnetic drill collar, so that the magnetic clinometer can be located in a suitable position in the non-magnetic drill collar to measure the inclination, otherwise the correct measurement results cannot be obtained.

  1. drill assembly
    The drill pipe is the hub connecting the surface and the underground. In rotary drilling, the drill string is used to transfer the energy needed to break rock, apply weight on bit to the bottom of the well, and transport drilling fluid in the well. When drilling with downhole power drilling tools, it withstands the reverse torque of downhole power drilling tools and delivers liquid energy to downhole power drilling tools at the same time. Drilling tools are composed of different components, and their composition varies with the drilling conditions and methods. The basic components are: kelly pipe, drill pipe, drill collar, drill pipe stabilizer and converter joint. With the continuous development of advanced drilling technology, the structure and performance of drill string are more and more demanding, and reliable drill string must be selected.

Common Basic Drill Assembly (BHA)

  1. Full eye drill assembly

Full hole drill assembly, also known as rigid fit drill assembly or rigid full hole drill assembly, is a kind of drill assembly installed at the lower part of the drill string with large stiffness and small gap between the hole diameter and the drill string outer diameter to prevent the hole inclination and hole curvature. The rigid full-hole drilling tool is generally composed of several centralizer with outer diameter similar to the drill bit diameter and a large drill collar with a certain length of outer diameter.

Its anti-skew principle is to install a certain number of centralizer on the lower drill string above the bit to centralize the drill collar; Improve the stiffness of the lower drill string and reduce its bending degree to eliminate the severe tilt of the bit, so that it can reduce and limit the increasing inclination force caused by the bending of the drill string. At the same time, the centralizer can be supported on the wall of the well to contend with the natural formation inclination force, so as to achieve the purpose of controlling the change of the well inclination in the minimum range.

In order to play the role of full hole drilling tool, there must be at least three stable points on the drilling tool, in addition to a centralizer near the drill bit, it should be placed on the above two centralizer to maintain three points in contact with the wall. If only two points of contact, the drill string can follow a curve, can not guarantee the straightness of the hole. If there are three points of contact, it can ensure the straightness of the hole and limit the lateral movement of the bit.

  1. Pendulum drill assembly

Pendulum drill tool is a kind of drill assembly designed to reduce the inclination of the well. It is to use the transverse component of the drill collar weight below the tangent point in the inclined well to push the bit to the lower side of the well wall to achieve the effect of gradually reducing the inclination of the well. This transverse component is like a pendulum, so it is called “pendulum force”, and the drill tool combined with this principle is called a pendulum drill.

3, tower drill assembly

A tower drill tool is a combination of drill collars whose diameter is gradually reduced from bottom to bottom on top of the drill bit. Drill collar should not be less than 12, the characteristics of this anti-inclination drilling tool is the bottom drill collar weight, stiffness, low center of gravity of the whole drill collar column, good stability, can produce a large pendulum reduction force. In soft formation, hole diameter is easy to expand, for centralizer full hole drilling tool or centralizer pendulum drilling tool, because of the large gap between hole diameter and centralizer, the anti-skew effect is poor.

In addition, the tower drill has the advantages of simple structure, convenient use, no need to calculate the centralizer position, and no problems such as the wear and repair of the centralizer and square drill collar, but the tower drill also has the shortcomings of small bottom clearance, easy to stick, large drill collar size, and convenient operation. The anti-skew effect of the tower drilling tool depends on the tower combination of the drilling tool. It requires low center of gravity, large diameter of bottom drill collar, large weight of whole drill collar and small size difference of each stage drill collar.


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