1、 Composition and function of drill string

(I) Composition of drill string

Drilling String is the general name of the steel pipe string above the drill bit and below the faucet It includes square kelly, drill pipe, drill Collar, various joints, stabilizers and other downhole tools

(II) Function of drill string

(1) Provide drilling fluid flow channel

(2) Provide weight on bit

(3) Transmission torque

(4) Tripping bit

(5) Measuring well depth

(6) Observe and understand downhole conditions (bit operation, borehole condition and formation condition)

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

(8) Drill-stem testing, also known as midway testing

1. Drill pipe

(1) Function: transmit torque and drilling fluid, and extend drill string

(2) Structure: pipe body+joint

(3) Specifications:

Wall thickness: 9-11 mm, generally 9.19 mm

Outer diameter: It depends on various drill pipe conditions, such as 127140 in common use. Length: generally about 9.5 m

Common drill pipe specifications (inner diameter, outer diameter, wall thickness, linear density, etc.)

 

 

Joint and thread

Thread connection conditions: the size is equal, the thread type is the same, and the male and female threads match

Features of drill pipe joint: larger wall thickness, larger outer diameter and higher strength

Drill pipe joint type:

Internal flat (IF), through-hole (FH), normal (REG); NC series 

● Internal flat type: mainly used for external thickening of drill pipe. It is characterized by the same inner diameter of drill pipe and small flow resistance of drilling fluid; But the outer diameter is large and easy to wear. 

● Through hole type: mainly used for internal thickening of drill pipe. Its characteristics are that the drill pipe has two inner diameters, and the flow resistance of drilling fluid is greater than that of the inner flat type, but its outer diameter is smaller than that of the inner flat type. 

● Normal type: mainly used for thickened drill pipe, drill bit and fishing tool. It is characterized by joint inner diameter< Inside diameter of thickened part< The inner diameter of the pipe body and the flow resistance of the drilling fluid are large, but the outer diameter is the smallest and the strength is large

This type of joint adopts V-type thread, but the thread type, thread pitch, taper and size are very different

● NC type series connector

NC23, NC26, NC31, NC35, NC38, NC40, NC44, NC46, NC50, NC56, NC61, NC70, NC77, etc

NC— National Coarse Thread, (USA) national standard coarse thread

xx— Indicates the pitch circle diameter of the base thread. The first two digits in inches are multiplied by 10

For example, the pitch diameter indicated by NC26 is 2.668 inches

NC thread is also V-type thread. Several NC type joints listed in Table 2-17 have the same pitch diameter as the old API standard joints

Taper, pitch and thread length can be used interchangeably

Difference between digital connector and old API connector

 

 

2. Drill collar

Structural features: screw threads are directly turned at both ends of the pipe body without special joints; Large wall thickness (38-53 mm), large weight and large rigidity

Main functions:

(1) Apply weight on bit

(2) Ensure the necessary strength under compression stress

(3) Reduce the vibration, swing and jump of the drill bit to make the drill bit work smoothly

(4) Control well deviation

Type: smooth drill collar, spiral drill collar, flat drill collar

Common dimensions: 6-1/4 ″, 7 ″, 8 ″, 9 ″

3. Kelly

Type: square, hexagonal

Features: large wall thickness and high strength

Main function: transmit torque and bear all weight of drill string

Common dimensions: 89mm (3.5 inches), 108mm (4.5 inches), 133.4mm (5.5 inches)

4. Stabilizer

Type: rigid stabilizer, non-rotating rubber sleeve stabilizer, roller stabilizer

Function: 1) anti-inclination

2) Control well trajectory

2、 Working state and stress analysis of drill string

(I) Working status of drill string

1. Under tripping condition

Vertical well: straight stretching and sliding

Inclined well: incline, bend and slide along with the wellbore

2. Under normal drilling conditions

The upper part is under tension and the lower part is under compression and bending; Rotates under torque

3. Rotary motion form of drill string

(1) The self-rotating drill string is like a flexible shaft, rotating around its own axis. Uniform wear, easy to fatigue damage

(2) The rotary drill string is like a rigid body, rotating around the borehole axis and sliding along the borehole wall. There is eccentric wear

(3) Combination of revolution and rotation The bending drill string rotates around the borehole axis and moves around its own axis

(4) The longitudinal vibration of the drill bit causes alternating stress

(5) Torsional vibration is caused by the change of the rotary resistance of the drill bit at the bottom of the well, which produces alternating torsional shear stress

(6) When the lateral shimmy reaches a certain critical speed, it may produce irregular shimmy and alternating bending stress

It is generally believed that the main form of bending drill string rotation is rotation, but it may also produce revolution or a combination of two forms of motion

Due to the effect of resistance during the rotation, the rotation of the drilling tool is unstable

(II) Stress analysis of drill string

1. Overview

(1) Tension caused by dead weight

(2) Pressure generated by WOB

(3) Buoyancy of drilling fluid

(4) Friction resistance

(5) Additional tension caused by circulating pressure drop

(6) Dynamic load generated during tripping

(7) Torque

(8) Bending stress

(9) Centrifugal force

(10) External extrusion force

(11) Alternating stress caused by vibration

2. The most stressed part of drill string:

1) Wellhead section: maximum tension and torque

2) Lower compression bending part: alternating axial stress, bending stress, torsional shear stress

3) Neutral point: (the point where the axial force on the drill string is equal to zero (N point, also known as neutral point)

Tensile and compressive alternating load

Reasons for bending of lower drill string:

The effect of WOB makes the lower drill string compressed. When the pressure reaches the critical pressure of the drill string, the drill string will lose its linear stable state and bend and contact with the wellbore

Multiple bending may occur under high pressure

Drill string design

1. Design content:

(1) Size selection

(2) Calculation of drill collar string length

(3) Strength design and verification of drill pipe string

2. Design principles

(1) Meet the requirements of strength (tensile strength, collapse strength, etc.) to ensure the safe operation of drill string

(2) Reduce the gravity of the whole drill string as much as possible, so as to drill deeper wells with the existing anti-load capacity

(I) Drill string size selection

1. Basis:

(1) Lifting capacity of drilling rig

(2) Hole size

(3) Geological conditions

(4) Process requirements

(5) Supply situation

2. Experience cooperation

 

 

Basic principles of selection:

1. Because the drill pipe is subject to the maximum torque and tension, the large size kelly should be selected as far as possible, which should be one level larger than the size of the drill pipe

2. When the lifting capacity of the drilling rig allows, it is advantageous to select large size drill pipe. Because the strength of large drill pipe is large, the water hole is large, and the drilling fluid

The flow resistance is small, and because the annulus is small, the drilling fluid upflow speed is high, which is conducive to carrying cuttings. The structure of drill string entering the well shall be as simple as possible to

For tripping operation. At present, 127 mm (5 in) drill pipe is mostly used in domestic oil fields

3. The size of the drill collar determines the effective diameter of the borehole. In order to ensure the smooth running of the casing or casing milling barrel in the drilled hole, the lowest part of the drill collar

The outer diameter of a section (generally not less than one column) shall not be less than the allowable minimum outer diameter. The allowable minimum outer diameter of drill collar is:

Allowable minimum drill collar outer diameter=2× Casing coupling outer diameter - drill diameter

When the stabilizer is used in the drill collar string, the drill collar with a slightly smaller outer diameter can be selected. The maximum outer diameter of drill collar selected in the drill collar string shall be based on the premise that it can be sleeved and milled in the possible fishing operation

Composite drill collar structure shall be adopted in the hole larger than 241.3mm. However, the outer diameter of the two adjacent drill collars should not exceed 25.4mm

4. The size of the drill collar is generally the same or similar to the outer diameter of the drill pipe joint, and sometimes the diameter of the drill collar is selected according to the anti-deviation measures. In recent years, large-diameter drill collars have been used more in the BHA, because the use of large-diameter drill collars has the following advantages: (1) use less drill collars to meet the requirements of the required drilling pressure, reduce drill collars, and also reduce the time to connect drill collars when tripping

(2) The rigidity of the drill string near the bit is improved, which is conducive to improving the working condition of the bit

(3) The clearance between the drill collar and the well wall is small, which can reduce the fatigue damage of the connecting part

(4) It is conducive to anti-inclination