Title Chapter 03 Dogleg Serverity Directional and Horizontal Drilling 262.7 KB 14
```                            Dogleg Severity
Introduction
Example 31
Example 32
Example 33
Example 34
Problems
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##### Document Text Contents
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Chapter 3 DOGLEG SEVERITY
INTRODUCTION

Dogleg severity is a measure of the amount of change in the inclination, and/or azimuth of a
borehole, usually expressed in degrees per 100 feet of course length. In the metric system, it is
usually expressed in degrees per 30 meters or degrees per 10 meters of course length. All
directional wells have changes in the wellbore course and, therefore, have some doglegs. If
not, it would not be a directional well. The dogleg severity is low if the changes in inclination
and/or azimuth are small or occur over a long interval of course length. The dogleg severity is
high when the inclination and/or azimuth changes quickly or occur over a short interval of
course length.

To show how a change in inclination can affect dogleg severity, consider the following example:

Example 3-1
Given: MD1 = 1,000 feet MD2 = 1,100 ft

I1 = 4° I2 = 6°

Determine: The dogleg severity.

Solution: The change in inclination is:

12 III

246I

The course length over which the change in inclination occurred is:

12 MDMDMD

ftMD 100'000,1'100,1

Calculation of dogleg severity:

MD
I

DLS

100
2

DLS

Therefore, the dogleg severity is 2°/100 feet.

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Suppose is equal to 8°, then: 2I

12 III

448I

12 MDMDMD

ftMD 100'000,1'100,1

MD
I

DLS

100
4

DLS

' 100/4DLS

The dogleg severity is 4°/100 feet. A greater change in inclination yields
a larger dogleg severity.

To show how the change in course length can affect dogleg severity, consider the following
example:

Example 3-2
Given: MD1 = 1,000 feet MD2 = 1,050 feet

I1 = 4° I2 = 6°

Determine: The dogleg severity.

Solution: 12 III

246I

12 MDMDMD

ftMD 50'000,1'050,1

50
2

DLS

2
2

50
2�\$

DLS

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D o g l e g S e v e r i t y

Figure 3-1. Chart for Determining Dogleg Severity

Doglegs are not necessarily a problem in directional wells. When a dogleg becomes a problem,
then it is considered severe. One of the immediate problems associated with doglegs is torque
and drag. More severe doglegs will cause higher torque and drag. The drill string will
experience less torque from a dogleg while drilling, because the collars are in compression
except in the case of a horizontal well. They accommodate themselves to the changes in hole
curvature. However, while tripping or reaming, the torque will be greater because the collars
are in tension. Care should be taken when tripping after a significant change in hole inclination
and/or direction. The assembly may go to the bottom, but it might not come back up through

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the dogleg. An assembly should never be forced to the bottom; it should be reamed to the
bottom.

Torque and drag are caused by the friction between the drill string and the borehole wall. When
the drill string is in tension, it tries to straighten while going around a dogleg. The drill string
exerts a force on the formation as shown in Figure 3-2. As tension on the drill string increases,
depth below dogleg increases and the lateral force increases; therefore, the torque and drag
increase.

The torque and drag can be reduced by several
different means. One method is to keep the
dogleg severity low. Once a severe dogleg exists
in the wellbore, its effect can be decreased
somewhat by reaming but only by a small amount.
Torque and drag can be reduced using lubricants
in the mud system. Oil and other commercially
available lubricants reduce the coefficient of
friction between the drill string and borehole wall;
thereby, reducing the torque and drag. Another
method is to reduce the tension in the drill string.
This can be accomplished by removing excess
collars, or replacing the collar with heviwate drill
pipe. The heviwate drill pipe is more flexible and
reduces the overall string weight while maintaining
the same available bit weight.

As drilling continues, the drill string tension in the
dogleg increases which increases the lateral force.
The lateral force causes the drill string to cut into
the wellbore wall at the dogleg. A keyseat is
formed if the lateral force is large enough to cut
into the wall. Soft formations require a lower force
than hard formations to form a keyseat.

Other problems associated with severe doglegs
are wearing of tool joints and worn spots in the
casing which can lead to collapse. Logging tools
and collars can become stuck in a keyseat.

Drill pipe fatigue is also associated with doglegs.
Most failures in drill pipe are fatigue failures
resulting from gradual progressive growth of minor
irregularities into major cracks even when the
stresses are less than the yield strength of the
metal. Figure 3-2 illustrates how a severe dogleg
can cause fatigue failures. Point “A” on the drill
pipe is in maximum tension while point “B” is in
minimum tension due to bending. (If there is no
weight hanging below the joint of drill pipe, point “A” would be in tension and point “B” would be
in compression.) As the pipe is rotated, the reference points go through cyclic stress reversals.
Point “A” goes from maximum tension to minimum tension and back to maximum tension on

Figure 3-2. Bending of Drill Pipe in a Dogleg,
Rotation causes Cyclic Stress Reversals

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feetDLS 100/3.18
5.4218

18000 o

Higher dogleg severities can be tolerated if the tension in the drill pipe is very low. Medium
radius horizontal wells can be drilled without causing significant fatigue damage to the drill pipe
because the tension in the dogleg is very low. The dogleg severity in a normal directional well
has to be lower at the kickoff point because the tension will be a maximum at that point. The
deeper the dogleg, the greater the dogleg severity that can be tolerated without causing fatigue.

PROBLEMS

1 Given the following survey data, calculate the dogleg severity.

MD1 = 100 feet MD2 = 200 feet
I1 = 1º I2 = 1º
A1 = 0º A2 = 180º

2 Given the following survey data, calculate the dogleg severity.

MD1 = 1200 feet MD2 = 1264 feet
I1 = 10º I2 = 11.5º
A1 = S48ºW A2 = S56ºW

NOMENCLATURE

A = Azimuth, degrees

DLS = Dogleg severity, degrees per 100 feet

pD = Outside diameter of the pipe, inches

I = Inclination, degrees

MD = Measured Depth

b = Bending stress, psi

= Denotes change in parameter

1 = Subscript denotes upper survey

2 = Subscript denotes lower survey

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REFERENCES

i Lubinski, A.; “Maximum Permissible Dog-Legs in Rotary Boreholes,” Journal of Petroleum

Technology, February, 1961.