This document provides an overview of directional well drilling concepts and techniques. It discusses the necessity of directional wells, types of wellbore trajectories, survey methods used to calculate trajectory, deflection tools and techniques for adjusting well path, and different mechanical and hydraulic methods for changing wellbore direction including whipstocks, jetting bits, and downhole motors. Key trajectory calculations like dogleg severity and methods for comparing actual vs planned well paths are also summarized.

1. Drilling Engineering 2 Course (2nd Ed.)

2. 1. Necessity of Directional well
2. well’s trajectory
3. Major Types of Wellbore Trajectories
4. Trajectory rule of thumbs and terms
5. Trajectory Calculation

3. 1. the Survey of a Well
2. Calculating the Survey of a Well
3. Deflection Tools and Techniques
4. Hydraulic Method (Jetting)
5. Mechanical Methods

5. survey stations
When drilling a well, inclination, azimuth and MD
are measured at various so called “survey stations”.
This is done with survey tools
to check the actual traverse of the well.
These measurements are then used for
(a) estimation of the real trajectory path,
(b) comparison with the planned well trajectory and
(c) planning necessary steps to re-direct the well
to reach the desired location.
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6. Survey tools
Sketch of a Single shot and Multishot tool Sketch of a gyroscope
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7. Survey tools
The tools to measure the inclination and
azimuth at the survey stations can be
as simple as dropping tools (totco, measures only
inclination, thus only used for vertical wells)
like single (one measurement per tool run) or
multishot magnetic instruments and gyroscopes, or
sophisticated
measurement while drilling tools that
• are assembled within the drillstring (close to the bit) and nearly
continuously measure the desired directional parameters or
logging while drilling tools that
• also make measurements of the formations penetrated for online
trajectory re-designing (e.g. following a geological horizon).
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8. full survey
The direction angles obtained by magnetic tools
must be corrected for true north and
the gyroscope corrected for drift
since the magnetic north
does not coincides with the true north.
Next Figure shows a map of these correction angles
for various locations.
With these corrected azimuth and inclination
values, a so called full survey
(containing TVD, horizontal departure, etc.) is calculated.
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9. Declination map
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11. Full survey methods
To obtain the full survey
from MD, inclination and
azimuth, various methods
that depend on different
models are proposed.
Below is a list of the most
popular ones:
Acceleration method
Average angle method
Angle-averaging method
Backward station method
Balanced tangential method
Circular arc method
Compensated acceleration
method
Mercury method
Minimum curvature method
 (the most accurate)
Quadratic method
Radius of curvature method
 (the most accurate)
Secant method
Tangential method
 (the simplest one but gives
inaccurate results)
Terminal angle method
Trapezoidal method
Vector averaging method
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12. Average angle method
α[◦] hole angle
𝜖[radian] azimuth
DMi [ft] measured depth between two survey stations
Li[ft] north/south coordinate between the two stations
Mi [ft] east/west coordinate between the two stations
The total north/south and east/west coordinates
and the TVD value are computed with:
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13. Radius of curvature method
where:
When < 0.25 [radians], F
can be set to 1.0
𝛽[radians]
curvature or the dogleg
F [1] ratio factor used
to smooth the wellbore
between the two survey
stations
𝛼1 inclination at station 1
𝛼2 inclination at station 2
𝜖1 azimuth at station 1
𝜖1 azimuth at station 2
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14. Minimum Curvature Method
where:
DL [/100 ft] dog leg
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15. Dogleg Severity
By definition, a dogleg is
a sudden change of inclination and/or
direction of a well’s trajectory.
For description purpose, the change is usually
expressed in a 100-[ft] interval ([◦/ 100 ft]) and
called “dogleg severity”.
As it has been seen in practice,
large dogleg severities can lead to
failure of drillpipe, drill collar or tool joints as well as
create so called “keyseats”
which result in stuck drillstrings.
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16. Dogleg Severity Calculations
To obtain the dogleg severity, the survey calculated
with one of the methods described above is used
along with following equations:
δ [◦/100 ft] dogleg severity
β [◦] total angle of change (turn)
αN [◦] new inclination
Δ𝜖 [◦] change in azimuth
∆α [◦] change in inclination
Lc [ft] length over which change of trajectory occurs
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17. drillpipe protector
To reduce the wear-
effect of large dogleg
severities, the add of
multiple steel or rubber
drillpipe protectors
which are
cylindrical pieces,
having an outside
diameter equal to
the outside diameter of
the tool joints, have
proven to be efficient.
drillpipe protectors
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20. actual vs. planned trajectory
The methods presented
above calculate
the trajectory path of the
well as it is drilled.
The actual trajectory is
constantly compared
with the planned one and
in case the actual one is
going of course
(which it always does
to some extend),
correction steps to bring
the trajectory on course
again have to be taken.
To correct
the course of the well
in case of minor
deflections,
an experienced driller can
vary the individual drilling
parameters
(WOB, RPM, etc.)
to adjust for
the of-going trajectory.
In case the trajectory is
largely of course,
a deflection tool
has to be run and
drilling in sliding mode
(e.g. positive displacement
motor (PDM)) carried out
to make the necessary
correction.
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21. The well direction determination
To determine
the direction the well is
drilled in sliding mode,
the bottom hole assembly
containing the deflection
tool is rotated
from the surface
by rotating
the whole drill string.
Then, either a so called
“Ragland vector diagram”
or the following
equations are applied
to compute the tool face
orientation.
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22. Calculation of the new inclination and
direction angles
The computational results have shown to be more
accurate than the graphical one gained from the
Ragland vector diagram. The tool face angle is given
by:
The new inclination and direction angles are:
for γ is right of high side of the borehole:
and for γ is left of high side of the borehole:
𝜖 𝑁 [◦] new direction of the trajectory
αN [◦] new inclination of the trajectory
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23. special deflection tools
Although rotary
assemblies can be
designed to alter the path
of the wellbore,
there are certain
circumstances
where it is necessary to use
special deflection tools,
for example kicking-off and
sidetracking.
These special tools include
jetting bits, whipstocks, and
downhole motors with a
deflection device.
In drilling operations,
the bit is forced under
weight and rotation to cut
a certain diameter hole.
As the bit penetrates along
the vertical axis,
it also moves laterally.
This movement can range
from very small to
considerable displacement.
This displacement
can be represented
in three dimensions.
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24. The various methods of deviations
Directional drilling is
to cause the bit to deviate in a controlled manner.
The various methods used
to induce the bit to build, drop and turn
can be classed into mechanical and hydraulic methods
besides the natural formation effects.
Mechanical techniques include whipstocks,
bottom hole assemblies, and down hole motors
with bending device.
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25. Different methods to deflect
the trajectory of a borehole
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26. Natural Formation Effects
The formations encountered when drilling oil wells
are very rarely homogeneous and isotropic.
One is more likely to find a sequence of different layers,
with each layer having its own drillability characteristics.
The bit may have to drill through
alternating layers of hard and soft rocks.
Furthermore,
these strata may not be lying evenly in horizontal beds
but instead be dipping at some angle.
The geology may be further complicated
by faulting and folding of the strata.
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27. Effect of dipping on well deviation
As the bit drills across a formation boundary,
it will tend to be deflected from its original course.
Experience has shown that
where the formations are steeply dipping
(greater than about 60◦)
the bit tends to drill parallel with the bedding planes.
Where the formation dip is less steep,
the bit tends to drill at right angles to the bedding planes.
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28. Bit walk & drilling related effects
In addition to changes in inclination
there may also be changes in direction
in which case the bit will tend to walk.
Under normal rotary drilling,
the bit will tend to walk to the right,
but with a downhole motor
the effect of reactive torque may force it to the left.
Drilling parameters such as:
weight on bit, RPM, and hydraulics,
will also affect the amount of deviation.
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30. Jetting technique
This method of changing the trajectory
of a wellbore
requires the use of a jetting bit
to wash away the formation.
Water or drilling mud is pumped
through a large jet that is oriented
in the direction of the desired trajectory change.
Jetting is a technique best suited to
soft-medium formation in which
the compressive strength is relatively low and
hydraulic power can be used
to wash away a pocket of the formation to initiate deflection.
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31. Bit design of jetting technique
The amount of inclination produced is also related to
the type of bottom hole assembly used with the jetting bits.
There are two commercial bits especially designed for
the jetting technique.
One is a two-cone bit
with an extended jet replacing the third cone and
the second one is a conventional three cone bit
with two small and one large “big eye” jet.
The actual design of the jetting process is a function of
hole size, pump capacity, expected formation hardness, and
the desired bit cleaning efficiency while drilling.
Compared to trajectory deflection
using a whipstock or downhole motors,
jetting is the most approximate method.
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32. Procedure of jetting technique
On any particular run,
the bit is mounted on an assembly, which
includes an orienting sub and a full-gauge stabilizer near the bit.
Once the bit touches the bottom,
the large nozzle is oriented in the required direction.
Maximum circulation rate is used
to begin washing without rotating the drill string.
The pipe is worked up and down with continuous jetting,
until a pocket is washed away.
At this stage the drill string can be rotated
to ream out the pocket
and continue building angle
as more weight is applied to the bit.
Surveys are taken frequently
to ensure that the inclination and direction are correct.
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33. pros and cons of jetting technique
Advantages of this method are:
several attempts can be made to initiate deflection
without pulling out of the hole,
a full gauge hole can be drilled from the beginning.
Disadvantages of this method are:
the technique is limited to soft-medium formations,
severe dog-legs can occur
if the jetting is not carefully controlled,
on smaller rigs there may not be enough pump capacity
to wash away the formation.
In summary, jetting is a very cost-efficient giving that
kicking-off takes place under suitable geological conditions.
In general it requires a good directional monitoring.
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35. mechanical methods
All mechanical methods rely on
the application of an appropriate side force
which causes the bit to deviate.
When the imposed side force on the bit is positive,
an angle is build up,
when it is negative, the force drops the angle.
Common mechanical techniques used
to deflect the bit are:
whipstocks,
downhole motors with bending device,
and bottom hole assembly.
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36. Whipstocks
The whipstock method to deviate a bit is
the oldest technique and,
if properly used, the most reliable one.
In comparison with other alternative methods,
it is the most time consuming one.
A whipstock can be
as simple as a kick-off sub at the end of
a conductor pipe or casing, or it can be
a more sophisticated retrievable jetting whipstock.
Although there are a number of variations
all whipstocks work on the principle of creating a curvature.
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37. Whipstock application
The successful use of a whipstock is largely
a matter of knowing when to run a whipstock
in relation to other mechanical or hydraulic devices.
In today’s industry,
the whipstock is predominately used for
sidetracking out the casing pipe,
which is called “casing whipstock”.
also it can be used to side-track out the open hole
when hydraulic jetting or running a mud motor
fails to deviate the well.
Which is called “open hole whipstock”.
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38. Deflection with a Whipstock
A whipstock can be
described as a steel
wedge with a chisel
shaped point at the
bottom.
This chisel shape
prevents the whipstock
from turning when
drilling begins.
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39. The whipstock installation
The whipstock, that is run down hole,
is attached to the lower end of the drill string
by means of a shear pin.
It is either set on the bottom or anchored and locked in
a packer which was previously installed in a casing string.
A modern whipstock has a tapered concave groove,
called the tool face,
which helps in orienting the whipstock.
Once it is installed down hole,
it guides the bit or the mill against the casing or
the open hole wall to drill in the desired direction.
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40. Sample of whipstocks
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41. 1. Dipl.-Ing. Wolfgang F. Prassl. “Drilling
Engineering.” Master of Petroleum
Engineering. Curtin University of Technology,
2001. Chapter 9

42. 1. Whipstock Running Procedures
2. Adjustable bent sub above motor
3. Motor housing with one or two bends
4. Offset Stabilizer on Motor
5. While Drilling Techniques
A. Data Transfer