Directional Drilling 2020-2021.pdf

[2020-2021]
Lecturer: Dr. Sahmi Eddwan Mohammed
UniversityofKirkuk-College of
Engineering-PetroleumDepartment
[
DIRECTIONAL, HORIZONTAL AND
MULTILATERAL DRILLING

Chapter Four Lecture- - -2021
PetroleumDepartment FourthStage Drilling Engineering II
Directional, Horizontal andMultilateral Drilling
1
 Introduction
Directional drilling has become a very important tool in the development of oil and gas
deposits. Current expenditures for hydrocarbon production have dictated the necessity of
controlled directional drilling to a much larger extent than previously.
Probably the most important aspect of controlled directional drilling is that it enables
producers all over the world to develop subsurface deposits that could be reached
economically in any other manner.
 Definition of Directional Drilling
Controlled directional drilling is the science and art of deviating a wellbore along a planned
course from a starting location to a target location, both defined with a given coordinate
system.
 Description of Directional Drilling
 Directional Drilling Applications
There are many reasons for drilling a non-vertical (deviated) well;
 Sidetracking.
 Inaccessible locations.
 Salt dome drilling.
Horizontal, Directional and Multilateral Drilling
Drilling a directional well basically
involves drilling a hole from one point in
space (the surface location) to another point
in space (the target) in such a way that the
hole can then be used for its intended
purpose.
A typical directional well starts off with a
vertical hole, then kicks off so that the
bottom hole location may end up hundreds
or thousands of feet or meters away from
its starting point.
With the use of directional drilling, several
wells can be drilled into a reservoir from a
single platform.

2
 Fault controlling.
 Multiple exploration wells from a single wellbore.
 Onshore drilling to offshore locations.
 Offshore multiwell drilling.
 Relief wells.
 Controlling vertical wells.
 Horizontal wells
 Extended reach wells
 Multilateral wells
 Short, medium, and long radius wells
 Applications of Directional drilling
There are many reasons for drilling a non-vertical (deviated) well. Some typical
applications of directionally controlled drilling are shown in Figure (5-1).
Fig. (5-1): Applications of directional drilling.

3
 Sidetracking
Sidetracking was the original
directional drilling technique used to
get past fish (obstructions).
Oriented sidetracks, the most common
type of sidetracking, are performed
when there are unexpected changes in
geology and obstructions in the path of
the wellbore.
Fig. (5-2): Sidetracking
 Inaccessible Locations
Inaccessible locations such as targets
located beneath cities, rivers or
environmentally sensitive areas make it
necessary to locate the drilling rig some
distance away from the target.
Fig. (5-3): Inaccessible locations.
 Salt Dome Drilling
Salt domes have been found to be
natural traps of hydrocarbons
accumulating beneath the overhanging
hard cap. A directional well is used to
reach the trapped reservoir to prevent
the problems associated with drilling a
well through the salt formation.
Fig. (5-4): Salt dome drilling.

4
 Multiple Exploration Wells
from a Single Wellbore
Multiple exploration wells from a single
wellbore use a single hole to drill
multiple new wells by deviating away
from the original well at a certain depth.
It allows the exploration of structural
locations without drilling another
complete well.
Fig. (5-6): Multiple Exploration Wells from A
Single Wellbore.
 Fault Controlling
Fault controlling is an application used
to drill a directional well into faulted
subsurface formations without crossing
the fault line.
Fig. (5-5): Fault drilling.
 Onshore Drilling to Offshore
Locations
Onshore drilling takes place when a
reservoir is located below large bodies
of water that are within drilling reach of
land. The wellheads are located on land,
and the borehole is drilled directionally
underneath the water to reach the
reservoir. This technique saves money
because land rigs are much cheaper than
offshore rigs. Fig. (5-7): Onshore Drilling to Offshore
Locations.

5
 Offshore Multiwell Drilling
Offshore multiwell drilling is the most
economical way to develop offshore
fields.
Several directional wells are drilled in
"clusters" on a multiwell offshore
platform.
Fig. (5-8): Offshore Multiwell Drilling.
 Relief Wells
Relief wells are used to kill wells that
are blowing by intercepting the
borehole. A carefully planned
directional well must be drilled with
great precision to locate and intercept
the blowing well’s borehole.
Fig. (5-9): Relief Wells.
 Controlling vertical wells
Directional techniques are used to
“straighten crooked holes”. when
deviation occurs in a well which is
supposed to be vertical, various
techniques can be used to bring the
well back to vertical.
This was one of the earliest
applications of directional drilling.
Fig. (5-10): Controlling vertical wells.

6
 Horizontal Wells
Horizontal wells are used to intersect a
producing formation horizontally to
better produce the reservoir. Horizontal
drilling increases the surface area of a
producing formation. For example, a
vertical well may give exposure to a
reserve with a depth of 20 to 30 ft (6 to
10 m) while a horizontal well drilled
into the same reservoir may give
exposure to 2000 to 3000 ft (600 to
1000 m).
Horizontal wells can make a platform
Fig. (5-11): Horizontal Wells.
 Extended Reach Wells
Extended reach wells are drilled to
reach reservoirs that have a horizontal
displacement in excess of 16400 ft (>
5000 m) from the starting point.
Fig. (5-12): Extended Reach Wells
 Multilateral Wells
Multilateral wells have several
wellbores running laterally and
originating from one original hole.
Fig. (5-13): Multilateral Wells

7
 Deflection Tools
The wellbore can be deflected from its current position using one of the following tools:
 Whipstocks
 Jetting action
 Downhole motors and bent sub
 Steerable positive displacement motor
 Rotary steerable systems
 Whipstocks
The whipstock is widely used as a deflecting medium for drilling multilateral wells. It
consists of a long inverted steel wedge (shute) which is concave on one side to hold and
guide a deflecting drilling or milling assembly. It is also provided with a chisel point at the
bottom to prevent the tool from turning, and a heavy collar at the top to withdraw the tool
from the hole, Figure (5-14).
Today, whipstocks are mainly used to mill casing windows for sidetracking existing wells.
There are two main types of Whipstocks:
 Short, Medium, and Long
Radius
Wells
Short radius wells, typically re-entries
of old vertical wells, have curves with a
143 ft (44m) radius or smaller that
cannot be drilled with conventional
motors. They are used to isolate higher-
/lower-pressured production zones or
water sands without setting and
cementing a liner. This type of drilling
is desirable when kicking off below a
problem formation.
Medium radius wells have curves with a
200-500 ft (61-152 m) radius that can be
drilled with conventional motors.
Long radius wells have curves with a
750 ft (229 m) radius or larger that can
also be drilled with conventional
motors.
Fig. (5-13): Short, Medium, and Long
Radius Wells.

8
1. The standard removable Whipstock which is used to kick off wells and for
sidetracking. The Whipstock is used with a drilling assembly consisting of a bit, a
spiral stabilizer, and an orientation sub, rigidly attached to the Whipstock by means of
a shear pin. To deflect the well, the whipstock and kick off assembly is run in hole and
oriented in the required direction. Weight is then applied to shear the pin and allow the
drilling bit to slide down the shute and drill in the set direction.
2. The Permanent Casing Whipstock is designed to remain permanently in the well.
3. Thru tubing whipstock.
 Jetting
This is an old technique which is rarely used today. It relies on hydraulics to deviate the
wellbore and is therefore only effective in soft formations. A special jet bit, is often used, but
it is possible to use a normal soft formation bit, using one very large nozzle and 2 small jet
nozzles. The large jet nozzle is the "toolface". The fluid coming out from the large nozzle
causes the maximum formation erosion and allows the well to be, effectively, deflected in
the direction of the jet coming out of the big nozzle. Jetting usually causes high dogleg
severities.
Fig. (5-14a): Whipstocks.
Fig. (5-14b): Establishing a deviation
by means of a whipstock.

9
Fig. (5-15a): Bit set-up for jetting.
 Downhole Motors with Bent Subs
A downhole motor with a bent sub, Figure (5-16), was a common method for
deflecting wells until replaced by steerable motors. The bent sub is run directly above the
motor and its pin is offset at an angle of 1-3 degrees. The bent sub has a scribe line cut on its
outside body (casing) above the pin offset. This scribe line is used to orient the BHA in the
Fig. (5-15b): Establishing a deviation by means of the jet bit: A, initial stage of the ‘jetting’ with an
increase in the deviation angle; B, penetration of the bit in rotary mode; C, further increase in the
deviation angle by jetting.

10
required direction. The orienting sub Figure (5-16) allows single shot surveys to be taken to
confirm the orientation of the BHA.
Deflection of the wellbore occurs when drilling is carried out with no surface rotation
to the drillstring. The drillbit is forced to follow the curve of the bent sub. The degree of
curvature depends largely on the bent sub offset angle and the OD of the motor. When the
required angles (inclination and/or azimuth) are obtained, this BHA is tripped out and
replaced with a rotary assembly.
Fig. (5-16): PDM BHA with bent sub.
 Steerable Positive Displacement Motors
The motor is designed with an in-built bent housing below the motor section; usually
the connecting rod housing. The bent housing angle is usually 0.25-1.5 degrees and is
designed to tilt the axis of the bit relative to the axis of the hole. The reader should note that
having only a small bit offset will create a considerable bit side force (deflecting force). A
steerable motor can be used in oriented mode (sliding) or rotary mode. In the sliding mode,
the drillstring remains stationary (rotary table or top-drive is locked) while the drillbit is
rotated by the motor. The course of the well is only changed when drilling in sliding mode as
the drillbit will now follow the curvature of the motor bent housing. In rotary mode, the
steerable motor becomes "locked" with respect to trajectory and the hole direction and

11
inclination are maintained while drilling. The use of steerable motors with the correct drillbit
and BHA reduces the number of round trips required to produce the desired
inclination/azimuth.
Single shot surveys are not usually accurate in orienting steerable motors due to the
high reactive torque produced by the motor. For this reason, most steerable motor assemblies
are run with an MWD (measurement while drilling) tool to provide real time survey and
orientation data. A steerable motor with an MWD tool is described as Steerable System.
Steerable motors are usually used to drill complete sections of a well, from current casing
shoe to next casing point.
Fig. (5-17): Typical composition of a system used in geosteering operations.

12
Fig. (5-18): Steerable drilling system.

13
 Rotary Steerable Systems
These systems do not use bent subs for affecting hole angles. Changes in hole angles are
brought about by the action of three pads contained within a non-rotating sleeve. The pads
are kept in constant contact with the formation by internal mud powered actuators. If no
angle change is required, the system is put in neutral mode by pushing the pads in every
direction thereby cancelling each other.
If changes in angle and direction are required, the electronics within the instruments
cause each pad to extend against the side the hole opposite the intended bias
direction, Figure (5-19). The resultant action of these forces then cause the bit to
build or drop angles as required. Signals can be sent from surface to the instrument
downhole as is the case with most current rotary steerable systems or the hole
inclination and direction are programmed into the instrument at surface and the
instrument then automatically corrects the hole trajectory without driller’s
intervention.
Fig. (5-19): Autotrak from Baker Inteq is an example of a rotary steerable system.
 Planning The Profile of the Well
There are basically three types of deviated well profile:
1) Build and Hold (slant)
2) S-shaped
3) Deep kick-off (J-shaped)

14
The build and hold profile is the most common deviated well trajectory and is the most
simple trajectory to achieve when drilling. The S-shaped well is more complex but is often
required to ensure that the well penetrates the target formation vertically. This type of
trajectory is often required by reservoir engineers and production technologists in
exploration and appraisal wells since it is easier to assess the potential productivity of
exploration wells, or the efficiency of stimulation treatments when the productive interval is
entered vertically, at right angles to the bedding planes of the formation. The deep kick-off
profile may be required when drilling horizontal wells or if it is necessary to drill beneath an
obstacle such as the flank of a Salt Diapir. This well profile is the most difficult trajectory to
drill since it is necessary to initiate the deviated trajectory in deeper, well compacted
formations.
Type -1: "Slant" profile (hole without
return to the vertical): In this type of hole
Features:
 Shallow kick-off point (KOP)
 Build-up section (which may have
more than one build up rate)
 Tangent section
Applications:
 Deep wells with large horizontal
displacements
 Moderately deep wells with
moderate horizontal displacement,
where intermediate casing is not
required
Fig. (5-20): Type 1 (Build and Hold) or slant

15
Disadvantages:
 Increased torque & drag
 Risk of keyseating
 Logging problems due to inclination
Type-2: "S-shaped hole" ("s" hole with
a section returning to vertical).
Features: There are several variations:
 Shallow KOP - Build, hold & drop
back to vertical
 Build-up section - Build, hold, drop &
hold
 Tangent section - Build, hold &
continuous drop through reservoir
 Drop-off section
Applications:
 Multiple pay zones
 Reduces final angle in reservoir
 Lease or target limitations
 Well spacing requirements
 Deep wells with small horizontal
displacements
Disadvantages:
 Increased torque & drag
 Risk of keyseating
 Logging problems due to inclination
Fig. (5-21): Type 2 (S Type Well)
Type-3: "Deep slant" hole:
Features:
 Deep KOP
 Build-up section
 Short tangent section (optional)
Applications:
 Appraisal wells to assess the extent of a newly
discovered reservoir
 Repositioning of the bottom part of the hole or re-
drilling
 Salt dome drilling
Disadvantages:
 Formations are harder so the initial deflection may
be more difficult to achieve
 Harder to achieve desired tool face orientation
with downhole motor deflection assemblies (more
reactive torque)
 Longer trip time for any BHA changes required
Fig. (5-22): Type 3 (Deep Kickoff and Build)

16
Fig. (5-23): Main configurations of a directional or horizontal well.
 DirectionalDrilling Terminology
1. Kick-off-point (K.O.P.): Depth at which wellbore deviation from vertical is initiated.
2. Build-up-rate (B.U.R.): Angular increase of the curvature expressed in degrees /
100ft (ex. 4o
/100ft).
3. Drop-off-rate (D.O.R.): Angular decrease of the curvature expressed in degrees /
100ft (ex. 4o
/100ft).
4. Slant section: Borehole section with constant inclination.
5. Dog leg severity: The change in wellbore inclination and/or direction in three
dimensions "usually expressed in o
/100ft.
6. Drift angle: The inclination angle of the wellbore in relation to vertical.
7. Vertical depth of the target: Hydrocarbon target.
8. Displacement: Distance on horizontal plane between the surface site and the target.
9. Well direction: Path of the well on horizontal plane between the starting point and the
target.

17
 Reference Systems and Coordinates
With the exception of Inertial Navigation Systems, all survey systems measure inclination
and azimuth at a particular measured depth (depths measured “along hole”). These
measurements are tied to fixed reference systems so that the course of the borehole can be
calculated and recorded.
These reference systems include:
 Depth references
 Inclination references
 Azimuth references
 Depth References
During the course of a directional well, there are two kinds of depths:
 Measured Depth (MD) is the distance measured along the actual course of the
borehole from the surface reference point to the survey point. This depth is always
measured in some way, for example, pipetally, wireline depth counters, or mud
logger’s depth counter.
 True Vertical Depth (TVD) is the vertical distance from the depth reference level to
a point on the borehole course. This depth is always calculated from the deviation
survey data.
In most drilling operations the rotary table elevation is used as the working depth
reference. The abbreviation BRT (below rotary table) and RKB (rotary kelly bushing)
are used to indicate depths measured from the rotary table. This can also be referred to
as derrick floor elevation. For floating drilling rigs the rotary table elevation is not
fixed and hence a mean rotary table elevation has to be used.
In order to compare individual wells within the same field, a common depth reference
must be defined and referred to (e.g. When drilling a relief well into a blow-out well,
the difference in elevation between the wellheads has to be accurately known).
Offshore, mean sea level (MSL) is sometimes used. Variations in actual sea level from
MSL can be read from tide tables or can be measured.
 Inclination References
The inclination of a well-bore is the angle (in degrees) between the vertical and the well bore
axis at a particular point. The vertical reference is the direction of the local gravity vector
and could be indicated by a plumb bob.

18
 Azimuth Reference Systems
For directional surveying there are three azimuth reference systems:
 Magnetic North
 True (Geographic) North
All “magnetic-type” tools give an azimuth (hole direction) referenced to Magnetic
North. However, the final calculated coordinates are always referenced to either True North
or Grid North.
 True (Geographic) North
This is the direction of the geographic North Pole which lies on the Earth’s axis of
rotation. Direction is shown on maps using meridians of longitude.
 Direction Measurements
Survey tools measure the direction of the wellbore on the horizontal plane with
respect to North reference, whether it is true or Grid North. There are two systems:
 Azimuth
In the azimuth system, directions are expressed as a clockwise angle from
0° to 359.99°, with North being 0°.Show figure (4-10).
 Quadrant Bearings
In the quadrant system Figure (5-24a), the directions are expressed as angles from
0°-90° measured from North in the two Northern quadrants and from South in the Southern
quadrants. The diagram in Figure (5-24b) illustrates how to convert from the quadrant
system to azimuth, and vice versa.
Fig. (5-24a): The Azimuth System Fig. (5-24b): The Quadrant System

19
Ex.(5-1): Determine the azimuth with respect to true north of the following wells:
Well Observed azimuth with Declination respect to magnetic north
Well No. Observed azimuth with respect to magnetic north Declination
1 N45o
E 3o
west
2 N45o
E 3o
west
3 S80o
W 5o
west
Solution:
True north = magnetic north ± (declination)
Well 1
Fig. (5-24c): Conversion from Quadrant to Azimuth
Systems

20
Quadrant Azimuth = N45o
E + (-3o
) =N42o
E
Azimuth with respect to true north = 42o
(Note: Azimuth is the angle measured with respect to true north)
Well 2
Quadrant Azimuth = N45o
E+ (+3o
) =N 48o
E
Azimuth with respect to true north = N48o
E
Well 3
Quadrant Azimuth = S80o
W + (-5) = S75o
W
Azimuth with respect to true north = 180o
+ 75o
= 255o
 Survey Calculations
Directional surveys are taken at specified intervals in order to determine the position of the
bottom of the hole relative to the surface location. The surveys are converted to a North-
South (N-S), East-West (E-W) and true vertical depth coordinates using one of several
calculation methods. The coordinates are then plotted in both the horizontal and vertical
planes. By plotting the survey data, the rig personnel can watch the progress of the well and
make changes when necessary to hit a specified target.
There are several methods that can be used to calculate survey data; however, some are more
accurate than others. Some of the most common methods that have been used in the industry
are:
1. Tangential,
2. Balanced Tangential,
3. Average Angle,
4. Radius of Curvature and
5. Minimum Curvature
Of these methods, the tangential method is the least accurate, and the radius of curvature and
the minimum curvature are the most accurate. The industry uses primarily minimum
curvature.
The first three calculation methods are based on the trigonometry of a right triangle;
therefore, a review of these trigonometric functions would be in order.

21
By definition, a right triangle has one angle which is equal to 90°. The sum of the other two
angles is 90°. Therefore, the sum of all three angles is 180°. Referring to the triangle in
Figure (5-25), the angles are A, B, and C with C being the right angle (90°).
C = 90°
A + B = 90°
A + B + C = 180°
In Figure (5-25), the length of the triangle sides are designated a, b, and c. Therefore we can
say that for a right triangle:
when c is the hypotenuse of the triangle. The hypotenuse is always the side opposite the right
angle (90º). The length of the hypotenuse can be determined by rearranging the equation to
read:
√
Fig. (5-25): Right Triangle.
The following equations also apply to a right triangle.

22
Ex. (5-2): Given: Well XYZ in Figure (5-26), assume the triangle represents the plan
view of a well. In this well, B is the surface location and A is the position of the bottom of
the hole. The length "b" would then be the East coordinate and is equal to 450 feet. The
length "a" would be the North coordinate and is equal to 650 feet. Note that the surface
coordinates are 0.00’ North and 0.00’ East.
Determine:
1. The closure distance (length “c”), and
2. The closure direction (angle B).
Solution: To aid in solving the problem, a plan view similar to Figure (5-26) should be
constructed and labeled. Then, use the trigonometric functions of a right triangle to solve the
problem.
1. Calculate the closure distance:
√
√
2. Calculate the closure direction. The direction of a borehole is always given in azimuth
from 0° to 360° or from the north or south such as:
N48 13’W, N10.72°E, S42°0’E, or S24.53°W
The direction can be express in degrees as a fraction or in degrees and minutes. In this
example, angle "B" would be the closure direction. Solving for angle "B":
B=34.70 o

23
Fig. (5-26): Horizontal Plan View of Well XYZ
North: 650’, East: 450’,
Closure Distance: 790.57’, Closure Direction: N34o
42’E (Azimuth 34.70º)
The closure direction can be expressed in azimuth as 34.70° or it can be expressed in the
quadrants. Converting the decimal to minutes:
Minutes =(60)(Decimal)
Minutes =(60 )(0.7)= 42' Minutes
Therefore, the closure distance and direction are: 790.57' and N34°42' E.
Common Nomenclature For Directional Wells
Directional companies use some common nomenclature for describing parts of a directional
well. However, not all directional contractors use the exactly the same nomenclature. Figure
(5-27) shows a typical build, hold and drop profile or a Type II wellbore and shows some of
the more common nomenclature.
The place where drilling depth measurements begin is the KB (Kelly Bushing), RT (Rotary
Table) or DF (Drilling Floor). If the well is drilled vertical before starting the directional
work, the place where directional drilling begins is the KOP or Kick Off Point. The part of
the wellbore where the inclination is increased is usually called the Build Section or Build

24
Curve. The point at which the building is complete is the EOB (End Of Build) or EOC (End
Of Curve). Usually, the inclination is held constant in the next portion of the well and is
called the Tangent Section or the Hold Section. The Drop Section is the portion of the
wellbore where the inclination is reduced. Not all wells have a drop section. It must be
remembered that not all wells fit a common directional profile and can vary significantly,
especially if the well has multiple targets.
Presented next is a brief explanation of the most commonly used survey calculation methods
and the appropriate calculations.
Fig. (5-26): Some Common Nomenclature For Directional Wells.
 Tangential
At one time the tangential method was the most widely used because it was the easiest. The
equations are relatively simple, and the calculations can be performed easily in the field.
Unfortunately, the tangential method is the least accurate method and results in errors greater
than all the other methods. The tangential method should not be used to calculate directional
surveys. It is only presented here to prove a point.

25
The tangential method assumes the wellbore course is tangential to the lower survey station,
and the wellbore course is a straight line. If you draw a line tangent to the inclination I2
(perpendicular to line CI2 in Figure (5-27), then the angle A becomes the same as the
inclination at the lower survey point. Because of the straight line assumption for the wellbore
course, the tangential method yields a larger value of horizontal departure and a smaller
value of vertical displacement when the inclination is increasing. This is graphically
represented in Figure (5-27).
Fig. (5-27): Illustration of Tangential Calculation Method
In Figure (5-27), Line is the assumed wellbore course. The dashed line AB is the change in
true vertical depth and the dashed line BI is the departure in the horizontal direction. The
opposite is true when the inclination is decreasing. In Type I, III and IV holes, the error will
be significant. In a Type II hole, the error calculated while increasing angle will be offset by
the error calculated while decreasing angle but only when the build and drop rates are
comparable. With the tangential method, the greater the build or drop rate, the greater the
error. Also, the distance between surveys has an effect on the quantity of the error. If survey
intervals were 10 feet or less, the error would be acceptable. The added expense of surveying
every 10 feet prohibits using the tangential method for calculating the wellbore course
especially when more accurate methods are available. 2AI2
The North-South, East-West coordinates are determined by assuming the horizontal
departure of the course length is in the same direction as the azimuth recorded at the lower
survey station, but this assumption is wrong. The actual wellbore course will be a function of
the upper and lower survey stations. Therefore, the tangential method results in an additional
error because an error already exists due to the method used to calculate the horizontal

26
departure. The error is compounded when the North-South, East-West coordinates are
calculated.
 Average Angle
When using the average angle method, the inclination and azimuth at the lower and upper
survey stations are mathematically averaged, and then the wellbore course is assumed to be
tangential to the average inclination and azimuth. The calculations are very similar to the
tangential method, and the results are as accurate as the balanced tangential method. Since
the average angle method is both fairly accurate and easy to calculate, it is the method that
can be used in the field if a programmable calculator or computer is not available. The error
will be small and well within the accuracy needed in the field provided the distance between
surveys is not too great. The average angle method is graphically illustrated in Figure (5-28).
The average angle method does have problems at low inclinations with large changes in
azimuth so it should not be used for vertical wells.
Fig. (5-28a): Illustration of Balanced Tangential Calculation Method.
Fig. (5-28b): Illustration of Average Angle Calculation Method.

27
 Radius Of Curvature
The radius of curvature method is currently considered to be one of the most accurate
methods available. The method assumes the wellbore course is a smooth curve between the
upper and lower survey stations. The curvature of the arc is determined by the survey
inclinations and azimuths at the upper and lower survey stations as shown in Figure (5-29).
The length of the arc between I1and I2 is the measured depth between surveys. In the
previous methods, the wellbore course was assumed to be one or two straight lines between
the upper and lower survey points. The curvature of the wellbore course assumed by the
radius of curvature method will more closely approximate the actual well; therefore, it is
more accurate. Unfortunately, the equations are complicated and are not easily calculated in
the field without a programmable calculator or computer. In the equations, the inclination
and azimuth are entered as degrees.
Fig. (5-29): Illustration of Radius of Curvature Calculation Method.
A closer inspection of the radius of curvature equations show that if the inclination or
azimuth are equal for both survey points, a division by zero will result in an error. In Figure
(5-29) the radius, r, will become infinitely long. In that case, the minimum curvature or
average angle methods can be used to make the calculations. It is also possible to add a small
number (such as 1 x 10-4
) to either survey point. The resulting error will be insignificant.
Generally, the radius of curvature calculations are used when planning a well. Using one of
the three previous methods to plan a well will result in substantial errors when calculating
over long intervals. This will be further explained in the section on planning a well.

28
 Minimum Curvature
The minimum curvature method is similar to the radius of curvature method in that it
assumes that the wellbore is a curved path between the two survey points. The minimum
curvature method uses the same equations as the balanced tangential multiplied by a ratio
factor which is defined by the curvature of the wellbore. Therefore, the minimum curvature
provides a more accurate method of determining the position of the wellbore. Like the radius
of curvature, the equations are more complicated and not easily calculated in the field
without the aid of a programmable calculator or computer. Figure (5-30) is a graphic
representation of the minimum curvature calculations. The balanced tangential calculations
assume the wellbore course is along the line I1A+AI2. The calculation of the ratio factor
changes the wellbore course to I1B+BI2 which is the arc of the angle B. This is
mathematically equivalent to the radius of curvature for a change in inclination only.
So long as there are no changes in the wellbore azimuth, the radius of curvature and
minimum curvature equations will yield the same results. If there is a change in the azimuth,
there can be a difference in the calculations. The minimum curvature calculations assume a
curvature that is the shortest path for the wellbore to incorporate both surveys. At low
inclinations with large changes in azimuth, the shortest path may also involve dropping
inclination as well as turning. The minimum curvature equations do not treat the change in
inclination and azimuth separately as do the radius of curvature calculations.
Fig. (5-30): Illustration of Minimum Curvature Calculation Method.

29
Table (5-1): Directional Survey Calculation Formula.

30

31
Ex. (5-3): Given: The survey data for Directional Well No. 1 are shown in Table (5-2).
Determine: The wellbore position at each survey point using the tangential, balanced
tangential, average angle, radius of curvature, and minimum curvature method.
Table (5-2): Survey for Example (5-3).

32

33

34
The same calculations are made at each survey depth, and the results are shown in Table (5-
3).
Table (5-3): Survey Calculations for Directional Well No. 1 using the Tangential Method.

35
 Balanced Tangential Method
Calculate the position of the wellbore at 1300 feet using the balanced tangential method
given the values at 1200 feet from Table (5-4).

36
4).
Table (5-4): Survey Calculations for Directional Well No. 1 using the Balanced Tangential Method.

37
 Average Angle Method
Calculate the position of the wellbore at 1400 feet using the average angle method and the
survey data at 1300 feet in Table (5-5).

38
5).
Table (5-5): Survey Calculations for Directional Well No. 1 using the Average Angle Method.

39
 Radius of Curvature Method
Calculate the position of the wellbore at 1500 feet using the radius of curvature method and
the survey data at 1400 feet in Table (5-6).

40
6).
Table (5-6): Survey Calculations for Directional Well No. 1 using the Radius of Curvature Method.

41
 Minimum Curvature Method
Calculate the position of the wellbore at 1600 feet using the minimum curvature method and
the survey data at 1500 feet in Table (5-7).

42

43
The same calculations are made at each survey depths, and the results are shown in Table (5-
8).
Table (5-8): Survey Calculations for Directional Well No. 1 using the Minimum Curvature Method.

44
The results of the survey calculations for Directional Well No. 1 in Example (5-3) are
compared in Table (5-9) and Table (5-10). The comparison shows a significant difference
when using the tangential method. The difference is much less pronounced with the other
four methods. Table (5-10) shows the difference in the calculated TVD, North and East
assuming the minimum curvature method is the most accurate. The average angle, balanced
tangential and radius of curvature methods are all within one foot of each other at total depth.
It must be remembered that as the distance between surveys increases, the average angle and
balanced tangential errors will increase significantly.
Table (5-9): Comparison of the Survey Calculation Methods for Example (5-3) Results.
Table (5-10): Relative Difference between the Survey Calculation Methods for Example (5-3) Results.
 Closure And Direction
The line of closure is defined as "a straight line, in a horizontal plane containing the last
station of the survey, drawn from the projected surface location to the last station of the
survey." The line of closure is identified in Figure (5-31). Simply stated, the closure is the
shortest distance between the surface location and the horizontal projection of the last survey
point. The closure is always a straight line because a straight line is the shortest distance
between two points. The closure is the polar coordinates at a given survey point as opposed
to north and east being rectangular coordinates.
When defining closure, the direction must also be given. Without indicating direction, the
bottomhole location projected in a horizontal plane could be anywhere along the

45
circumference of a circle with the radius of the circle being equal to the closure distance. The
direction and closure exactly specifies where the bottom of the hole is located in relation to
the surface location.
The closure distance and direction are calculated using the following equations assuming that
the wellhead coordinates and zero feet North and zero feet East:
√
If the wellhead coordinates are not zero North and East, the wellhead coordinates must be
subtracted from the These are the same equations used for calculating an angle and
hypotenuse of a right triangle.
Ex. (5-4): Given: To illustrate the use of these equations, the closure and direction of the
Directional Well No. 1 in Example (5-3) for the results of the minimum curvature method
are calculated below
From Table (5-8), the coordinates of the last survey point in the example well are:
North = 1543.05 ft
East = 639.8 ft
Solution:
√
√
Since the bottomhole location is in the northeast quadrant, the closure distance and direction
are: 1670.43 ft N22.52E
Then, the horizontal projection of the bottom of the hole is 1670.43 feet away from the
surface location in the N22.52E direction.
 Vertical Section
The vertical section is the horizontal length of a projection of the borehole into a specific
vertical plane and scaled with vertical depth. When the path of a wellbore is plotted, the
vertical section is plotted versus TVD. The closure distance cannot be plotted accurately

46
because the plane of closure (closure direction) can change between surveys. The vertical
plot of a wellbore is in one specific plane. Figure (5-31) graphically shows the difference
between the closure distance and vertical section. The closure distance and vertical section
are equal only when the closure direction is the same as the plane of the vertical section.
Fig. (5-31): Graphic Representation of the Difference between Closure Distance and Vertical Section
in the Horizontal Plane.
The vertical section azimuth is usually chosen as the azimuth from the surface location to the
center of the target. If multiple targets are present and changes in azimuth are required to hit
each target, the vertical section azimuth is usually chosen as the azimuth from the surface
location to the end of the wellbore.
The vertical section is calculated from the closure distance and direction. The equations for
calculating the vertical section can be seen in Table (5-1) and are as follows:
VS= cos (AZvs-AZcl)(Closure Distance)
Ex. (5-5): Given: The data of Directional Well No. 1 from the previous examples.
The plane of the vertical section is 10°.
Calculate: The vertical section at the last survey point.
From the previous example:

47
Closure Distance = 1670.43 feet
Closure Direction = 22.52°
Calculate the vertical section:
VS= cos (AZvs-AZcl)(Closure Distance)
VS= cos (10-22.62)(1670.43)= 1630.71 ft
Therefore, the distance of 1630.71 feet would be plotted on the vertical section. Figure (5-
32) and Figure (5-33) are respectively the plan view and vertical section for Example (5-3).
Fig. (5-32): Plan View for Directional Well No. 1 of Example (5-3).

48
Fig. (5-33): Vertical Section for Directional Well no. 1 in Example (5-3).

49
Problems
 Dogleg Severity
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 dogleg
severity. 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:

50
Ex. (5-6)

51
To show how the change in course length can affect dogleg severity, consider the following
example:
Ex. (5-7)
The dogleg severity is 4°/100 feet. Example (5-6) and Example (5-7) show that for the same
change in inclination, a shorter course length will result in a greater dogleg severity.
The previous examples were simplified cases in which only the inclination was changed and
the azimuth remained constant. A change in azimuth also affects dogleg severity.
Unfortunately, the effect on dogleg severity due to a change in azimuth is not as easy to
understand or calculate. A 2° change in azimuth in a 100 foot course length will not yield a
dogleg severity of 2°/100 feet unless the inclination is 90°. At low inclinations a change in
azimuth will have a small dogleg severity. As the inclination increases, the dogleg severity
will also increase for the same change in azimuth. Three equations for calculating dogleg
severity using both inclination and azimuth are shown below.

52
The first two equations are very long and it is easy to make a mistake in the calculations.
Last equation is more simple but not very accurate below an inclination of 5°. The
nomenclature is the same as for the survey calculations.
In three equations above, the “100” changes the dogleg severity to “per 100 feet”. In the
metric system, the “100” should be changed to “30” for dogleg severity in degrees per 30
meters or “10” for dogleg severity in degrees per 10 meters.
To illustrate the effect azimuth has on dogleg severity, consider the following problem
Ex.(5-8): A 10° azimuth change at inclinations of 1°, 10°, 20°, 30°, 40°, 50°, 60°, 70°, 80°,
and 90°. Determine: The dogleg severity at each inclination.
Solution: To make the problem easier to understand, a table can be set up with the necessary
information.

53
Table (5-11): Data for Example (5-8).
Calculate the dogleg severity at 1° using first equation. In this example, the inclination
remains constant at 1°. The azimuth will change from 10° to 20° over a course length of 100
feet.

54
Calculate the dogleg severity at a constant inclination of 10° using first equation.
Calculate the dogleg severity at a constant inclination of 20º using first equation.
The dogleg severity for the remaining constant inclinations was calculated and is shown in
Table (5-12).
At an inclination of 1°, the dogleg severity is 0.17°/100 feet for a 10° change in azimuth. At
an inclination of 50°, the dogleg severity is 7.66°/100 feet for the same change in azimuth.
The results in Table (5-12) show that the dogleg severity increases as the inclination

55
increases for the same change in azimuth. The equation used to calculate the dogleg
severities in Table (5-12) can also be used to calculate the dogleg severity in Example (5-8).
Table (5-12): Calculated Dogleg Severity.
Problems

56
 Planning A Directional Well
The geometry of a directional well can be defined with three parameters:
 Build rate
 Hold inclination, (drop inclination), and
 Kickoff point (KOP)
The directional well configuration can be determined by assuming any two of the above
three parameters and then, calculating the third. The assumption of a particular parameter
requires good understanding for an intelligent selection. Hold inclination and kickoff point
are easier to calculate than the build rate.
The build-rate can be chosen to minimize fatigue in drill pipe, minimize keyseat possibility,
or help to minimize torque and drag. If drilling a horizontal well, the build rate may be
selected based on steerability of the bottomhole assembly.
The hold inclination can be chosen based on any number of concerns. At low inclinations, it
may be difficult to maintain the direction of the wellbore. Bit walk is greater at low
inclinations because the direction can change significantly with small changes in dogleg
severity. Above 30 degrees, it is more difficult to clean the hole with 45o to 60o being the
hardest to clean. Above 60o, open hole logs may no longer fall. If the hole is not very clean,
open hole logs may not fall at inclinations above 50o. In cased hole, wireline tools will not
fall at inclinations greater than 70o
. Tubing conveyed perforating or coiled tubing conveyed
perforating will be required.
The kickoff point may be selected based on hole conditions and target constraints. Many
times it is desirable to case the build curve to minimize the possibility of a keyseat; therefore,
the kickoff point may be based on casing seats. It may be desirable to drill some troublesome
formations such as lost circulation or sloughing before kicking the well off. MWD tools do
not tolerate large quantities of LCM for extended periods of time. In sloughing formations,
stuck pipe may lead to loss of very expensive directional tools. If the troublesome formations
are too deep, it may be desirable to be drilling a hold section in these formations.
Generally, the build rate is chosen trying to keep below the endurance limit of the drill string
in order to minimize the possibility of fatigue damage. The higher in the hole the kickoff
point, the lower the dogleg severity needs to be in order to minimize fatigue in the drill string
through the build section.

57
It may not always be possible to drill a directional well and not cause some fatigue in the
drill string or to keep the inclination below 30o. It depends upon the target departure. With
high departure targets, high inclinations will be required. After all, the objective of the
directional well is to hit the target or to hit multiple targets.
 Determining Directional Well Plan
The majority of today's directional well planning is performed on computers. Computers are
fast and can incorporate both changes in build and drop rates and changes in direction. All
directional drilling service companies offer this service; therefore, a final well plan should be
generated by a computer.
However, there are times when a directional driller or engineer may need to estimate the
inclination needed to achieve a specified departure, or he may need to change the well plan
while drilling the well. Sidetracking around a fish is an example. At these times a computer
may not be available. Presented here is a simple method of planning a directional well which
can be used in the office or field. With little practice, this method can be used to plan and
plot a directional well. If possible, the final plan should be processed by the directional
drilling contractor on a computer. In this chapter, we will look at two dimensional planning
only. Three dimensional planning (incorporating direction changes) is beyond the scope of
this manual.
It should be remembered that if you can plan a directional well, then you can incorporate
changes to a directional plan in the field. The same equations are used to predict the
inclination and azimuth required to hit the target. The equations are also used to revise the
directional program when the target changes as in geosteered wells.
The first (and simplest) well to consider is a Type I well. The Type I well has a vertical hole
to a relatively shallow depth. Then, at the kickoff point, the well is deviated to a specified
inclination. At the end of built point, the hold or tangent section of the hole is drilled at the
specified inclination until the total depth is reached. Buildup charts can be used to determine
the hold inclination, measured depth (MD), true vertical depth (TVD), and horizontal
departure (DEP) in the building or dropping section of the hole. They can also be easily
calculated. The trigonometric solution of a right triangle is used to determine the MD, TVD,
and DEP in the hold section of the hole. An example problem follows which will
demonstrate the procedures.

58
Ex. (5-9):
Given: Target Depth - 9800 feet TVD
Kickoff Point - 2000 feet TVD
Horizontal Departure - 2926 feet
Direction of Departure- N20°E
Rate of Build - 2°/100 feet
Total Depth of Well - 10000 feet TVD
Determine: True vertical depth for each section
Measured depth for each section
Horizontal departure for each section
North coordinate at target and TD
East coordinates at target and TD
Closure distance at target and TD
Closure direction at target and TD
Construct a vertical section and horizontal plan view.
Solution: Section 1 - Vertical to KOP
From the information given, the kickoff point is 2,000 feet. Since this is a vertical hole, there
is no horizontal departure, and the MD is the same as the TVD. The following shows the
data for this section of the hole. In reality, the hole will not be perfectly vertical but for
planning purposes, it sufficient to assume that it is vertical.
Section MD (feet) TVD (feet) Dep. (feet)
Vertical to KOP 2000 2000 0
Section 2 - Build
To determine the angle necessary to achieve the desired horizontal departure of 2926 feet,
the 2°/100 feet buildup graph is used. To use this graph, one must determine the TVD

59
remaining in which to accomplish the horizontal departure. The TVD remaining in this
example is the total TVD to the target minus the TVD to the kickoff point or: 9800' - 2000' =
7800' TVD remaining.
In 7800 feet of TVD, the hole must have a horizontal departure of 2926 feet. Using the
2°/100 feet buildup graph Figure (5-34), enter the graph at 2926 feet on the horizontal
departure scale (bottom). Draw a line up until it meets the TVD depth (vertical scale) of
7800 feet. Read the angle of inclination running through this point. That inclination is 22°.
Therefore, if the inclination is increased at 2°/100 feet to 22° and then maintained; the
horizontal departure will be 2926 feet after drilling 7800 feet of true vertical depth. When the
hole is kicked off at 2000 feet TVD, the inclination should be built to 22° at a rate of 2°/100
feet. The 22° inclination is maintained until a TVD of 9800 feet is reached which will hit the
target. Drilling is continued at 22° to a total depth of 10000 feet TVD.
It takes 1100 feet of measured hole to increase the inclination from 0° to 22°. The true
vertical depth for the 1100 feet of drilling is 1073.17 feet and the horizontal departure is
208.6 feet. These numbers were calculated using the radius of curvature method.
Build 1100 1073.17 208.6
Section 3 - Hold to Target and TD
The MD of the hold section of the hole can be calculated using the geometry of a right
triangle with the hypotenuse being the measured depth. The remaining horizontal departure
and true vertical depth can be calculated by subtracting the TVD and DEP to the end of the
build section from the total.
TVD Remaining = 9800’ – 2000’ – 1073.17’ = 6726.83’
DEP Remaining = 2926’ - 208.60’ = 2717.40’
Figure (5-35) is a right triangle which represents the hold section of the hole with Angle “A”
being the inclination (22°). Side “b” is the TVD (6726.83 feet). Side “a” is the horizontal
departure (2717.4 feet). Side “c” is the MD which must be calculated. From the
trigonometric functions of a right triangle, we know that:

60
From this equation, we can solve for the measured depth.
Therefore, the measured depth of the hold section of the hole to the target is 7254.01 feet.
Now, the horizontal departure and measured depth of the well must be calculated from 9800
feet TVD to 10000 feet TVD which is the remaining portion of the well to be drilled. It can
also be calculated using the trigonometric functions of a right triangle. The inclination is 22°
and the TVD remaining is as follows:
TVD Remaining = 10000’ - 9800’ = 200’’

62
Fig.(5-34): Graph for 2o
/100 feet Buildup.

63
Fig. (5-35): Right Triangle Representing Hold to Target Section.
In the triangle shown in Figure (5-36), the Angle “A” is equal to the inclination (22°). Side
“b” is equal to TVD (200 feet). Side “c” (MD) and side “a” (DEP) must be determined. The
horizontal departure can be determined from the tangent of Angle “A.”
From this equation, we can solve for the measured depth.
Therefore, the horizontal departure is equal to 80.81 feet. The measured depth can be
determined from the cosine of angle “A”.
c = 215.71 ft

64
Fig. (5-36): Right Triangle Representing Hold to TD Section in.
Therefore, the measured depth is equal to 215.71 feet. The information from the hole portion
of the hole can be entered in Table (5-13).
Table (5-13): Directional Profile for Example (5-9).
Section MD (feet) TVD (feet) Dep (feet)
Build 1100 1073.17 208.6
Hold to target 7254 6726.83 2717.4
Hold to TD 215.71 200 80.81
Total 10569.72 10000 3006.81
The well is to be drilled to a vertical depth of 2000 feet. Then the well is kicked off at a rate
of 2°/100 feet until an inclination of 22° is reached. The inclination is maintained at 22°, and
the well is drilled to a TD of 10569.72 feet MD or 10000 feet TVD.
To make the directional program easier to understand, a vertical and horizontal plan view of
the wellbore course can be drawn. The vertical section is shown in Figure (5-37) and was
constructed from Table (5-13). To construct the figure, the true vertical depth will be on the
vertical scale, and the horizontal departure will be on the horizontal scale. As shown, each
section of the well is plotted on the graph. The first section to be plotted is the vertical
section to the kickoff point. In that section, the TVD is 2000 feet and the horizontal departure
is 0.00 feet. The point is plotted representing the location of the well at 2000 feet TVD. This

65
point will be called point "B". Point "A" is the location at the surface. A line is drawn from 0
to 2000 feet TVD which represents the wellbore course.
The next point to plot is at the end of the build section. That point can be located by the TVD
and DEP at the end of the build section. From Table (5-13), the TVD is equal to 1073.07 feet
plus 2000 feet. The DEP is equal to 208.6 feet plus 0.00 feet. Therefore, the TVD and DEP
are 3073.19feet and 208.6 feet, respectively. This point can now be plotted on the graph and
is called point "C". Since the inclination increases from 0° to 22°, a smooth curve should be
drawn from point "B" to point "C".
The next section is the hold section to the target at a true vertical depth of 9800 feet. The
TVD and DEP can be calculated by summing the values in Table (5-13) through the hold to
target section.
TVD = 2000' + 1073.17' + 6726.83'
TVD = 9800'
DEP = 0.00' + 208.60' + 2717.40' = 2926 ft
Note that the TVD and DEP are equal to the values specified in the problem at the target
depth. This point can be plotted and is called point "D". A straight line is drawn from point
"C" to point "D".
The last section is hold to TD. The TVD and DEP are again calculated by summing all the
values of TVD and DEP to total depth. This has already been done in Table (5-13).
Therefore, the TVD is 10,000 feet and the DEP is 3006.81 feet. Point "E" can now be
plotted. A straight line is drawn from "D" to "E". The vertical section is labeled as shown in
Figure (5-37).

66
Fig. (5-37): Vertical Section Well DEF, Happy Oil Company, N20E Plain.
The next step is to determine the closure distance and direction, the North coordinate, and
the East coordinate. The closure direction is given as N20ºE. The closure distance is the
horizontal departure at any point in the well. At target depth the closure distance is 2926 feet,
and at total depth, the closure distance is 3006.81 feet. The North and East coordinates can
now be determined from the solution of a right triangle at both the target depth and total
depths as shown in Figure (5-37) and Figure (5-38). In the right triangle, “b” will represent
the North coordinate and “a” will represent the east coordinate. The closure or horizontal
departure is represented by “c”, and angle “A” is the closure direction and is N20ºE or an
azimuth of 20°. The following are the calculations for the North and East coordinates.

67
Target Depth, North Coordinate
b= 2749.54 ft
North = 2749.54 ft
Target Depth, East Coordinate
b= 1000.75 ft
East = 1000.75 ft
Total Depth, North Coordinate
b= 2825.48 ft
North = 2825.48 ft
Total Depth, East Coordinate
b= 1028.39 ft
East = 1028.39 ft

68
Fig. (5-38): Right Triangle Representing the Horizontal View of the Well in at Target Depth.
Fig. (5-39): Right Triangle Representing the Horizontal View of the Well in at Total Depth.

69
Table (5-14): Results of Example (5-9).
Closure
distance
(feet)
Closure
direction
(degree)
North (feet) East (feet)
Targetdepth 2926 N 20o
E 2749.54 1000.75
Total depth 3006.81 N 20o
E 2825.48 1028.39
Fig. (5-40): Horizontal Plan View of the well in Example (5-9).
Enough information is now available to make a horizontal plan view. The horizontal plan
view for the example well is shown in Figure (5-40), and was constructed from the
information in Table (5-14).

70
The directional plan for a Type III well is very similar to the plan for a Type I well. The only
difference is the kickoff point because a Type III well is deeper and there is no hold section,
therefore, an example will not be given for a Type III well.
The Type II well has a vertical section to a relatively shallow depth. At the kickoff point, the
well is deviated to a desired inclination, and the inclination is maintained until the drop point
is encountered. The well is then brought back to vertical, and drilling continues to TD. The
drop section of the hole is the difference between the Type I and Type II hole. Again, the
best way to illustrate the calculation of a well plan is through an example problem:
Ex. (5-10): Given: Target Depth - 9800 feet TVD
Kickoff Point - 2000 feet TVD
Horizontal Departure - 2926 feet
Direction of Departure- S 40°W
Rate of Build - 2.5°/100 feet
Rate to Drop - 1.5°/100 feet
Total Depth of Well - 10,000 feet TVD
Determine: True vertical depth for each section
Measured depth for each section
Horizontal departure for each section
North coordinate at target and TD
East coordinates at target and TD
Closure direction at target and TD
Closure distance at target and TD
Construct a horizontal plan view and vertical section.
Solution: Section 1 - Vertical to KOP

71
The kickoff point is specified as being 2000 feet. Since, the well is vertical, the measured
depth (MD) will be equal to the true vertical depth (TVD), and the horizontal departure
(DEP) is 0.00 feet. The data is in tabular form as follows:
Section 2 - Build
The 2.5° build-up and 1.5° drop-off graph is used to determine the inclination necessary to
achieve the desired horizontal departure. To use this graph calculate the true vertical depth
remaining.
TVD Remaining = 9800’ – 2000’ = 7800’
Enter Figure (5-41) at 2926 feet on the horizontal departure scale. Follow the 2926 feet line
until it intersects the 7800 feet TVD line (on the vertical scale). An inclination of
approximately 24º is read from the graph. Note that it is not exactly 24º.
Therefore, the inclination will be built at a rate of 2.5° /100 feet until an inclination of 24° is
reached. The measured depth, true vertical depth, and horizontal departure can be obtained
from the 2.5°/100 feet buildup table. The following is read from the table.
Build 960 932.17 198.14
Section 3- Drop
The measured depth, true vertical depth, and horizontal departure for the drop section of the
hole can also be determined using the buildup charts. Even though the inclination is
decreasing, the values of measured depth, true vertical depth, and horizontal departure are
the same as long as the inclination returns to zero. Therefore, we may determine these values
from the 1.5° buildup chart (drop rate). Enter the table at 24° and read the following
information:
Drop 1600 1553.62 330.23

72
Fig. (5-41): Graph for 2.5o
Build-up and 1.5o
Drop-Off per 100 feet.
Section 4 - Hold
All the values of measured depth, true vertical depth, and horizontal departure to the target
are known with the exception of the hold section. The true vertical depth of the hold section
can be determined by subtracting the total TVD at the target. As specified in the problem, the

73
TVD at the target is 9800 feet. The total TVD of the vertical to KOP, Build, and Hold
sections is:
TVD (Total) = TVD (Vertical) + TVD (Build) + TVD (Drop)
TVD (Total) = 2000' + 932.17' + 1553.62' = 4485.79'
Therefore, the TVD in the hold section is:
TVD (Hold) = TVD (Target) - TVD (Total)
TVD (Hold) = 9800' - 4485.79' = 5314.21'
The horizontal departure in the hold section can be determined the same way.
DEP (Total) = DEP (Vertical) + DEP (Build) + DEP (Drop)
DEP (Total) = 0.00' + 198.14' + 330.23' = 528.37'
The DEP in the hold section is:
DEP (Hold) = DEP (Target) - DEP (Total)
DEP (Hold) = 2926' - 528.37' = 2397.63'
The hold section of the hole can be represented as a right triangle with the hypotenuse as the
measured depth Figure (5-42). Angle “A” is the inclination (24°), side “a” represents the
horizontal departure, and side “b” represents the true vertical depth. The measured depth can
be determined from the trigonometric functions of a right triangle.
√
√
Note that if the inclination of the hold section is calculated from the departure and TVD, the
hole angle is 24.28º. It was not possible to read the graph accurately enough to get 24.28º.
For a final program, it would be best to calculate the hold angle or to do the directional plan
on a computer.
The data for the hold section is as follows:
Hole 5830 5314.21 2397.63

74
Fig. (5-42): Right Triangle Representing the Hold Section of Example (5-10).
Section 5 - Vertical to TD
Vertical to TD is the section from the target, 9800 feet TVD, to total depth, 10000 feet TVD
(through the producing formation and enough room for the shoe joints). Since the hole is
vertical, the measure depth is equal to the true vertical depth of 200 feet and the horizontal
departure is 0.00 feet. The data for all five sections are shown in Table (5-15).
Table (5-15): Directional Profile for Example (5-10).
Section MD (feet) TVD (feet) Dep (feet)
Build 960 932.17 198.12
Hold 5830 4314.21 2397.63
Drop 1600 1553.62 330.23
Vertical to TD 200 200 0
Total 10590 10000 2926

75
A vertical section and horizontal plan view of the well can be drawn. The vertical section is
shown in Figure (5-43). The true vertical depth is on the vertical scale and the horizontal
departure is on the horizontal scale. The vertical section can be plotted using the data in
Table (5-15).
Point "A" is the surface location of the well where MD, TVD, and DEP are equal to 0.00
feet.
Point "B" is at the end of the vertical to KOP section. From Table (5-15), MD and TVD are
equal to 2000 feet. The DEP is 0.00 feet because the inclination is 0°. This point can be
plotted by moving down to 2000 feet on the TVD scale and moving across 0.00 on the DEP
scale.
Point "C" is at the end of the build section. The point is located by summing the MD, TVD,
and DEP from the surface to the end of the build section.
MD = 2000' + 960.00'
MD = 2960'
TVD = 2000' + 932.17'
TVD = 2,932.17'
DEP = 0.00' + 198.14'
DEP = 198.14 feet
Point "C" is plotted on the graph by moving down 2932.17 feet on the TVD scale (from
surface) and moving across 198.14 feet (from 0.00 feet) on the DEP scale.
Point "D" is at the end of the hold section and is determined by summing the MD, TVD, and
DEP from surface to the end of the hold section.
MD = 2000' + 960.00' + 5830.05'
MD = 8790.05'

76
Fig. (5-43): Vertical Section, Well DEF, Happy Oil Company, S40W Plain.
TVD = 2000' + 932.17' + 5314.21'
TVD = 8246.38'
DEP = 0.00' + 198.14' + 2397.63'
DEP = 2595.77

77
Point "D" is plotted on the graph by moving down the TVD scale 8246.38 feet and moving
across the DEP scale 2595.77 feet.
Point "E" is at the end of the drop section and the MD, TVD, and DEP are determined by
summing those values.
MD = 2000' + 960' + 5830.05' + 1600'
MD = 10390.05'
TVD = 2000' + 932.17' + 5314.21' + 1553.62'
TVD = 9800'
DEP = 0.00' + 198.14' + 2,397.63' + 330.23'
DEP = 2926'
Point "E" is then plotted the same as the previous points. (Note that at the target, the problem
specified the TVD to be 9800 feet and DEP to be 2926 feet.)
Point "F" is at the end of the vertical to TD section.
MD = 2000' + 960' + 5830.05' + 1600' + 200'
MD = 10590.05'
TVD = 2000' + 932.17' + 5314.21' + 1553.62' + 200'
TVD = 10000'
DEP = 0.00' + 198.14' + 2397.63' + 330.23' + 0.00'
DEP = 2926'
Point "F" is plotted and the vertical plan view is labeled as shown.
The horizontal plan view Figure (5-44) is constructed using the closure distance and
direction. In this problem the closure distance and direction are given as 2926 feet and S 40°
W. The North and East coordinate are equal to the length of side “b”, and the East coordinate
is equal to the length of side “a”. Angle “A” is equal to the closure direction S 40° W or an
azimuth of 220°. The calculations are shown in Figure (5-45).
North Coordinate

78
b= -2241.45 ft
North = -2241.45 ft
Fig. (5-44): Horizontal Plan View of the Well in Example (5-10).

79
East Coordinate
b= -1880.8 ft
East = -1880.8 ft
Fig. (5-45): Right Triangle Representing Horizontal View of the Well in Example (5-10).
The coordinates are the same for the target depth and total depth since the inclination
between the two points is 0°.
The results are shown in Table (5-16). The horizontal plan view can now be constructed as
shown in Figure (5-44).

80
Table (5-14): Results of Example (5-9).
Closure
distance
(feet)
Closure
direction
(degree)
North (feet) East (feet)
Target depth 2926 N 40o
E -2241.45 -1880.8
Total depth 2926 N 40o
E -2241.45 -1880.8
Problems
 Horizontal Well
A horizontal well is defined as a well with an inclination angle of 90 degrees from the
vertical. A vertical well is one with zero inclination angles.
 Types of Horizontal Wells
There are three types of horizontal wells:
1. Short radius
2. Medium radius
3. Long radius

81
Fig. (5-46): Types of horizontal well.
 Short Radius Wells (SRW)
The main features of this type are the very high build-up rate of 60 – 150 degrees /100 ft
with a radius range of 40-100 ft.
 Medium Radius Wells (MRW)
The build-up rate for this type is usually 8-30 degrees/100ft with a radius range of 200 to 700
ft. The horizontal drain is usually between 1000 – 3500 ft.
 Long Radius Wells (LRW)
This is the most common type of horizontal wells especially offshore. The build-up rate is
usually from 2 to 6 degrees/100ft. The most common BHA used is a steerable system
containing a single bent sub with a downhole motor.

82
 Reasons for drilling horizontal well
1. Nacurally fractured reservoir
2. Formation with water and gas coning
3. Heavy oil reservoirs / thermal application
4. Depleted gas/Gas storage reservoirs
5. Low permeability shale gas reservoirs
6. Water flooding/ Co2 inection

83
 Multilateral wells
A multilateral well is a well that has two or more drainage holes (or secondary
laterals or branches or legs) drilled from a primary well bore (or trunk or main bore or
mother bore or back bore). Both trunk and branches can be horizontal, vertical or
deviated.
 Types of multilateral wells
 Multibranched
 Forked
 Laterals into horizontal holes
 Laterals into vertical holes
 Stacked-laterals
 Dual opposing laterals
 Advantages of Multi-Laterals
1. Increased production from a single well due to increased reservoir exposure
2. Accelerated production
3. Reduction of surface well equipment and surface facility costs
4. Multi-laterals provide flexible selectivity and easy monitoring of oil and gas wells
 Main Applications of Multi-Lateral Wells
1. Tight reservoirs
2. EOR tools
3. Slot recovery
4. Injection/Production from same well
5. Complex drainage reservoirs
6. Structural delineation from first few wells
7. Exploration wells keepers, if main well was dry
Fig. (5-47): Multilateral well configurations

84
 Multilateral Well Planning Considerations
The following is a partial list of some of the most important considerations in planning a
multilateral well:
1. Drilling methods
2. Junction design
3. Well control issues
4. Drilling issues
5. Milling problems
6. Completion requirements
7. Multi-lateral requirements
8. Abandonment
 Drilling Planning Issues
There are three main drilling techniques:
1. Long radius
2. Medium radius
3. Short radius
The planning issues to consider when drilling a lateral are:
1. Hole size
2. Hole angle
3. Kick off methods
4. Flow control and isolation
5. Formation damage and clean up of the lateral
6. Drainage patterns for optimum production
 Kick Off Methods
A lateral can be kicked off using one of three methods:
1. Open hole
2. Cased hole
3. Composite casing
 Open Hole Kick Offs
In open hole kick off a cement plug is first placed in the open hole where the kick off
window is desired. Once the cement plug is set, a kick off assembly is to build up angle
away from the mother bore.
Further build up assemblies or a hold up assembly is then run to drill to final total depth.

85
 Advantages of Open Hole Kick Offs
1. Simple and relatively cheap
2. No whipstocks (they are unreliable in open hole)
3. Suitable for vertical or deviated wells
4. Lateral can be same size as parent hole
5. Plug can be drilled out to access lower zone
6. No need for extra equipment or personnel
 Disadvantages
1. Have to wait on cement to set approx. 24 hours
2. Possible contamination of drilling fluid
3. Must have a good cement job to enable kick off
 Cased Hole Kick Offs
In cased hole kick offs, a window is first cut at the position where kick off is desired.
Thereafter, the same procedure as for open hole kick offs.

86
Cased hole kick off
 Composite Casing
In composite casing, a sacrificial casing joint is run as part of the intermediate string (say 9
5/8 "). The casing joint is then milled and a kick off assembly is run to start the multilateral
section.
Kick off composite casing
 Factors Affecting Junction Design
A junction is the point where the lateral meets the main bore.
A junction in a multi-lateral well provides:
1. Isolation of lateral and main bore from surrounding formations; and

87
2. Allows re-entry into the lateral
When designing a junction, one must think of all the factors that affect well stability,
performance and completion design.
Briefly, the junction design should consider:
 junction stability
 location of target (s) below junction
 laterals placed in zone of similar pressure and fluid properties
 production plans: commingled flow or separate flow
 casing size
 completion design
 surface location and access
 lifecycle well requirements
The junction stability depends on
 Fracture gradient at junction
 Pore pressure
 Mechanical properties of the material making up the junction
 Reactive formations around the junction
Multilateral junction.
 Technology Advancement Of Multilaterals (Taml)
The industry has agreed on a classification for the complexity of junction construction. The
classification is given the name: Technology Advancement of Multilaterals (TAML) Levels
and has values from 1 to 6; with one having the simplest design and six the most complex.
The six classes are:

88
 Level 1
Both the main bore and lateral (s) are open and the junction is unsupported. The lateral is
usually constructed in a consolidated formation from the low side of the well.
 Level 2
The main bore is cased and cemented and the lateral is open or possibly a liner dropped in
the lateral. The junction integrity and stability depends on the type of formation. The
Junction is constructed with either downhole milling or by installing a pre-milled window
joint.
 Level 3
The main bore is cased and cemented; the lateral is cased but not cemented. In this system,
mechanical integrity at the junction is required but not hydraulic integrity.
Intervention and sand control are usually the main design considerations. The junction is
constructed by mechanically attaching a liner to the main bore casing.
 Level 4
The main bore and lateral are both cased and cemented. The junction is constructed by one
of three methods;
 Performing a washover operation that removes the lateral extension and whipstock
from the wellbore thereby allowing access to the lower lateral.
 After the liner is placed in the lateral and across the junction, a hole is milled through
the liner and whipstock to expose the lower main bore.
 Low-side perforations of the lateral liner and whipstock

89
 Level 5
Pressure integrity at the junction is achieved by using the completion equipment (cement is
not acceptable).
The junction construction here is similar to that in level 4 with the added use of completion
equipment to achieve hydraulic integrity at the junction.
In addition, packers are placed above and below the junction and in the lateral to provide
complete pressure integrity at the junction. In all at least three packers: lateral isolation
packer, main bore completion packer below the junction and a main bore production packer
above the junction are required.
 Level 6
Pressure integrity at the junction is achieved with casing. The mechanical and hydraulic
integrity are achieved when the ML system is installed.
Level 6: Downhole splitter. Large main bore with two (smaller) lateral bores of equal size
coming out of a mechanical splitter.

90
Levels I and II were the earliest form of ML completion and have achieved standardization
and popularity in the industry, but are only effective in hard competent formations. The
technical complexity for levels 3-6 is far greater.

Directional Drilling 2020-2021.pdf

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Directional Drilling 2020-2021.pdf

Similar to Directional Drilling 2020-2021.pdf (20)

Recently uploaded

Recently uploaded (20)

Directional Drilling 2020-2021.pdf