Introduction to
Directional Drilling
Wayne Longstreet
Drilling Manager - Dragon Oil (Turkmenistan) Ltd
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1
Objective of Directional Drilling
Controlled directional drilling is
the science of deviating a
wellbore along a planned course
to a subsurface
target whose location
is a given lateral
distance and direction
from the vertical.
Slide
2
Historical Background
• Originally only as a remedial operation.
• Now primarily a reservoir optimization tool.
• First well surveying in 1920’s in Oklahoma.
Acid Bottle Inclinometer.
• 1929 a directional inclinometer with
magnetic needle first used. Acid bottle
technique proved to have error margin of
@10 degrees (low).
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3
Historical Background
(cont’d)
• As more accurate survey tools developed, it
was found most boreholes were “crooked”.
• Thus the emerging science of geology as
given a boost when it was realized that the
measured depth of producing zones was in
many cases different from the vertical depth.
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4
Historical background
(cont’d)
• 1930’s saw the first controlled directional
well drilled in Huntington Beach,
California. 1933 used to drill under
Sunnyside Cemetery in Long Beach.
• 1934 first relief well drilled in Conroe,
Texas.
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5
Historical background
(cont’d)
• Today the technology incorporates:
Horizontal, EM-MWD, SAG-D, Multi-Laterals,
Extended Reach Drilling, Downhole
Adjustable Gauge Stabilizers, Downhole
Adjustable Motors, Bicentric Bits, 3D Wells,
Underbalanced Wells, River Crossings, CTU
Drilling.
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6
Reasons for Drilling
Directional Wells
Slide
7
Relief Wells
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8
Faulted Formations
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9
Multiple Wells from a Single Structure
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10
Reach Inaccessible Locations
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11
Straight Hole Control & Sidetracking
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12
Horizontal and Multi-Laterals
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13
Steam Assisted Gravity Drainage
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14
Up and Down Dip Laterals
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15
Draining Multiple Reservoirs
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16
More Efficient Drainage of a
Single Reservoir
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17
Geological Considerations
• Knowledge of the
local geology is
essential to the
directional driller.
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18
Where Under the
Earth Are We?
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19
Surveying
• Regardless of the type of survey instrument
(single-shot, multishot, steering tool, SRO Gyro, MWD, EMMWD)
three pieces of information are known.
– Survey Measured Depth.
– Borehole Inclination.
– Borehole Azimuth (corrected for true north).
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20
Directional Surveying Permits
• The determination of the bottomhole location
relative to the surface location or another
reference system.
• The location of excessive doglegs or hole
curvature.
• The monitoring of inclination and azimuth
during drilling.
• The orientation of deflection tools.
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21
Survey Calculation Methods
• Average Angle
• Radius of Curvature.
• Minimum Radius of Curvature.
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22
Average Angle
• Calculates the average of the angles at the
top and bottom of the course section and
assumes this to be the inclination and
direction of the wellbore.
• Oldest and least accurate method.
• Easiest to calculate by hand.
• Accurate when BUR is small and survey
stations are close together.
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23
Radius of Curvature
• Each course length is defined by points at the
top and bottom, and the wellbore is assumed
to be curved in either or both the vertical and
horizontal projections.
• More complex calculations than average
angle.
• Accurate when stations are far apart with
higher rates of curvature.
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24
Minimum Radius of Curvature
• Projects the actual dogleg and accounts for
the severity of the dogleg on the drillstring.
• Most accurate method of calculation in use
today.
• The use of computers and programmable
calculators have made this the only real tool
used today.
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25
Radius of Uncertainty
• All tools have a range of accuracy.
• The assumption is made that errors will
average out, but this still leaves us with a
cone of potential error.
• Critical in thin sections and extended
horizontal wells.
• Use of LWD, Geosteering, and geology help
the directional driller.
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26
Relative Accuracy of Methods
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27
Calculation
Method
Error on TVD
(ft)
Error on
Displacement
Tangential
Balanced Tangential
Average Angle
Radius of Curvature
Minimum Radius
Mercury (STL = 15’)
-25.38
-0.38
+0.19
0.00
0.00
-0.37
+43.09
-0.21
+0.11
0.00
0.00
-0.04
The Earth’s Magnetic Field
• Theory #1: Rotation of the earth’s mantle in
relation to the liquid core is thought to
produce electrical currents.
• Theory #2: The internal circulation currents
(similar to phenomenon observed at the
periphery of the sun) of the liquid iron in the
earth’s core acts as the source according to
the principle of a self-excited dynamo.
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28
Magnetic Declination
• The angle between magnetic north and
geographic north (true north) is defined as
the angle of declination.
• All surveys are converted to true north.
• Angles of declination to the west of true
north can be written as negative numbers, to
the east as positive numbers.
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29
Visual
Magnetic North
True North
Angle of
Declination
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30
Magnetic Interference
• Caused by:
– Drill String.
– Fish left in hole.
– Nearby casing.
– Geology (Iron Pyrite, Hematite)
– Magnetic “hot spot” in Drill Collar.
– Fluctuations in the earth’s magnetic field.
(minor)
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31
Minimizing Drill String Interference
• Eliminate magnetism by using “nonmag” collars (monel).
– The connection area can be
magnetized due to mechanical torque.
(azimuth errors in 10’s of degrees)
Never space within 2’ of connection.
– Do not space in the center. Collars are
bored from both ends leaving a ridge in
center and potential magnetic hot spot.
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32
Drill String Interference
(cont’d)
• Non-mag stabilizers are magnetic near the
blades (hard facing can be very magnetic).
• Amount of non-mag BHA is affected by:
– Latitude.
– Hole Inclination.
– Distance from North/South hole azimuth.
– Location (Alaska has used as much as 165 ft
above magnetometer).
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33
True Vertical Depth vs Measured Depth
MD
TVD
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34
Vertical Section and Closure
• VS is the length of the horizontal
displacement defined by it’s azimuth in
relation to the target.
• Closure is the length of the horizontal
displacement passing through the survey
point.
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35
Azimuth
• Wellbore direction measured in the
horizontal plane and expressed in
degrees from the North direction starting
at 0 and continuing clockwise to 360.
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36
Dog Leg Severity
• AKA BUR.
• Expressed in degrees per unit of length.
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37
Basic Well Planning
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38
Defining Objectives
• Careful planning is essential for success.
• Each well will have specific objectives
defined by the reservoir or business units.
• The design must be tailored to meet all of
the objectives.
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39
Location
• DD involves drilling a hole from one point in
space (surface location) to another point in
space (target).
• Local coordinate system must be known so
the target can be accurately correlated to
the target.
• Most directional plans will use wellhead
location as 0.
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40
Target Size
• During drilling the trajectory is constantly
monitored in relation to the target.
• Costly decisions are constantly being made
to ensure that the well objectives are met.
• Today’s technology allows us to drill
extremely accurate wells.
• Cost of the well is largely dependent upon
accuracy required.
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41
Cost
vs
Accuracy
• Operators often adopt arbitrary target sizes
or tolerances which do not reflect the
geological realities of the reservoir.
• Many needless correction runs have been
made.
• Hard Boundaries must be clearly defined.
Legal limits, fault lines, pinch outs.
• Communication and Team Work required.
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42
Wellbore Profile
• Given surface location, target location,
target tvd, and rectangular coordinates, it is
possible to determine the geometric well
profile from surface to bottomhole target.
• General directional well types:
– Straight, Build and Hold, “S” Wells, Slant
Wells, Horizontal, and Multi-Lateral.
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43
Determining KOP
• Kick Off Point is the depth at which the well
will be deviated off the vertical.
• Selection of KOP is made by considering
the geometrical wellpath and geological
characteristics.
• Optimum inclination is determined by
maximum permissible BUR and location of
the target.
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44
Determining Build & Drop Rates
• Maximum permissible build/drop rate is
determined by:
– Total depth of well, and hole size.
– Torque & Drag limitations. High DLS results in
higher T&D except in horizontal wells.
– Geology - high bur’s not always possible in soft
formations.
– Limitations of tools, casing, drill strings.
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45
Types of Directional Wells
• Build and Hold
• “S” Wells
• Slant Wells
• Horizontal Wells
• Multi-Laterals
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46
Anti-Collision
• As platform and pad drilling becomes more
popular anti-collision planning is critical.
• Radius of uncertainty.
• Lead angle for rotary drilling.
• Accurate well plan map essential.
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47
Survey Tools
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48
Steering Tools
• Continuous readout when drilling without
rotation. Some available now that will allow
slow rotation - unreliable.
• Jointed pipe operations require a “wet
connect” system.
• Wet connects are unreliable and time
consuming.
• System of choice for coil drilling.
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49
Magnetic Single & Multi Shots
• Camera or Electronic (digital).
• Very Accurate.
• Still the basic surveying tool.
• Multi-shot surveys used to legally confirm
open hole wells drilled with MWD. Pump
down - time out.
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50
Gyroscopes (Gyros)
• Rate Integrating Gyro (North Seeking) most
common today.
• Very accurate.
• Used to legally confirm wellbore location of
cased holes.
• Most common gyro errors are caused by
initial alignment or drift.
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51
Gyro Drift
•
Drift values may range as follows:
– 0.5 to 1 degree/min for cheap gyros (toys).
– A few degrees per hour for directional gyros.
– 1/100th degree per hour for inertial gyros using
gimbal flotation.
– 1/1000 degree per hour for some inertial gyros with
spherical spinning rotors, supported by electrical
fields. Used in space flight.
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52
Mud Pulse MWD
• Use coded mud pulses to transmit tool data
to surface in digital (binary) form.
• Pulses are converted to electrical energy by
a transducer at surface and decoded by
computer.
• Three types: Negative Pulse, Positive
Pulse, Standing Wave Generator.
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53
EM - MWD
• Uses the same triaxial inclinometers and
triaxial magnetometers as conventional
MWD.
• Transmits data to surface using Electromagnetic telemetry.
• Dependent on depth and formation
resistivity.
• Two commercial systems available.
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54
Directional Drilling
Tools
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55
Spiral Drill Collars
• Spiral grooves reduce
the wall contact by 40%
with a weight reduction
of only 4%
• Reduces chances of
becoming differentially
stuck.
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56
Non-Magnetic Drill Collars
• Usually flush, not spiraled.
• Made from high quality
stainless steel.
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57
HWDP
• Less rigid than drill collars.
• More common in the modern
era of higher build rates and
horizontal wells.
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58
Stabilizers
• Indispensable part of
rotary directional BHA’s.
• Non-rotating styles
available,
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59
Downhole Adjustable Stabilizers
• Essential for extended
reach directional
drilling.
• Adjustable by changing
pump pressure or by
cycling pumps.
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60
Roller Reamer
• Designed to
maintain hole gauge.
• Either 3 point or 6
point.
• Near bit roller
reamers help
prolong bit life,
without adding
torque associated
with near bit
stabilizers.
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61
Underreamers & Hole Openers
• Used to wipe out
bridges and keyseats,
opening directional pilot
holes, opening holes for
casing, drilling out skin
damage.
• Underreamer is
hydraulically operated.
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62
Keyseat Wiper
• Run between the top
drill collar and the drill
pipe.
• Greater diameter than
the DC’s.
• If stuck, release, jar out,
rotate and back ream to
eliminate keyseat.
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63
Bent Sub
• Run on top of a straight
motor.
• Not in common use
after the advent of
steerable assemblies
• If bored for mule shoe it
becomes an orienting
sub.
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64
Steerable Motor
• Most common tool in
use today.
• Versatile. Rotary or
slide drilling.
• Saves many trips for
BHA changes
experienced in the past.
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65
Motor Assembly Cross Section
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66
Dump Valve Assembly
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67
Power Section Assembly
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68
Surface Adjustable Bend
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Bearing Assembly
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70
Deflection Methods
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71
Whipstocks
• Most commonly used in
re-entries.
• Controllable and
versatile.
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72
Jetting
• Used in soft
formations.
• Erratic results
and severe
doglegs.
• Not in common
use today.
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73
Bent Sub-Straight Motor
• Not commonly used
today, since the
advent of steerable
assemblies.
• Good for low build
rates.
• Non-rotatable.
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74
Steerable Assemblies
• Very versatile and
controllable.
• Modern method of
directional drilling.
• Motor bend less than 2
degrees.
• Offset pads are run for
high build rates.
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