Chapter 1
I NTRODUCTION
At one time, it was assumed all oil wells were essentially vertical or the bottom of the hole was
directly under the drilling rig. Unfortunately, this is not true. The petroleum industry did not
become fully aware of deviated well problems until the development of the Seminole, Oklahoma
field. The wells in this field were drilled very close together. As a result of the deviation
tendencies, wells were drilled into other drilling wells and wells which were already producing.
Also, wells were encountering the producing formation at different measured depths. The true
vertical depths were similar, but measured depths varied significantly.
In the late 1920's, survey instruments were developed that could measure both inclination and
azimuth. Deviations as high as 46º from vertical were measured in the Seminole wells. The
average deviation from vertical was approximately 13°.
The first controlled directional well was drilled in California in 1930 to tap offshore oil reserves.
Unfortunately, there was a dispute as to who owned the oil offshore. Operators were drilling
across lease lines in order to drain oil owned by someone else, resulting in legal problems.
In the 1930's, wells were directionally drilled to tap oil reserves that would otherwise be
inaccessible. Directional drilling was employed to produce oil from under a cemetery. Oil was
produced from under the ocean by placing the rig on the shore and directionally drilling into the
offshore oil deposits.
Little attention was paid to directional drilling until a relief well was drilled to kill a blowout near
Conroe, Texas. In that instance, a blowout had occurred while drilling; and as a result, a 170
foot diameter crater was created around the well. The drilling rig sank and was lost.
Approximately 6,000 barrels of oil per day were flowing from the crater. A relief well was drilled
near the surface location of the blowout. Directional drilling techniques were used to intersect
the producing formation near the blowout, and the blowout was killed by pumping fluid down the
relief well and into the blowout well. Since then, directional drilling has been widely accepted.
Today, the on-going research and development of new tools and techniques are making
directional drilling more accurate and economical.
Controlled directional drilling is defined by the API as: The art and science involving the
intentional deflection of a wellbore in a specific direction in order to reach a predetermined
objective below the surface of the earth. Today, it is much more science than art.
USES OF DIRECTIONAL DRILLING
Sidetracking is one of the primary uses for directional drilling. Sidetracking is an operation
which deflects the borehole by starting a new hole at any point above the bottom of the old hole
as in Figure 1-1. The primary reason for sidetracking is to bypass a fish which has been lost in
the hole; however, there are several other reasons for sidetracking. A sidetrack can be
performed so the bottom of the hole can intersect a producing formation at a more favorable
position such as up dip above the oil-water contact. A well can be sidetracked to alleviate
problems associated with water or gas coning. A sidetrack can be performed in an old well to
move the location of the bottom of the hole from a depleted portion of the reservoir to a portion
that is productive, such as, across a fault or permeability barrier. Sidetracking an exploration
Copyright © 2007 OGCI/PetroSkills. All rights reserved.
1-1
Horizontal and Directional Drilling
Chapter 1
well can lead to a better geologic understanding of an area (Figure 1-2) especially where the
geology is complicated. Sidetracking and directional drilling can be more economical than
multiple exploration wells if the upper portion of the well is expensive to drill.
In horizontal wells, it is a common practice to sidetrack existing vertical wells most frequently
utilizing the short radius method. A whipstock is set inside the casing and the well sidetracked.
Then the formation is drilled horizontally to increase productivity. Multiple sidetracks can be
drilled from the same well, which are termed multilaterals.
Most often, a sidetrack is accomplished by setting a cement plug in the hole and dressing off
the plug to a depth at which the sidetrack will commence. The sidetrack can be either "blind" or
"oriented". In a blind sidetrack, the direction of the sidetrack is not specified and is not
considered a directional well. In either case, a deflecting tool is used to drill out the old hole and
start a new hole.
Figure 1-1. Sidetracking a Stuck Bottomhole Assembly.
1-2
Copyright © 2007 OGCI/PetroSkills. All rights reserved
Horizontal and Directional Drilling
Introduction
Figure 1-2. Multiple Sidetracks
Straight hole drilling is a special case of directional drilling where an attempt is made to keep
the hole vertical. Some reasons for wanting to keep the hole vertical are:
1. To keep from crossing lease lines;
2. To stay within the specifications of a drilling contract;
3. To stay within the well spacing requirements in a developed field (Figure 1-3).
In some areas of the world, deviation from vertical is caused by the natural formation
tendencies. Packed hole assemblies are employed to keep the dogleg severity within reason.
Pendulum assemblies are used to keep the inclination as low as possible though with limited
success at lower inclinations. If the inclination is already too great to hit a previously specified
target, pendulum assemblies, and sometimes downhole motors are used to bring the hole back
within range of the target. It should be noted here that sometimes targets are unduly restricted.
Controlling the inclination of a well costs significantly more than letting it deviate and keeping
the dogleg severity within reason. If there are no restrictions on bottomhole location, the well
should be allowed to deviate.
Copyright © 2007 OGCI/PetroSkills. All rights reserved.
1-3
Horizontal and Directional Drilling
Chapter 1
Figure 1-3. Straight Hole Drilling
Controlled directional drilling is used when drilling multiple wells from an artificial structure
such as offshore platforms, drilling pads, or man made islands (Figure 1-4). The economics of
building one offshore platform for each well would be prohibitive in most cases. However, since
wells can be directionally drilled, forty or more wells can be drilled from a single platform.
Without controlled directional drilling, most offshore drilling would not be economical. Some
fields are developed using drilling pads where multiple wells are drilled from one location due to
economic or environmental pressures. Where the environment is concerned, roads and
1-4
Copyright © 2007 OGCI/PetroSkills. All rights reserved
Horizontal and Directional Drilling
Introduction
production facilities may not be allowed for each surface location with a vertical well. As oil
companies become more environmentally conscious, it may be politically advantageous to
develop fields from drilling pads in sensitive areas. In areas of shallow water depth, multiple
wells can be drilled from artificial islands. Subsea wells are drilled from a template on the ocean
floor. In all cases, location construction expenses and rig move expenses are reduced. Also,
due to the close proximity of the wells, production costs are lower. However for most land wells,
it is usually more economical to drill vertical wells rather than drill directional wells from a pad.
Figure 1-4. Multiple Wells from an Artificial Structure
Copyright © 2007 OGCI/PetroSkills. All rights reserved.
1-5
Horizontal and Directional Drilling
Chapter 1
There are special cases when multiple sands are drilled with a single wellbore. Where
steeply dipping sand zones are sealed by an unconformity, fault, or salt dome overhang, a
number of vertical wells would be required to produce each sand, which are separated by a
permeability barrier. However, all the sand zones can be penetrated with one directionally
drilled well thereby greatly reducing the cost of production (Figure 1-5).
Figure 1-5. Drilling Multiple Sands from a Single Wellbore
1-6
Copyright © 2007 OGCI/PetroSkills. All rights reserved
Horizontal and Directional Drilling
Introduction
There are times when oil deposits lie under inaccessible locations such as towns, rivers,
shorelines, mountains, or even production facilities (Figure 1-6). When a location cannot be
constructed directly above the producing formation, the wellbore can be horizontally displaced
by directional drilling. This allows production of an otherwise inaccessible hydrocarbon deposit.
Figure 1-6. Inaccessible Location
Copyright © 2007 OGCI/PetroSkills. All rights reserved.
1-7
Horizontal and Directional Drilling
Chapter 1
Directional drilling is also applicable in fault drilling (Figure 1-7). It is sometimes difficult to drill
a vertical well in a steeply dipping, inclined fault plane. Often, the bit will deflect when passing
through the fault plane, and sometimes the bit will follow the fault plane. To avoid the problem,
the well can be drilled on the upthrown or downthrown side of the fault and deflected into the
producing formation. The bit will cross the fault at enough of an angle where the direction of the
bit cannot change to follow the fault.
Figure 1-7. Fault Drilling
1-8
Copyright © 2007 OGCI/PetroSkills. All rights reserved
Horizontal and Directional Drilling
Introduction
Many oil fields are associated with the intrusion of salt domes. Directional drilling has been
used to tap some of the oil which has been trapped by the intrusion of the salt. Instead of
drilling through the salt overhangs, the wells can be directionally drilled adjacent to the salt
dome and into the underlying traps as shown in Figure 1-8. However, since the development of
salt saturated and oil based muds, the amount of directional drilling has decreased. It is difficult
to drill long intervals of salt with fresh water muds. Directionally drilling around the salt,
alleviates a lot of the problems associated with drilling salt.
Figure 1-8. Salt Dome Drilling
Copyright © 2007 OGCI/PetroSkills. All rights reserved.
1-9
Horizontal and Directional Drilling
Chapter 1
A highly specialized application for directional drilling is the relief well. If a well blows out and is
no longer accessible from the surface, then a relief well is drilled to intersect the uncontrolled
well near the bottom (Figure 1-9). Water or mud are then pumped through the relief well and
into the uncontrolled well. Since it is sometimes required that the relief well intersect the
uncontrolled well, the directional drilling has to be extremely precise and requires special tools.
Survey data is not accurate enough to intersect a wellbore at depth. Proximity logging is
required when drilling relief wells.
Figure 1-9. Relief Well Drilling
Horizontal drilling is another special application of directional drilling and is used to increase
the productivity of various formations (Figure 1-10). One of the first applications for horizontal
drilling was in vertically fractured reservoirs. In fractured reservoirs, a significant quantity of the
production comes from fractures. Unless a vertical well encounters a fracture system,
production rates will be low. A horizontal well has a much greater chance of encountering a
1-10
Copyright © 2007 OGCI/PetroSkills. All rights reserved
Horizontal and Directional Drilling
Introduction
prolific fracture system. The Austin Chalk in Texas is a classic example of using horizontal
drilling techniques to produce a fractured reservoir. Horizontal wells are a very common way to
produce some formations.
Figure 1-10. Horizontal Drilling
Horizontal drilling is used to produce thin oil zones with water or gas coning problems. The
horizontal well is optimally placed in the oil leg of the reservoir. The oil can then be produced at
high rates with much less pressure drawdown because of the amount of formation exposed to
the wellbore.
Horizontal wells are used to increase productivity from low permeability reservoirs by increasing
the amount of formation exposed to the wellbore. Additionally, numerous hydraulic fractures
can be placed along a single wellbore to increase production and reduce the number of vertical
wells required to drain the reservoir.
Copyright © 2007 OGCI/PetroSkills. All rights reserved.
1-11
Horizontal and Directional Drilling
Chapter 1
Horizontal wells can be used to maximize production from reservoirs which are not being
efficiently drained by vertical wells. These wells usually have permeability streaks in
combination with natural fractures. The horizontal well can connect the portions of the reservoir
that are productive.
Directional drilling can also be used to drill multilateral wells. Multilaterals are additional wells
drilled from a parent wellbore as illustrated in Figure 1-11. Multilaterals can be as simple as an
open hole sidetrack or it can be more complicated with a junction that is cased and has
pressure isolation and reentry capabilities. Multilaterals are used where production can be
incrementally increased with less capital costs. Multilaterals can be used offshore where the
number of slots are limited. It is also used to place additional horizontal wells in a reservoir.
Figure 1-11. Multilateral Wells Drilled From a Platform.
1-12
Copyright © 2007 OGCI/PetroSkills. All rights reserved
Horizontal and Directional Drilling
Introduction
Another application of directional drilling is what is commonly termed extended reach drilling.
As illustrated in Figure 1-12, extended reach drilling is where wells have high inclinations and
large horizontal displacements for the true vertical depth drilled. Extended reach drilling is used
to develop reservoirs with fewer platforms or smaller sections of a reservoir where an additional
platform cannot be economically justified. Extended reach drilling will become more popular as
the cost of platforms in deeper water and severe environments becomes more expensive.
Definition of Extended Reach Drilling
0
2000
80 Degrees
4000
TVD Below KOP, feet
70 Degrees
6000
60 Degrees
8000
50 Degrees
10000
12000
Extended Reach
Right of Line
14000
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
Departure, feet
Figure 1-12. Extended Reach Drilling Definition after Mobil 1 .
Copyright © 2007 OGCI/PetroSkills. All rights reserved.
1-13
Horizontal and Directional Drilling
Chapter 1
Advances in technology have allowed operators to drill extended reach wells with very high
HD/TVD ratios (the ratio of the horizontal displacement to true vertical depth). Wells have been
drilled with HD/TVD ratios in excess of 6/1 as illustrated in Figure 1-13. In these wells the
horizontal departure was more than six times the true vertical depth with the total measured
depth exceeding 32,800 feet (10,000 m).
Figure 1-13. Extended Reach Wells Drilled By BP2 .
TYPES OF DIRECTIONAL WELLS
There are four basic types of directional wells. Most wells can be categorized by one of the four
basic types or a combination thereof.
A Type I well (Figure 1-14) is often called a build and hold. The Type 1 well is drilled vertically
from the surface to kickoff point at a relatively shallow depth. At that point, the well is steadily
and smoothly deflected until a maximum angle and the desired direction are achieved. Then,
casing is run and cemented if desired. The established angle and direction are maintained
while drilling to the target depth. One or more strings of casing can be run if necessary.
Usually this method is employed when drilling shallow wells with single producing zones.
The Type II well (Figure 1-15) is often called an “S” curve. It is similar to the Type I because the
well is deflected at a relatively shallow depth, and surface casing is frequently (but not always)
run through the build curve. The angle and direction are maintained until a specified depth and
1-14
Copyright © 2007 OGCI/PetroSkills. All rights reserved
Horizontal and Directional Drilling
Introduction
horizontal departure has been reached. Then, the angle is steadily and smoothly dropped until
the well is near vertical. Intermediate casing is usually run through the section of the hole
where the angle was dropped. Drilling continues in the vertical hole below the intermediate
casing to the target. Type II wells are generally used where multiple pay zones are
encountered. Also, after the well has been returned to vertical, directional drilling services are
no longer required. Since most of the directional drilling is done in the more shallow portions of
the hole where trips are shorter and penetration rates are higher, the overall cost of the well is
reduced. A disadvantage of the Type II is that it will generate more torque and drag for the
same horizontal departure.
Target
Departure
Target
Depth
Figure 1-14. Basic Hole Pattern for a Type I Well
Copyright © 2007 OGCI/PetroSkills. All rights reserved.
1-15
Horizontal and Directional Drilling
Chapter 1
Target
Departure
Target
Depth
Figure 1-15. Basic Hole Pattern for a Type II Well
1-16
Copyright © 2007 OGCI/PetroSkills. All rights reserved
Horizontal and Directional Drilling
Introduction
The Type III well (Figure 1-16) is a continues build to target. It is similar to the Type I well
except the kickoff point is at a deeper depth, and surface casing is set prior to deflecting the
well. The well is deflected at the kickoff point, and inclination is continually built through the
target interval. The inclinations are usually high and the horizontal departure low. This type of
well is generally used for multiple sand zones, fault drilling, salt dome drilling, and stratigraphic
tests. It is not used very often.
Target
Departure
Target
Depth
Figure 1-16. Basic Hole Pattern for a Type III Well
Copyright © 2007 OGCI/PetroSkills. All rights reserved.
1-17
Horizontal and Directional Drilling
Chapter 1
Type IV wells can be categorized as horizontal or extended reach wells (Figure 1-17). Design
of these wells can vary significantly, but they will have high inclinations and large horizontal
departures. Horizontal wells will have an inclination greater than 80°.
Target
Depth
Target
Departure
Figure 1-17. Basic Hole Pattern for a Type IV Well
1-18
Copyright © 2007 OGCI/PetroSkills. All rights reserved
Horizontal and Directional Drilling
Introduction
There are some directional well designs that do not fit any of the above types. A term that is
frequently used is “Designer Wells.” Designer wells are wells that with several targets and the
targets are widely spaced. They require significant changes in azimuth along with changing
inclination and have a highly engineered well plan.
REFERENCES
1
Tolle, G. and Dellinger, T.; "Mobil Identifies Extended Reach Drilling Advantages, Possibilities
in North Sea," Oil & Gas Journal, May 26, 1986, pp 78-86.
2
Mason, C.J.; Lopez, J.; Meling, S.; Munger, R and Fraser, A; SPE 84447, “ Casing Running
Challenges for Extended-Reach Wells,” SPE ATC, Denver, October, 2003.
Copyright © 2007 OGCI/PetroSkills. All rights reserved.
1-19