horizontal well 2

7/28/2019 Horizontal Well 2

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CHAPTER-3

HORIZONTAL DRILLING

Horizontal drilling has become one of the most valuable technologies

ever introduced into the upstream oil business. Along with other advances

over the last fifteen years such as massive fracturing and 3-D seismic,

horizontal drilling has had a significant impact on oil and gas production. A

considerable number of articles and papers have been written on horizontal

drilling, but the focus has been on the hardware and drilling side of the

technology. Relatively little has been published on the reserves that have

been found. This lack of reserve and economic information is partly due to

operators wanting to retain a competitive advantage in their drilling. But it is

also due to the difficulty in applying traditional reserve estimation methods

to a situation which petroleum engineers do not fully understand. Volumetric

analysis is complicated because of a lack of knowledge on extent of the

fracture systems. Pressure transient and reservoir simulation methods are

made difficult by the same poor understanding of the reservoir character.

Analogy has not been effective since there simply are none to this new

technology. That leaves performance analysis as a reserve estimating tool.

And it is only recently that enough history is available to reliably examine

the reserve potential of horizontal wells.


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HISTORY OF HORIZONTAL DRILLING

Little horizontal drilling occurred until the early 1980s, by which time

improved down hole drilling motors and the invention of other necessary

supporting equipment, materials, and technologies, particularly down hole

telemetry equipment, had brought some kinds of applications within the

imaginable realm of commercial viability. Tests indicating that commercial

horizontal drilling success could be achieved in more than isolated instances

were carried out between 1980 and 1983 by the French firm Elf Aquitaine in

four horizontal wells drilled in three European fields. These included the

Rospo Mare Oil Field, located offshore Italy in the Mediterranean Sea,

where output was very considerably enhanced. Early horizontal production

well drilling was subsequently undertaken by British Petroleum in Alaskas

Prudhoe Bay Field, in a successful attempt to minimize unwanted water and

gas intrusions. Horizontal drilling has since been undertaken with increasing

frequency by many more operators. Domestic horizontal wells have now been

planned and completed in at least 57 counties or offshore areas located in or

off 20 States. They have been almost entirely focused on crude oil

applications. In 1990, worldwide, more than 1,000 horizontal wells were

drilled. Some 850 of them were targeted at Texas Upper Cretaceous Austin

Chalk Formation alone. Less than 1 percent of the domestic horizontal wellsdrilled were completed for gas, as compared to 45.3 percent of all

successful wells (oil plus gas) drilled. Of the 54.7 percent of all successful

wells that were completed for oil, 6.2 percent were horizontal wells. Market


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penetration of the new technology has had a noticeable impact on the drilling

market and on the production of crude oil in certain regions.

Chronological Order

First horizontal well 1929 Economic viability in 1980s

o Rospo Mare Field (1982)o Prudhoe Bay (1984)o Bima and Arjuna Fields (1986 - 1987)o Austin Chalk (1985 - 1987)

Through 2000o 23,385 Horizontal Wells From 69 Countries

United States 10,966 Canada 9,655

DEFINITION AND THEORY

To attain higher performances, the oil and gas companies are demanding

greater drilling efficiency in conditions such as extended reach and

horizontal drilling. The improved production and return on investment can be

achieved from fewer wells, but better quality wells.


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Figure 2 Theoretical vertical profile for a buildup rate of 1/10 m

(3/100 ft) for a well reaching horizontal. (Courtesy Inst. Fr. du Petr.)


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Figure 3 - Theoretical vertical profile for a buildup rate of 2/10 m

(6/100 ft) for a well reaching horizontal. (Courtesy Inst. Fr. du Petr.)

Figure 1 and figure 2 show the theoretical vertical profile for a

buildup to horizontal with respectively 1/10 m (3/100ft) and 2/10 m

(6/100ft). In the first case, the distance below kick-off point (KOP) to

reach horizontal is 570 m (2870ft) TVD, with ameasured depth of 900m

(2952ft). In the second case, the corresponding lengths are 290m (951ft)

and 450m (1,476ft).


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To follow accurately the theoretical trajectory, MWD techniques

must be used. When the borehole is near horizontal, logging or surveying

tools cannot be lowered by gravity anymore. They must be pumped down the

drill pipes for directional measurements. Conventional logging has to be

carried out by conveying the logging sondes downhole at the tip of the drill

string. The logging operation becomes long, expensive and dangerous. A much

more efficient way is to survey the trajectory and record the logs while

drilling. The logging data can be used to ascertain that the borehole is being

drilled in the anticipated pay zone. If not, immediate remedial action is

taken to steer the well towards the pay zone. The most advanced

technique in use today is the geosteeringtechnique.

Geosteering is usually done with a mud motor. A mud motor with bent

sub allows changing of orientation and inclination without pulling the drill

string out. Steering is done by rotating it a small angle. In classical

geosteering the sensors for inclination, azimuth, drilling parameters, and

logging are located above the mud motor and the distances may be in the

order. Although radial measurements can be performed to verify that the

borehole is being drilled in the pay zone, it is often too late to make a

correction and the borehole leaves the pay zone.

The new geosteering system offers measurements at the bit (below

the mud motor) of inclination, rpm, azimuthal gamma ray, azimuthal

resistivity, and bit resistivity. The signals are transmittedelectromagnetically to the MWD sub located above the mud motor, then

relayed to surface with the standard mud pressure transmission system. To

summarize, the following is recorded just above the drill bit:


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inclination revolution per minute azimuthal gamma ray azimuthal resistivity bit resistivity

Above the mud motor, the following is recorded:

weight-on-bit torque inclination azimuth tool face neutron density Pe

Other parameters, such as alternator voltage (for flow rate),

temperature and pressure, can also be monitored.

An example of three horizontal wells drilled in a 2m (6ft) in the North

Sea is shown in Figure 4.

Well No. 1was drilled with inclination and azimuth data only. The sensorswere located above the mud motor. Only a short section (63m; 207ft) was

drilled in the reservoir.


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Figure 4 North Sea geosteering example. (Courtesy Anadrill.)

Well No. 2 was geologically steered by adding gamma ray and

resistivity capability. Only a short section is out of the reservoir, making a

total of 168 m (552 ft) in the reservoir.

Well No. 3was steered with the new geosteering system. A smooth

trajectory was obtained with the whole interval in the reservoir. The last

section was dipped intentionally to investigate the lower reservoir.

We see that horizontal drilling can be carried out satisfactorily if the

following is available:

sophisticated MWD/LWD technology computer capability positive displacement motor


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Suggestions have been made recently to drill horizontal brunchesin the

horizontal portion of the horizontal wells. If this technique is developed, the

drainage capacity of horizontal wells will be even better.

Types of Horizontal Wells

Horizontal drilling is performed using long radius of curvature to

reach horizontal: 1 to 2/10m (3to 60/100ft). Attempts have been made

for years to improve production with medium, short and ultra short radius of

curvature. The sketch of Figure 5shows the four types of curvatures.

Figure 5 Schematics ofdifferent types of wells or drains: (a) ultra short

radius; (b) short radius; (c) medium radius; (d) long radius.

Table 1gives the range of values of radius, angle change with depth,

and usual horizontal lengths drilled in each case.


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Table 1 Characteristics of Horizontal Wells

Ultra short radius wells are usually called drain holes. They are

drilled with special equipment and completed with 1 to 2 in. tubing. The

tubing is perforated and severed where it reached the main vertical hole. No

MWD or LWD operations are carried out in these drain holes.

Shortradiuswells are usually drilled from a cased or uncased vertical

well. Articulated drill collars are used to drill to 90 orbeyond. A second

stabilized assembly is used to drill the rest of the hole, usually in 4 or 6

in. diameter.

No MWD or LWD equipment exists to date to log these wells.

However, service companies are developing equipment for MWD purpose in

40-ft (12-m) curvature radius or 1.5/ft (4.6/m). The equipment consists of

articulated mud motors and inclination and azimuth sensors.


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Medium radius and long radius wells are drilled with conventional

oilfield tools. Both MWD and LWD are usable in these wells. Downhole

motors are mostly used in medium radius wells to avoid fatigue of the BHA.

Long radius wells have been drilled with both mud motors and rotary

techniques.

Air drilling operationscan also be utilized for directional drilling and

MWD/ LWD technologies. Due to the absence of reliable downhole

pneumatic motors, this technology is not yet fully developed.

Example 1


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Dogleg Severity (Hole Curvature) Calculations 2

Currently there are several analytical methods for calculating dog-leg

severity.

These methods include:

Tangential Radius of curvature Average angle Trapezoidal (average tangential) Minimum curvature

Example 2 (Tangential Method)


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Example 3 (Radius of Curvature Method)


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Example 4 (Vectorial Method)

Example 5 (Average Tangential Method)


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Brief Summary for Definition and Theory of Horizontal Drilling Wells 2,4

Petroleum engineers categorize horizontal wells according to the

radius of the arc described by the well bore as it passes from the vertical

to the horizontal. Wells with arcs of 3 to 40 foot radius are defined asshort-radius horizontal wells. Medium-radius wells have arcs of 200 to 1,000

foot radius, while long-radius wells have arcs of 1,000 to 2,500 feet. The

required horizontal displacement, the required length of the horizontal

section, the position of the kickoff point (from the vertical), and completion

constraints are generally considered when selecting a radius of curvature.

Short-radius horizontal wells are commonly used when re-entering

existing vertical wells in order to use them as the physical base for the

drilling of add-on arc and horizontal hole sections. The steel casing (lining)

of an old vertical well facilitates attainment of a higher departure (or "kick


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off") angle than can be had in an uncased hole, so that a short-radius profile

can more quickly attain horizontality, and thereby rapidly reach or remain

within a pay zone. The small displacement required to reach a near-

horizontal attitude favors the use of short-radius drilling in small lease

blocks, while a need to avoid extended drilling in a difficult overlying

formation may call for use of a short-radius well that kicks off near the

bottom of, or below, the difficult formation. Short-radius horizontal drilling

also has certain economic advantages over longer radius drilling. These

include a lower capital cost, the fact that the suction head for downhole

production pumps is smaller, and that use of an MWD system is frequently

not required if long horizontal sections are not to be drilled. A current

drawback to use of a short-radius horizontal well is that adequate tools do

not yet exist to reliably do producing zone isolation, logging, remedial, or

stimulation work in short-radius holes. Most therefore have to be completed

open hole (no casing), and to allow this the reservoir rock must be physically

competent, or serious production problems will result.

Medium-radius horizontal wells allow the use of larger hole diameters,

near-conventional bottom hole (production) assemblies, and more

sophisticated and complex completion methods. It is also possible to log the

hole. Although the drilling of medium-radius horizontal wells does require

the use of an MWD system, which increases drilling cost, medium-radius

holes are perhaps the most popular current option. They can be drilled on

leases as small as 20 acres. Long-radius holes can be drilled using either

conventional drilling tools and methods, or the newer steerable systems.

Long-radius wells, in the form of deviated wells (not, however, deviated to


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the horizontal), have existed for many years. They are not suited to leases

of less than 160 acres due to their long lateral displacements before

reaching the horizontal.

The attainable horizontal displacement, particularly for medium- and

long-radius wells, has grown significantly, as operators and the drilling and

service contractors have devised, tested, and refined their procedures, and

as improved equipment has been designed and used. Routinely achievable

horizontal displacements have rapidly climbed from 400 to over 8,000 feet.

EXAMPLE 1

A single build horizontal well is to be planned with the following data.At what

depth is the KOP and at what depth does the horizontal section begin?

Build Gradient:14 deg/100 ft to 90 deg

TVD of the horizontal section:8000ft

SOLUTION

Build 409.26ft14

18000RTVD ===

7590.7ft409.268000KOP ==

The Departure beginning of the horizontal section is 409.26 ft

EXAMPLE 2

A combination horizontal well with a tangent section is to be planned with

the following data.At what depth is the KOP ?

Upper Build:5deg/100 ft to 75 deg.

Tangent Length:350 ft

Lower Build:12deg/100 ft to 90 ft


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TVD of the horizontal section:7350 ft

75deg.1 = 15deg.2 = 90deg.3 =

[ ]

1213.7ftTVDInclined

sin(75)sin(90)477.46350cos(75))1145sin(75TVDInclined

477.46ft12

18000R1145.92ft5

18000BG100R

)sin(sinRTcossinRTVDInclined

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132111

=

++=

=====

++=

6137ft12137350KOP ==

horizontal well 2

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