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| Source: Anadarko
Petroleum Corporation |
Once a natural gas or oil well is drilled, and it has
been verified that commercially viable quantities of
natural gas are present for extraction, the well must
be 'completed' to allow for the flow of petroleum or
natural gas out of the formation and up to the surface.
This process includes strengthening the well hole with
casing, evaluating the pressure and temperature of the
formation, and then installing the proper equipment
to ensure an efficient flow of natural gas out of the
well.
There are three main types of conventional natural
gas wells. Since oil is commonly associated with natural
gas deposits, a certain amount of natural gas may be
obtained from wells that were drilled primarily for
oil production. These are known as oil wells. In some
cases, this "associated" natural gas is used
to help in the production of oil, by providing pressure
in the formation for the oils extraction. The associated
natural gas may also exist in large enough quantities
to allow its extraction along with the oil. Natural
gas wells are wells drilled specifically for natural
gas, and contain little or no oil.
Condensate wells are wells that contain natural gas,
as well as a liquid condensate. This condensate is a
liquid hydrocarbon mixture that is often separated from
the natural gas either at the wellhead, or during the
processing of the natural gas. Depending on the type
of well that is being drilled, completion may differ
slightly. It is important to remember that natural gas,
being lighter than air, will naturally rise to the surface
of a well. Because of this, in many natural gas and
condensate wells, lifting equipment and well treatment
are not necessary.
Completing a well consists of a number of steps; installing
the well casing, completing the well, installing the
wellhead, and installing lifting equipment or treating
the formation should that be required. Click on the
links below to learn about these aspects of the well
completion process:
Well Casing
Installing well casing is an important part of the
drilling and completion process. Well casing consists
of a series of metal tubes installed in the freshly
drilled hole. Casing serves to strengthen the sides
of the well hole, ensure that no oil or natural gas
seeps out of the well hole as it is brought to the surface,
and to keep other fluids or gases from seeping into
the formation through the well. A good deal of planning
is necessary to ensure that the proper casing for each
well is installed. Types of casing used depend on the
subsurface characteristics of the well, including the
diameter of the well (which is dependent on the size
of the drill bit used) and the pressures and temperatures
experienced throughout the well. In most wells, the
diameter of the well hole decreases the deeper it is
drilled, leading to a type of conical shape that must
be taken into account when installing casing. To review
the drilling of a natural gas well and the history of
drilling practices, including casing, click here.
There are five different types of well casing. They
include:
- Conductor Casing
- Surface Casing
- Intermediate Casing
- Liner String
- Production Casing
Conductor Casing
Conductor casing is installed first, usually prior
to the arrival of the drilling rig. The hole for conductor
casing is often drilled with a small auger drill, mounted
on the back of a truck. Conductor casing, which is usually
no more than 20 to 50 feet long, is installed to prevent
the top of the well from caving in and to help in the
process of circulating the drilling fluid up from the
bottom of the well. Onshore, this casing is usually
16 to 20 inches in diameter while offshore casing usually
measures 30 to 42 inches. The conductor casing is cemented
into place before drilling begins.
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| A Small Auger Drill |
| Source: USGS |
Surface Casing
Surface casing is the next type of casing to be installed.
It can be anywhere from a few hundred to 2,000 feet
long, and is smaller in diameter than the conductor
casing. When installed, the surface casing fits inside
the top of the conductor casing. The primary purpose
of surface casing is to protect fresh water deposits
near the surface of the well from being contaminated
by leaking hydrocarbons or salt water from deeper underground.
It also serves as a conduit for drilling mud returning
to the surface, and helps protect the drill hole from
being damaged during drilling. Surface casing, like
conductor casing, is also cemented into place. Regulations
often dictate the thickness of the cement to be used,
to ensure that there is little possibility of freshwater
contamination.
Intermediate Casing
Intermediate casing is usually the longest section
of casing found in a well. The primary purpose of intermediate
casing is to minimize the hazards that come along with
subsurface formations that may affect the well. These
include abnormal underground pressure zones, underground
shales, and formations that might otherwise contaminated
the well, such as underground salt-water deposits. In
many instances, even though there may be no evidence
of an unusual underground formation, intermediate casing
is run as insurance against the possibility of such
a formation affecting the well. These intermediate casing
areas may also be cemented into place for added protection.
Liner Strings
Liner strings are sometimes used instead of intermediate
casing. Liner strings are commonly run from the bottom
of another type of casing to the open well area. However,
liner strings are usually just attached to the previous
casing with 'hangers', instead of being cemented into
place. This type of casing is thus less permanent than
intermediate casing.
Production Casing
Production casing, alternatively called the 'oil string'
or 'long string', is installed last and is the deepest
section of casing in a well. This is the casing that
provides a conduit from the surface of the well to the
petroleum producing formation. The size of the production
casing depends on a number of considerations, including
the lifting equipment to be used, the number of completions
required, and the possibility of deepening the well
at a later time. For example, if it is expected that
the well will be deepened at a later date, then the
production casing must be wide enough to allow the passage
of a drill bit later on.
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| Installing Well Casing |
| Source: ChevronTexaco
Corporation |
Well casing is a very important part of the completed
well. In addition to strengthening the well hole, it
also provides a conduit to allow hydrocarbons to be
extracted without intermingling with other fluids and
formations found underground. It is also instrumental
in preventing blowouts, allowing the formation to be
'sealed' from the top should dangerous pressure levels
be reached. For more technical information on blowouts
and their prevention, click here.
Once the casing has been set, and in most cases cemented
into place, proper lifting equipment is installed to
bring the hydrocarbons from the formation to the surface.
Once the casing is installed, tubing is inserted inside
the casing, from the opening well at the top, to the
formation at the bottom. The hydrocarbons that are extracted
run up this tubing to the surface. This tubing may also
be attached to pumping systems for more efficient extraction,
should that be necessary.
Completion
Well completion commonly refers to the process of finishing
a well so that it is ready to produce oil or natural
gas. In essence, completion consists of deciding on
the characteristics of the intake portion of the well
in the targeted hydrocarbon formation. There are a number
of types of completions, including:
- Open Hole Completion
- Conventional Perforated Completion
- Sand Exclusion Completion
- Permanent Completion
- Multiple Zone Completion
- Drainhole Completion
The use of any type of completion depends on the characteristics
and location of the hydrocarbon formation to be mined.
Open Hole Completion
Open hole completions are the most basic type and are
only used in very competent formations, which are unlikely
to cave in. An open hole completion consists of simply
running the casing directly down into the formation,
leaving the end of the piping open, without any other
protective filter. Very often, this type of completion
is used on formations that have been treated with hydraulic
of acid fracturing.
Conventional Perforated Completion
Conventional perforated completions consist of production
casing being run through the formation. The sides of
this casing are perforated, with tiny holes along the
sides facing the formation, which allows for the flow
of hydrocarbons into the well hole, but still provides
a suitable amount of support and protection for the
well hole. The process of actually perforating the casing
involves the use of specialized equipment designed to
make tiny holes through the casing, cementing, and any
other barrier between the formation and the open well.
In the past, 'bullet perforators' were used, which were
essentially small guns lowered into the well. The guns,
when fired from the surface, sent off small bullets
that penetrated the casing and cement. Today, 'jet perforating'
is preferred. This consists of small, electrically ignited
charges, lowered into the well. When ignited, these
charges poke tiny holes through to the formation, in
the same manner as bullet perforating.
Sand Exclusion Completion
Sand exclusion completions are designed for production
in an area that contains a large amount of loose sand.
These completions are designed to allow for the flow
of natural gas and oil into the well, but at the same
time prevent sand from entering the well. Sand inside
the well hole can cause many complications, including
erosion of casing and other equipment. The most common
method of keeping sand out of the well hole are screening,
or filtering systems. This includes analyzing the sand
experienced in the formation and installing a screen
or filter to keep sand particles out. This filter may
either be a type of screen hung inside the casing, or
adding a layer of specially sized gravel outside the
casing to filter out the sand. Both of these types of
sand barriers can be used in open hole and perforated
completions.
Permanent Completion
Permanent completions are those in which the completion,
and wellhead, are assembled and installed only once.
Installing the casing, cementing, perforating, and other
completion work is done with small diameter tools to
ensure the permanent nature of the completion. Completing
a well in this manner can lead to significant cost savings
compared to other types.
Multiple Zone Completion
Multiple zone completion is the practice of completing
a well such that hydrocarbons from two or more formations
may be produced simultaneously, without mixing with
each other. For example, a well may be drilled that
passes through a number of formations on its way deeper
underground, or alternately, it may be efficient in
a horizontal well to add multiple completions to drain
the formation most effectively. Although it is common
to separate multiple completions so that the fluids
from the different formations do not intermingle, the
complexity of achieving complete separation is often
a barrier. In some instances, the different formations
being drilled are close enough in nature to allow fluids
to intermingle in the well hole. When it is necessary
to separate different completions, hard rubber 'packing'
instruments are used to maintain separation.
Drainhole Completion
Drainhole completions are a form of horizontal or slant
drilling. This type of completion consists of drilling
out horizontally into the formation from a vertical
well, essentially providing a 'drain' for the hydrocarbons
to run down into the well. In certain formations, drilling
a drainhole completion may allow for more efficient
and balanced extraction of the targeted hydrocarbons.
These completions are more commonly associated with
oil wells than with natural gas wells.
The Wellhead
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| A Wellhead |
| Source: NETL - DOE |
The wellhead consists of the pieces of equipment mounted
at the opening of the well to regulate and monitor the
extraction of hydrocarbons from the underground formation.
It also prevents leaking of oil or natural gas out of
the well, and prevents blowouts
due to high pressure formations. Formations that are
under high pressure typically require wellheads that
can withstand a great deal of upward pressure from the
escaping gases and liquids. These wellheads must be
able to withstand pressures of up to 20,000 psi (pounds
per square inch). The wellhead consists of three components:
the casing head, the tubing head, and the 'christmas
tree'.
The casing head consists of heavy fittings that provide
a seal between the casing and the surface. The casing
head also serves to support the entire length of casing
that is run all the way down the well. This piece of
equipment typically contains a gripping mechanism that
ensures a tight seal between the head and the casing
itself.
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| The 'Christmas Tree' |
| Source: NGSA |
The tubing head is much like the casing head. It provides
a seal between the tubing, which is run inside the casing,
and the surface. Like the casing head, the tubing head
is designed to support the entire length of the casing,
as well as provide connections at the surface, which
allow the flow of fluids out of the well to be controlled.
The 'christmas tree' is the piece of equipment that
fits atop the casing and tubing heads, and contains
tubes and valves that serve to control the flow of hydrocarbons
and other fluids out of the well. It commonly contains
many branches and is shaped somewhat like a tree, thus
its name, christmas tree. The christmas tree is the
most visible part of a producing well, and allows for
the surface monitoring and regulation of the production
of hydrocarbons from a producing well.
Lifting and Well Treatment
Once the well is completed, it may begin to produce
natural gas. In some instances, the hydrocarbons that
exist in pressurized formations will naturally rise
up through the well to the surface. This is most commonly
the case with natural gas. Since natural gas is lighter
than air, once a conduit to the surface is opened, the
pressurized gas will rise to the surface with little
or no interference. This is most common for formations
containing natural gas alone, or with a light condensate.
In these scenarios, once the christmas tree is installed,
the natural gas will flow to the surface on its own.
In order to more fully understand the nature of the
well, a potential test is typically run in the early
days of production. This test allows well engineers
to determine the maximum amount of natural gas that
the well can produce in a 24 hour period. From this,
and other knowledge of the formation, the engineer may
make an estimation on what the MER, or 'most efficient
recovery rate' will be. The MER is the rate at which
the greatest amount of natural gas may be extracted
without harming the formation itself. Another important
aspect of producing wells is the 'decline rate'. When
a well is first drilled, the formation is under pressure
and produces natural gas at a very high rate. However,
as more and more natural gas is extracted from the formation,
the production rate of the well decreases. This is known
as the decline rate. Certain techniques, including lifting
equipment and well stimulation, can increase the production
rate of a well.
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| A Horse Head Pump |
| Source: ChevronTexaco
Corporation |
In some natural gas wells, and oil wells that have
associated natural gas, it is more difficult to ensure
an efficient flow of hydrocarbons up the well. The underground
formation may be very 'tight', making the movement of
petroleum through the formation and up the well a very
slow and inefficient process. In these cases, lifting
equipment or well treatment is required.
Lifting equipment consists of a variety of specialized
equipment used to help 'lift' petroleum out of a formation.
This is most commonly used to extract oil from a formation.
Because oil is found as a viscous liquid, it takes some
coaxing to extract it from underground. Various types
of lifting equipment are available, but the most common
lifting method is known as 'rod pumping'. Rod pumping
is powered by a surface pump that moves a cable and
rod up and down in the well, providing the lifting pressure
required to bring the oil to the surface. The most common
type of cable rod lifting equipment is the 'horse head'
or conventional beam pump. These pumps are recognizable
by the distinctive shape of the cable feeding fixture,
which resembles a horse's head.
Well Treatment
Well treatment is another method of ensuring the efficient
flow of hydrocarbons out of a formation. Essentially,
this type of well stimulation consists of injecting
acid, water, or gases into the well to open up the formation
and allow the petroleum to flow through the formation
more easily. Acidizing a well consists of injecting
acid (usually hydrochloric acid) into the well. In limestone
or carbonate formations, the acid dissolves portions
of the rock in the formation, opening up existing spaces
to allow for the flow of petroleum. Fracturing consists
of injecting a fluid into the well, the pressure of
which 'cracks' or opens up fractures already present
in the formation. In addition to the fluid being injected,
'propping agents' are also used. These propping agents
can consist of sand, glass beads, epoxy, or silica sand,
and serve to prop open the newly widened fissures in
the formation. Hydraulic fracturing involves the injection
of water into the formation, while CO2 fracturing
uses gaseous carbon dioxide. Fracturing, acidizing,
and lifting equipment may all be used on the same well
to increase permeability.
These techniques are mostly applicable to oil wells,
but have also been used to increase the extraction rate
for gas wells. Because it is a low-density gas under
pressure, the completion of natural gas wells usually
requires little more than the installation of casing,
tubing, and the wellhead. Unlike oil, natural gas is
much easier to extract from an underground formation.
However, as deeper and less conventional natural gas
wells are drilled, it is becoming more common to use
stimulation techniques on gas wells.
Click on the following links to learn more:
The next step in the process of producing natural gas
is processing. This involves taking the 'raw' natural
gas obtained from underground, removing impurities,
and ensuring that the gas is ready for use prior to
being transported to its destination.
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