Speight J.G. Natural Gas: A Basic Handbook
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2.4
Production
55
tubing to the surface. This tubing may also be attached to pumping
systems for more efficient extraction, should that be necessary.
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 several types
of
completions, including:
1.
2.
3.
4.
5.
6.
Openhole 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.
Openhole completion
is the most basic type and is only used in very
competent formations, which are unlikely
to
cave in.
An
openhole
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
forma-
tions that have been treated with hydraulic or acid fracturing.
Conventional perforated completion
consists
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 wellbore, but still provides a suit-
able amount
of
support and protection for the wellbore. The process
of actually perforating the casing involves the use
of
specialized
equipment designed to make tiny holes through the casing, cement,
and/or 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,
jet perforating
is preferred and consists
of
small, electrically ignited
shot small bullets that penetrated the casing and
cement.
Currently,
56
Chapter
2
Origin
and Production
charges, lowered into the well that, when ignited, poke tiny holes
through to the formation,
in
the same manner as bullet perforating.
Sand-exclusion completion
is 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
wellbore can cause many complications, including erosion of casing
and other equipment. The
most
common method
of
keeping
sand
out of the wellbore are screening
or
filtering systems. This includes
analyzing the sand 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 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 openhole and perforated completions.
Permanent completion
is the type
of
completion in which the comple-
tion, and wellhead, is 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 compZetion
is the practice
of
completing a well such that
hydrocarbons from
two
or more formations may be produced simul-
taneously, without mixing with each other. For example, a well may
be drilled that passes through several 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 com-
plexity
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 wellbore. When it
is
nec-
essary to separate different completions, hard rubber packing instru-
ments are used to maintain separation.
Drainhole completion
is a form of horizontal or slant drilling and con-
sists 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
com-
pletion may allow for more efficient and balanced extraction
of
the
2.4
Production
57
natural gas. This type
of
completion is more commonly associated
with oil wells than with natural gas wells.
2.4.2 The Wellhead
This 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 pres-
sures
of
up to
20,000
psi (pounds per square inch). The wellhead con-
sists
of
three components: the casinghead, the tubinghead, and the
Christmas
tree.
The
casinghead
consists
of
heavy fittings that provide a seal between
the casing and the surface. The casinghead 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. The tub-
inghead is much like the casinghead and provides a seal between the
tubing, which is
run
inside the casing, and the surface. Like the cas-
inghead, the tubinghead 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
(so
named because
of
its many branches that make
it appear somewhat like a 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. The
Christmas
tree
is the most visible part
of
a pro-
ducing well, and allows for the surface monitoring and regulation of
the production
of
hydrocarbons from a producing well.
2.4.3 Natural
Gas
Production
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. Because natural gas is lighter
58
Chapter
2
Origin
and
Production
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.
To more fully understand the nature
of
the well, a potential test is typ-
ically
run
in the early days of production. This test allows well engi-
neers 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
of
the
most
efFcient recovery rate
(MER,
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. As more natural gas is extracted from
the formation, the production rate
of
the well decreases (the
decline
rate).
Certain techniques, including lifting equipment and well stimu-
lation, can increase the production rate of a well.
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, or compact,
making the movement
of
petroleum and natural gas through the for-
mation 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. The most common lifting
method is known as
rod
pumping
in which the pumping mechanism 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 (balanced conventional beam, sucker
rod)
pump. These
pumps are recognizable by the distinctive shape of the cable feeding
fixture, which resembles a horse’s head.
2.4.4
Well
Treatment
Another method
of
ensuring the efficient flow of hydrocarbons out
of
a formation
is
well treatment, which
is
a
type
of
well stimulation that
consists
of
injecting acid, water,
or
gases into the well to open up the
2.5
References
59
formation and allow the petroleum to flow through the formation
more easily. Acidizing a well consists of injecting acid (usually hydro-
chloric 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
and 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
carbon dioxide fracturing uses gaseous carbon dioxide. Fracturing,
acidizing, and lifting equipment may all be used
on
the same well to
increase permeability.
For
example, C0,-sand fracturing involves
using a mixture of sand and liquid carbon dioxide to fracture forma-
tions, creating and enlarging cracks through which oil and natural
gas may flow more freely. The carbon dioxide then vaporizes, leaving
only sand in the formation, holding the newly enlarged cracks open.
Because there are no other substances used in this type of fracturing,
there are no
leftovers
from the fracturing process that must be
removed. This type of fracturing effectively opens the formation and
allows for increased recovery
of
natural gas. In addition
it
does not
damage the deposit, generates no underground wastes, and protects
groundwater resources.
Because natural gas is a low-density gas under pressure, the comple-
tion of such wells usually requires little more than the installation
of
casing, tubing, and the wellhead. Unlike crude 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.
2.5
References
Speight,
J.G.,
1993,
Gas Processing: Environmental Aspects and Methods,
Butterworth-Heinemann, Oxford, England.
Speight,
J.G.,
2007,
The
Chemistry
and
Technology
of
Petroleum,
4th
Edition,
CRC
Press, Taylor and Francis Group, Boca Raton, Florida.
CHAPTER
3
Composition and Properties
Natural gas is found throughout the world in reservoirs deep beneath
the surface of the earth and floor of the oceans (Chapter 2). It forms
as pockets of gas in crude oil reservoirs or is trapped in porous rock
formations. Natural gas found in crude oil reservoirs is
associated
nat-
ural
gas
and
non-associated natural
gas
is found in reservoirs without
crude oil (Mokhatab et al., 2006 and references cited therein; Speight,
2007; and references cited therein).
Natural gas is colorless, odorless, tasteless, shapeless, and often
quoted as being lighter than air, but this statement
is
not
quite true
and is qualified later in this chapter. It is gaseous at any temperature
higher than
-161°C.
When it is at its natural state, it is not possible to
see or smell natural gas. For safety reasons, an odorant (a
rnercuptan,
also called a
thiol)
is added to natural gas
so
that it can be smelled if
there is a gas leak.
A
mercaptan is a chemical odorant that smells a
little like rotten eggs.
3.1
Composition
Natural gas is a combustible mixture of hydrocarbon gases (Mokhatab
et al., 2006). While natural gas is formed primarily of methane,
it
can
also include ethane, propane, butane, and pentane. The composition
of natural gas can vary widely before
it
is refined (Table
3-1)
(Speight,
1993).
In
its purest form, such as the natural gas that is delivered to
the consumer is almost pure methane.
The primary component of natural gas is methane (CH,), the shortest
and
lightest hydrocarbon molecule,
It
also
contains heavier gaseous
61
62
Chapter
3
Composition and Properties
Table 3-1 Range
of
Composition
of
Natural Gas
Gas Composition Range
Methane CH4
70-90%
Ethane
C2H6
Propane C3H8
0-20%
Butane
C4H10
Pentane and higher hydrocarbons C,Hl,
0-10%
Carbon dioxide CO,
0-8%
Oxygen
0-2
043.2%
Nitrogen N2
0-5%
Hydrogen sulfide, carbonyl sulfide
H,S,
COS
0-5%
Rare gases: Argon, Helium, Neon, Xenon
A,
He, Ne, Xe trace
hydrocarbons, such as ethane (C,H,), propane (C,H,) and butane
(C4Hlo), as well as other sulfur containing gases in varying amounts
(see also Natural Gas Condensate). Natural gas also contains and is
the primary market source of helium.
Briefly, natural gas contains hydrocarbons and non-hydrocarbon gases.
Hydrocarbon gases are methane (CH,), ethane (C,H,), propane (C,H,),
butanes (C4Hlo), pentanes (CsHl,), hexane (C,H14), heptane (C7H16), and
sometimes trace amounts of octane (C8H18), and higher molecular
weight hydrocarbons. Some aromatics [BTX-benzene (C,H,), toluene
(C,H,CH,), and the xylenes (CH,C,H,CH,)] can also be present, raising
safety issues due to their toxicity. The non-hydrocarbon gas portion
of
the natural gas contains nitrogen
(N,),
carbon dioxide (CO,), helium
(He), hydrogen sulfide (H,S), water vapor (H,O), and other sulfur com-
pounds (such as carbonyl sulfide
(COS)
and mercaptans (eg, CH,SH)
and trace amounts
of
other gases. Carbon dioxide and hydrogen sulfide
are commonly referred to as
acid gases
because they form corrosive
compounds in the presence of water. Nitrogen, helium, and carbon
dioxide are also referred to as
diZuents,
because none of them burn, and
thus they have no heating value.
Mercury
can also
be present either
as
a metal in vapor phase or as an organo-metallic compound in liquid
3.1
Composition
63
fractions. Concentration levels are generally very small, but even at
very small concentration levels, mercury can be detrimental due its
toxicity and its corrosive properties (reaction with aluminum alloys).
Within the natural gas family, the composition of
associated
gas
(a by-
product
of
oil production and the oil recovery process, Figure
3-1)
is
extremely variable, even within the gas from a particular petroleum
reservoir. After the production fluids are brought to the surface, they
are separated at a tank battery at or near the production lease into a
hydrocarbon liquid stream (crude oil or condensate), a produced
water stream (brine or salty water), and a gas stream.
The gas stream traditionally has high proportions of
natural
gas liquids
(NGLs) and is referred to as
rick gas.
Natural gas liquids are constitu-
ents such as ethane, propane, butane, and pentanes and higher
molecular weight hydrocarbon constituents. The higher molecular
weight constituents (i.e., the C5' product) are commonly referred to
as
natural gasoline.
Rich gas will have a high heating value and a high
hydrocarbon dew point. When referring to natural gas liquids in the
gas stream, the term
gallon per thousand
cubic
feet
is
used as a measure
of high molecular weight hydrocarbon content. On the other hand,
the composition of non-associated gas (sometimes called
well
gas)
is
deficient in natural gas liquids. The gas is produced from geological
formations that typically do not contain much,
if
any, hydrocarbon
liquids (Figure
3-2).
Naturally, the properties of associated gas and non-associated gas
differ considerably when both gases are in the unrefined state, but
once refined to produce methane, the properties
of
the methane are
evident.
Generally, the hydrocarbons that have a higher molecular weight
than methane, carbon dioxide, and hydrogen sulfide are removed
from natural gas prior to its use as a fuel. However, because the com-
position of natural gas is never constant, there are standard test
methods by which the composition and properties
of
natural gas can
be determined (Table
3-2)
and, thus, prepared for use. It is not the
intent to cover the standard test methods in any great detail in this
text because descriptions of the test methods are available elsewhere
(Speight,
2002).
Organic
sulfur
compounds and hydrogen sulfide
are
common
con-
taminants that must be removed prior to most uses. Gas with a
64
Chapter
3
Composition and Properties
Figure
3-1
An
anticlinal reservoir containing oil and (associated)
gas.
Figure
3-2
An
anticlinal reservoir containing
natural
gas.
3.1
Composition
65
Table
3-2
Standard Test Methods for Natural Gas (ASTM,
2006)
D
1070-03
Standard Test Methods for Relative Density of Gaseous
Fuels
D1071-83(2003) Standard Test Methods for Volumetric Measurement of
Gaseous Fuel Samples
D1072-90(1999) Standard Test Method for Total Sulfur
in
Fuel Gases
D1142-95(2000) Standard Test Method for Water Vapor Content of
Gaseous Fuels by Measurement of Dew-Point Temperature
~~
D1826-94(2003) Standard Test Method for Calorific (Heating) Value of
Gases in Natural Gas Range by Continuous Recording
Calorimeter
D1945-03 Standard Test Method for Analysis of Natural Gas by Gas
Chromatography
D1946-90(2000) Standard Practice for Analysis
of
Reformed Gas by Gas
Chromatography
D1988-91(2000) Standard Test Method for Mercaptans in Natural Gas
Using Length-of-Stain Detector Tubes
D3588-98(2003) Standard Practice for Calculating Heat Value,
Compressibility Factor, and Relative Density of Gaseous
Fuels
D3956-93(2003) Standard Specification for Methane Thermophysical
Property Tables
D3984-93(2003) Standard Specification for Ethane Thermophysical
Property Tables
D4084-94(1999) Standard Test Method for Analysis of Hydrogen Sulfide in
Gaseous Fuels (Lead Acetate Reaction Rate Method)
D4150-03 Standard Terminology Relating to Gaseous Fuels
D4362-93(2003) Standard Specification for Propane Thermophysical
Property Tables
D4468-85(2000) Standard Test Method for Total Sulfur in Gaseous Fuels by
Hydrogenolysis and Rateometric Colorimetry
D4650-93(2003) Standard Specification for Normal Butane
Thermophysical Property Tables
D4651-93(2003)
Standard Specification
for
Isobutane Thermophysical
Property
Tables