Speight J.G. Natural Gas: A Basic Handbook
Подождите немного. Документ загружается.
4
Chapter
1
History
and
Uses
impurities are also removed, such as hydrogen sulfide (the refining
of
which can produce sulfur, which is then also sold separately). After
refining (Chapter 7), the clean natural gas is transmitted through a
network of pipelines that deliver natural gas to its point of use.
Two new and possibly large sources
of
methane that may extend the
availability
of
natural gas are
methane hydrates
(also called
gas
hydrates)
and coal-bed methane (Berecz and Balla-Achs, 1983; Sloan,
1997; Gudmundsson et al., 1998; Max,
2000;
Sloan,
2000).
Their pro-
duction technologies have only recently been developed, and these
sources are now becoming economically competitive.
Methane-rich gases are also produced by the anaerobic decay of non-
fossil organic material and are referred to as biogas. Sources
of
biogas
include swamps, which produce swamp gas; marshes, which produce
marsh gas; landfills, which produce landfill gas, as well as sewage
sludge and manure, by way
of
anaerobic digesters,
in
addition to
enteric fermentation, particularly in cattle.
Although natural gas is a vital component
of
the world’s supply
of
energy and one
of
the most useful
of
all energy sources, it must be
understood that the word
gas
has a variety
of
different uses, and
meanings. Fuel for automobiles is also called
gas
(being a shortened
version
of
gasoline),
but that is a totally different fuel. The
gas
used
in
a barbecue grill is actually propane (C,H,), which, while closely asso-
ciated with
and
commonly found
in
natural gas and petroleum, is not
really natural gas.
Natural gas
is
the flammable gaseous mixture that occurs alone or
with petroleum in reservoirs and is predominantly methane (CH,)
and some
of
the higher molecular weight paraffins (CnH2n+2) generally
containing up to five carbon atoms (Table
1-1).
Briefly, methane is
the simplest member
of
the hydrocarbon series and has one carbon
atom and four hydrogen atoms (Figure
1-1).
In
its purest form, the natural gas delivered to the consumer is almost
pure methane, and the remaining hydrocarbons and non-hydrocarbons
have been removed though refining. The non-hydrocarbon constitu-
ents include, but are not limited to, carbon dioxide
(Cod,
hydrogen sul-
fide (H,S), nitrogen
(N3,
and helium (He).
All
of
the hydrocarbon constituents
of
natural
gas
are
combustible,
but non-flammable non-hydrocarbon components (carbon dioxide,
1.1
introduction
5
Figure
1-1
Simplified representation
of
methane as (a) a two-dimensional
formula and
(b)
a three-dimensional formula.
Table
1-1
Range
of
Composition
of
Natural Gas
Propane C3H8
0-20%
Butane
C4HIO
Pentane and higher hydrocarbons
C&*,
0-10%
Carbon dioxide
CO,
&8%
Oxygen
Nitrogen
N2 0-5%
Hydrogen sulfide, carbonyl sulfide
H,S,
COS
0-5%
Rare gases: Argon, Helium, Neon, Xenon
A,
He, Ne, Xe trace
hydrogen sulfide, nitrogen, and helium) detract slightly from the
heating value
of
natural gas. However, they are valuable, and
in
cer-
tain natural gases where their concentrations are relatively high, they
may be extracted commercially.
6
Chapter
1
History and
Uses
Natural gas is colorless, shapeless, and odorless in its pure form. Quite
uninteresting except that natural gas is combustible, and when
burned it gives
off
a great deal of energy. Unlike other fossil fuels,
however, natural gas is clean burning and emits lower levels of
potentially harmful by-products into the air. It is the environmental
aspect
of
natural gas use that is currently elevating its use as an
important energy source.
There are several general definitions that
have
been applied
to
natural
gas science and technology. For example,
lean
gas
contains methane
as the major constituent. On the other hand,
wetgas
contains consid-
erable amounts
of
the higher molecular weight hydrocarbons. Nat-
ural gas is considered
dry
when it is almost pure methane, having had
most
of
the other commonly associated hydrocarbons removed.
When other hydrocarbons are present, the natural gas is
wet.
To
fur-
ther define the terms
dry
and
wet
in
quantitative measures, the term
dry
natural
gas
indicates that there is less than
0.1
gallon
(1
gallon, US,
=
264.2 m3) of gasoline vapor (higher molecular weight paraffins) per
1,000
ft3
(1
ft3
=
0.028
m3). The term
wet
natural
gas
indicates that
there are such paraffins present
in
the gas, in fact more than
0.1
ga1/1,000
ft3.
Sour
gas
contains hydrogen sulfide, whereas
sweet
gas
contains very
little,
if
any, hydrogen sulfide.
Residue
gas
is
natural gas from which
the higher molecular weight hydrocarbons have been extracted and
casing-head
gas
is derived from petroleum but is separated at the sepa-
ration facility at the wellhead.
Natural gas is considered as an environmentally friendly clean fuel,
offering important environmental benefits when compared to other
fossil fuels. The superior environmental qualities over coal
or
crude
oil are that emissions of sulfur dioxide are negligible, and nitrous
oxide and carbon dioxide emissions are lower. This helps to reduce
problems of acid rain, ozone layer, or greenhouse gases. Natural gas is
also a very safe source of energy when transported, stored, and used.
Natural gas is produced in many countries around the world and most
of those countries produce both oil and natural gas; a few produce
only natural gas. The ten largest natural gas producing countries are:
United States, Canada, Russia, United Kingdom, Algeria, Netherlands,
Iran, Indonesia, Norway, and Uzbekistan
(EIA,
2004). Because trans-
portation costs
add
to
the
cost
of
natural
gas,
in
most
countries
natural gas is consumed within the country or exported to a
1.2
History
7
neighboring country by pipeline. Technology for liquefying natural
gas
so
that it can be transported in tankers
is
improving (Chapter
4).
As
technology continues to expand the options for gas transportation,
demand for natural gas is expected to grow.
A
common misconception about natural gas is that resources are
being depleted at an alarming rate and the supplies are quickly
run-
ning out.
In
fact, there is a vast amount of natural gas estimated still
to be retrieved from a variety of reservoirs (Chapter
2).
However,
many proponents of the depletion theory believe that price spikes
indicate that natural gas resources are depleted beyond the point
of
no
return. However, price spikes of any commodity are not always
caused by waning resources but can be the outcome of other forces at
work in the marketplace.
1.2
History
By definition, natural gas is a naturally occurring gaseous fossil fuel
that is found in oil fields, and natural gas fields, and coal beds. For clar-
ification,
natural gas
is not the same as
town
Bus,
although the history of
nuturd
gas
cleaning
has its roots in
town
gas
cleaning
(Chapter
5).
Town gas is a generic term referring to manufactured gas produced for
sale to consumers and municipalities. The terms
coal
gas,
manufac-
tured
gas,
producer
gas,
and syngas
(synthetic
natural
gas,
SNG)
are also
used for gas produced from coal. Depending
on
the processes used for
its creation, town gas is a mixture of hydrogen, carbon monoxide,
methane, and volatile hydrocarbons with small amounts of carbon
dioxide and nitrogen as impurities.
Prior to the development of natural gas supplies and transmission in
the United States during
1940s
and
1950s,
virtually all fuel and
lighting gas was manufactured, and the by-product coal tar was an
important feedstock for the chemical industry. The development of
manufactured gas paralleled that of the industrial revolution and
urbanization.
The uses of natural gas did not necessarily parallel its discovery. In
fact, the discovery
of
natural gas dates from ancient times in the
Middle East. During recorded historical time, there was little or
no
understanding of what natural gas was; it posed somewhat of a mys-
tery to man. Sometimes, such things
as
lightning strikes
would
ignite
natural gas that was escaping from under the earth’s crust. This would
8
Chapter
1
History and Uses
create a fire coming from the earth, burning the natural gas as it
seeped out from underground. These fires puzzled most early civiliza-
tions, and were the root
of
much myth and superstition. One
of
the
most famous
of
these types
of
flames was found in ancient Greece,
on
Mount Parnassus about 1000 BC.
A
goat herdsman came across what
looked like a burning spring, a flame rising from a fissure in the rock.
The Greeks, believing it to be of divine origin, built a temple
on
the
flame. This temple housed a priestess who was known as the Oracle of
Delphi, giving out prophecies she claimed were inspired
by
the
flame.
These types
of
springs became prominent in the religions
of
India,
Greece, and Persia. Unable to explain where these fires came from,
they were often regarded as divine,
or
supernatural. The energy value
of
natural gas was not recognized until about 900 BC in China, and
the Chinese drilled the first known natural gas well
in
211 BC. The
Chinese formed crude pipelines out
of
bamboo shoots to transport
the gas, where it was used to boil sea water, separating the salt and
making it drinkable.
Natural gas was discovered and identified in America as early as 1626,
when French explorers discovered natives igniting gases that were
seeping into and around Lake Erie.
In
Europe, natural gas was
unknown until it was discovered in Great Britain in 1659, although it
was not commercialized until about 1790. Around 1785, natural gas
produced from coal was used to light houses, as well as streetlights.
Manufactured natural gas
of
this type (as opposed to naturally-
occurring gas) was first brought to the United States in 1816, when it
was used to light the streets
of
Baltimore, Maryland. This manufac-
tured gas was much less efficient, and less environmentally friendly,
than modern natural gas that comes from underground.
In
1821 in Fredonia, United States, residents observed gas bubbles
rising
to
the surface from a creek. William Hart,
considered as
America’s “father
of
natural gas,” dug there the first natural gas well in
North America (Speight, 1993, Chapter
1
and references cited therein).
The American natural gas industry got its beginnings in this area.
In
1859, Colonel Edwin Drake (a former railroad conductor who adopted
the title “Colonel” to impress the townspeople) dug the first well.
Drake hit oil and natural gas at 69 feet below the surface of the earth.
More recently, natural gas was discovered as a consequence
of
pros-
pecting for crude
oil.
It
was often
an
unwelcome
by-product,
as
natural gas reservoirs were tapped
in
the drilling process and workers
1.2
History
9
were forced to stop drilling to let the gas vent freely into the air. Now,
and particularly after the crude oil shortages
of
the seventies, natural
gas has become an important source
of
energy in the world.
Throughout the 19th century, natural gas was used almost exclusively
as a source
of
light, and its use remained localized because
of
lack of
transport structures, making it difficult to transport large quantities
of
natural gas long distances. There was an important change in 1890
with the invention
of
a
leak-proof pipeline coupling, but transporta-
tion
of
natural gas to distant customers did not become practical until
the 1920s as a result
of
technological advances
in
pipelines. More-
over, it was only after World War
I1
that the use
of
natural gas grew
rapidly because
of
the development
of
pipeline networks and storage
systems.
The techniques to discover gas are essentially those used
to
discover
crude oil. When using seismic techniques, gas slows down the
velocity
of
the seismic waves to produce a characteristic and stronger
reflection. Over time, as more knowledge
of
a hydrocarbon province
is obtained, better recognition of the characteristics and amplitude
of
the seismic reflection from gas can lead
to
a greater chance
of
success.
Natural gas, like crude oil and often
in
conjunction with crude oil,
has been generated over geological time from deep-lying source rock,
sometimes called the
kitchen,
which contains organic debris. It is now
generally accepted that the deeper and hotter the source rock, the
more likelihood
of
gas being produced. However, there is consider-
able discussion about the heat
to
which the organic precursors have
been subjected, and it is
no
way certain that cracking temperatures
(300"C,
572°F) played a role in natural gas formation. Maturation
of
the organic debris through temperature effects occurred over geolog-
ical time (millennia), and shortening the time to laboratory time and
increasing the temperature to above and beyond the cracking temper-
ature (at which the chemistry changes) does not offer conclusive
proof
of
high temperatures (Speight,
2007).
At some point during or after the maturation process, the gas and
crude oil migrated from the source rock upward, sideways, or
in
both
directions (subject to the structure
of
the accompanying and overlying
geological formations). Eventually, the gas and crude oil were trapped
in reservoirs in the subsurface formations that were often many miles
from
the
source
rock.
It
is
rare
that
the
source
rock
and
the
reservoir
were
one
and the same. Thus, a natural gas field may have
a
series
of
10
Chapter
1
History
and Uses
layers
of
crude oil/gas and gas reservoirs in the subsurface. In some
instances, the natural gas and crude oil parted company leading to the
occurrence of reservoirs containing only gas (non-associated gas).
1.3
Composition and Properties
Although the composition and properties
of
natural gas are presented
elsewhere in more detail (Chapter
3),
it is pertinent to give a brief
overview here of the composition and properties of natural gas. The
primary component of natural gas is methane (CH,), and it also con-
tains gaseous hydrocarbons such as ethane (C,H,), propane (C,H,),
and butane (C4Hlo), as well as other non-hydrocarbon gases. In fact,
natural gas is the primary market source of the valuable rare gas
helium (He), which is used in cryogenics, as a deep-sea breathing gas,
for inflating balloons and airships, and as a protective gas for many
industrial purposes, such as arc welding. Inhaling a small amount of
helium temporarily changes the quality
of
a person’s voice, but cau-
tion must be exercised because helium is an asphyxiant.
Gas with a significant amount
of
sulfur impurities, such as hydrogen
sulfide, is termed
sour
gas
or acid gas, and must be cleaned before con-
sumption (Chapter
7).
Processed natural gas available to end-users is
tasteless and odorless, however, before gas is distributed to end-users, it
is odorized by adding small amounts of thiols (sulfur-containing com-
pounds having the general formula R-SH) to assist in leak detection.
Natural gas can also be hazardous to life and property through an
explosion and/or asphyxiation. For example,
if
natural gas leaks go
undetected (hence the reason for adding odorants such
as
thiols), fire-
ball-type explosions can result.
In
addition, natural gas, being an
asphyxiant, can kill organisms (including humans)
if
it displaces air
to the point where the oxygen content
of
the surrounding atmo-
sphere is insufficient to support life.
Natural gas is often stated to be lighter than air, but this
is
only partly
true. In comparison with air, the relative density
of
methane is lighter
than the density of air and tends to dissipate into the atmosphere.
The other hydrocarbon constituents of natural gas have a higher den-
sity than air and, therefore, do not dissipate into the atmosphere but
will collect on the ground. Being colorless (invisible) and odorless,
the
hydrocarbons
can
pose
a
danger
to
life
in
the
immediate
vicinity
either through asphyxiation or explosion.
1.4
Reservoirs
11
When natural gas (i.e., methane) is confined, such as within a house
or any building, gas concentrations can reach explosive mixtures and,
if
ignited, result in destructive blasts. Methane has a lower explosive
limit
of
5%
in
air, and an upper explosive limit of
15%.
By definition, the
lower expZosive limit
of
a gas or vapor at ordinary
(ambient) temperature is the percent by volume
of
the gas vapor
in
air and is the lower limit at which the gas explodes
or
inflames.
Con-
versely, the
upper
explosive
limit
of
a
gas
or
vapor
at
ordinary (ambient)
temperature is the percent by volume
of
the gas vapor in air and is the
upper limit at which the gas explodes
or
inflames. Similarly, the
Zower
flammability limit
is the minimum concentration by volume
of
a com-
bustible substance that is capable
of
propagating a flame under speci-
fied conditions and the
upper
pammability limit
is the maximum
concentration by volume of a combustible substance that is capable
of
continued propagation
of
a
flame under the specified conditions.
However, these limits are
to
be used as a guide only, because under a
variety
of
other circumstances, methane and the other hydrocarbon
constituents of natural gas are explosive and flammable.
1.4
Reservoirs
Although there are several ways that methane, and thus natural gas,
may be formed (Chapter
2),
it is usually found underneath the surface
of
the earth. Once formed, natural gas migrates through the under-
ground sediments through fissures and faults until the gas enters a
geological formation
(reservoir)
that retains or
traps
the gas.
Reservoirs vary in size from a few hundred meters to tens
of
kilome-
ters across, and tens
to
hundreds
of
meters thick, with the gas trapped
against an impermeable layer similar to crude oil traps (Speight,
2007).
Reservoirs are not underground spaces in which clouds of gas
exist, but rather are made
of
porous and permeable rocks (such as
sandstone) that can hold significant amounts
of
gas within their pore
spaces.
Reservoirs generally comprise a geological formation that is made up
of layers of porous, sedimentary rock, such as sandstone,
in
which the
gas can collect. However, for retention
of
the gas each trap must have
an
impermeable base rock and an impermeable cap
rock
to
prevent
further movement
of
the gas. Such formations,
known
as
reservoirs
or
12
Chapter
1
History
and
Uses
Pups
(i.e., naturally-occurring storage areas) vary
in
size and can
retain varying amounts
of
gas.
There are several types of these formations, but the most common is,
characteristically, a folded rock formation such as an anticline as occurs
in many petroleum reservoirs (Figure
1-2),
which traps and holds nat-
ural gas.
On
the other hand, a reservoir may be formed by a geological
fault that occurs when the normal sedimentary layers sort
of
split verti-
cally,
so
that impermeable
rock
shifts
down
to trap natural gas in the
more permeable limestone or sandstone layers. Essentially, the geolog-
ical formation that layers impermeable rock over more porous, oil-and
gas-rich sediment has the potential to form a reservoir.
The properties
of
the reservoir rocks allow the natural gas within
them to flow through the pore spaces
to
a producing well. Some reser-
voirs may be only hundreds of feet below the surface. Other reservoirs
are thousands, even tens of thousands of feet underground.
In
the
United States, several reservoirs have been discovered at depths
greater than
30,000
feet. Many offshore wells are drilled
in
thousands
of feet of water and penetrate tens of thousands
of
feet into the sedi-
ments below the sea floor.
Natural gas reservoirs, like crude oil reservoirs, exist
in
many forms,
such as the dome (syncline-anticline) structure (Figure
1-2),
with
water below, or a dome of gas with a crude oil rim and water below
the oil. When the water is
in
direct contact with the gas, pressure
effects may dictate that
a
considerable portion of the gas
(20%
or
more) is dissolved
in
the crude oil as well as
in
the water.
As
gas is pro-
duced (or recovered) from the reservoir, the reservoir pressure
declines allowing the dissolved gas to enter the gas phase.
In
addi-
tion, and because of the variability of reservoir structure, gas does not
always flow equally to wells placed throughout the length, breadth,
and depth
of
the reservoir and at equal pressure. Recovery wells must
be distributed throughout the reservoir to recover as much of the gas
as efficiently as possible.
As
the gas pressure in the reservoir declines, the reservoir energy (i.e., res-
ervoir pressure) declines, and the gas requires stimulation for continued
production. Furthermore, reduction
in
the gas pressure may allow com-
paction
of
the reservoir rock by the weight of rock above, eventually
resulting in subsidence of the surface above the reservoir.
This
can be a
gradual process or a sudden catastrophic process
depending
on
the
struc-
tures of the geological formation above the reservoir.
1.4
Reservoirs
13
Figure
1-2
An
anticlinal reservoir containing oil and associated gas.
A
reservoir containing
wet
gas with a large amount
of
valuable
natural
gas liquids
(any hydrocarbons other than methane, such as ethane,
propane, and butane) and even light crude oil and condensate has to
be treated carefully. When the reservoir pressure drops below the crit-
ical point for the mixture, the liquids may condense out and remain
in the reservoir. Thus, it is necessary to implement a
cyclingprocess
in
which the wet gas is produced to the surface, the natural gas liquids
are condensed as a separate stream, and the gas is compressed and
injected back into the reservoir to maintain the pressure.
Many reservoirs contain oil, gas, and water (Figure
1-2)
that are segre-
gated according to density, with gas (lowest density) being
on
top,
then oil (median density), then water (highest density)
on
the
bottom. However, other parameters, such as fluidhock properties and
solubility restrict complete gravitational separation, and each layer
might be a mixture of typically gas, oil, and water with each being
more predominant in its own particular level.
The larger subsurface traps are the easiest to locate, and
in
mature pro-
duction areas of the world most
of
these large reservoirs have already
been found. Improved seismic techniques (Chapter
3)
have improved
the discovery of the smaller and difficult-to-find reservoirs.
In
fact,
technological innovation has greatly enhanced the industry’s ability to
find and recover more gas and not only makes it easier to find new
deposits
of
oil
and
gas
but
also
to
recover more
gas
from
each
reservoir.
For example, new drilling techniques have made it feasible to intersect