Speight J.G. Natural Gas: A Basic Handbook
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CHAPTER
2
Origin
and
Production
Natural gas occurs in the porous rock
of
the earth’s crust either alone
(non-associated natural gas)
or with accumulations
of
petroleum
(associ-
ated natural gas).
In
the latter case (Figure 2-1), the gas forms the
gas
cap,
which is the gas trapped between the liquid petroleum and the
impervious cap rock
of
the petroleum reservoir. When the pressure
in
the reservoir is sufficiently high, the natural gas may be dissolved in
the petroleum and is released upon penetration of the reservoir as a
result
of
drilling operations. The most preferred type of natural gas is
the
non-associated gas.
Such gas can be produced at high pressure,
whereas associated, or dissolved, gas must be separated from petroleum
at lower separator pressures, which usually involves compression.
The composition
of
natural gas varies (Table
2-1,
Chapter
1
and
Chapter
3),
and it is not surprising that there are several general defi-
nitions that have been applied to natural gas that usually relate not
only to the mode
of
formation
of
the gas over geological time and the
existence
of
the gas in the reservoir but also to the composition
of
the
gas. Thus, as with petroleum, natural gas from different wells varies
widely in composition (Speight,
1993,
2007),
and the proportion of
non-hydrocarbon constituents can vary greatly depending upon the
mode
of
formation, maturation conditions, and whether the gas is
wet, dry, sweet,
or
sour.
For example,
lean gas
is gas in which methane is the major constit-
uent and the amount of higher molecular weight hydrocarbons is low
(<0.1
ga1/1,000 ft’).
On
the other hand,
wet gas
contains considerable
amounts
(>0.1
gal/1,000 ft3)
of
the higher molecular weight hydrocar-
bons, such as ethane, propane,
and
butane.
Sour gas
contains
35
36
Chapter
2
Origin
and
Production
Figure
2-1
An
anticlinal reservoir containing oil and (associated) gas.
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
casinghead
gas
is derived from petroleum but is separated at the separation
facility at the wellhead. Another product is
gas condensate,
which con-
tains relatively high amounts of the higher molecular weight liquid
hydrocarbons. These hydrocarbons may occur
in
the gas phase
in
the
reservoir.
2.1
Origin
There are many different theories as to the origins
of
fossil fuels, spe-
cifically natural gas and petroleum. The most generally accepted
theory is that natural gas and petroleum are formed when organic
matter or
debris
(such as the remains of a plants or animals) is com-
pressed under the earth, at very high pressure for a very long time.
Millions of years ago, the remains of plants and animals decayed and
built up in thick layers. Over time, as sediment, mud, and other
debris piled on top
of
the organic matter, metamorphosis occurred
and
the sediment, mud, and other
debris were
changed
to
rock,
causing pressure to be put
on
the organic matter. The increasing
2.1
Origin
37
pressure compressed the organic matter and, combined with other
subterranean effects, decomposed the individual constituents into gas
and petroleum.
Thus, the events that are believed to have occurred in the formation
of natural gas and petroleum are:
400
to
300
million years ago: tiny sea plants and animals died
and were buried
on
the ocean floor; over time
they
were
covered with layers of silt and sand.
300
to
100
million years ago: the organic debris started
to
change by simple chemical reactions.
100 to
50
million years ago: the organic debris was buried
deeper and deeper; pressure increased and (possibly)
temperature increased (but, as previously noted, the level
of
the temperature is largely unknown and, at best, very
speculative).
50 to
1
million years ago: the organic debris reacted, under
the prevalent conditions underground, to produce methane
and other hydrocarbon products that eventually became
natural gas, which migrated to reservoirs where it was trapped
and awaiting discovery.
The deep-lying source rock that contains the organic precursors is
often referred to as the
kitchen.
In
theory, the deeper and hotter the
kitchen, the more likelihood
of
gas being produced. The increase of
heat with depth
is
due
to
the presence
of
a
geothermal gradient
that is
generally
on
the order
of
25 to
30"C/km
(0.008
to
O.O09"C/ft
of depth
or
8
to
9"C/l,OOO ft
of
depth;
0.015
to 0.016"F/ft
of
depth or 15 to
16"F/1,000 ft
of
depth), i.e. about one degree
for
every one hundred
feet below the surface
of
the earth.
However, this does not mean that non-associated gas will always be
found at greater depths than crude oil. Gas and crude oil have
migrated from the kitchen, sideways and upwards, until they are
trapped in reservoirs in the subsurface formations. Thus, a field may
have
a
series
of
layers of crude oil/gas and gas reservoirs over a
thousand meters subsurface.
The methane (natural gas) produced
in
this
manner
is
referred
to
as
themogenic
methane
(natural gas), because it is presumed that the
38
Chapter
2
Origin
and Production
pressure effects and the increased depth of the precursors include the
influence
of
heat to convert the organic matter
to
natural gas and
petroleum includes the effects
of
heat. However, the actual tempera-
ture is not known and, like the remainder of the theory, is at best
speculative. Experimentalists will note that laboratory studies at spec-
ified temperatures will convert organic materials to methane and
other hydrocarbons gases. Be that as it may, the temperatures used in
the laboratory are often high
on
the basis that the increase in temper-
ature makes up for the lack
of
geological time in the laboratory.
The
experimentalists often ignore that higher temperatures change the
chemistry
of
the process and the findings may
not
be completely
true. The fact remains that the thermogenic origin of natural gas and
other fossil fuels is not conclusively proven.
This theory answers many questions, namely the fact that gas and oil
are only found under a small portion
of
the earth. While there are
other theories, this one seems to be the most widely accepted.
Methane (natural gas) can also be formed through the transformation
of
organic matter by tiny microorganisms. This type of methane is
referred to as
biogenic methane (biogenic
natural gas).
In
this process,
the methane-producing microorganisms
(methanogens)
chemically
break down organic matter to produce methane. The
methanogens
are
commonly found
in
areas near the surface of the earth that are lacking
oxygen. Formation
of
methane in this manner usually occurs close to
the surface of the earth, and the methane produced is usually lost into
the atmosphere.
In
certain circumstances, however, this methane can
be trapped underground, recoverable as natural gas.
An
example of
biogenic methane is landfill gas. Waste-containing landfills produce a
relatively large amount
of
natural gas from the decomposition
of
the
waste materials that they contain. New technologies are allowing this
gas to be collected to add to the supply
of
natural gas.
A
third way in which methane (and natural gas) may be formed is
through
abiogenic processes.
Extremely deep under the earth’s crust,
there exist hydrogen-rich gases, and as these gases rise towards the sur-
face of the earth, they may interact through the catalytic activity
of
minerals in the absence
of
oxygen. This interaction may result in a
reaction to form products that are found in the atmosphere (including
nitrogen, oxygen, carbon dioxide, argon, and water).
If
these gases are
under very high pressure as they move towards the surface
of
the
earth, they are likely to form methane
(abiogenic methane),
similar
to
thermogenic methane.
2.2
Exploration
39
Because of the manner in which it is formed, and the diverse nature
of
the precursors, natural gas contains constituents other than
methane. While the major constituent of natural gas is indeed
methane, there are components such as saturated hydrocarbons
(CnHZn+,), aromatic hydrocarbons (derivatives of benzene), carbon
dioxide
(CO,),
hydrogen sulfide
(H,S),
and mercaptans (thiols, R-SH),
as well as trace amounts
of
gases such
as
carbonyl sulfide
(COS)
(Table
2-1).
In cases where the natural gas does not contain hydrogen
sulfide, there may also be a relative lack
of
carbon dioxide.
2.2
Exploration
Historically,
in
the modern fossil fuel era, natural gas was discovered
as a consequence of petroleum exploration. The methods used to dis-
cover natural gas reservoirs are essentially those used to petroleum
reservoirs. In addition, in the early days
of
the natural gas industry,
the only way of locating natural gas reservoirs was to search for sur-
face evidence of the underground formations (capable
of
being reser-
voirs) such as seepages of gas emitted from underground. However,
because such a low proportion of natural gas reservoirs actually seep
to the surface, this made for a very inefficient and difficult explora-
tion process.
As
the demand for fossil fuel energy has increased dra-
matically over the past years, so has the necessity for more accurate
methods of locating these deposits.
The only way of being certain that a natural gas reservoir exists is to
drill an exploratory well. This consists
of
actually digging into the
earth’s crust to allow geologists to study the composition of the
underground rock layers
in
detail.
In
addition to looking for natural
gas reservoirs by drilling an exploratory well, geologists also examine
the drill cuttings and fluids to gain a better understanding
of
the geo-
logic features of the area. However, drilling an exploratory well is
expensive and, time consuming. Therefore, exploratory wells are only
drilled
in
areas where other data have indicated a high probability of
gas-containing formations.
The search for natural gas now begins with geologists locating the
types of rock that are usually found near to natural gas and petroleum
reservoirs. The methods used include seismic surveys that are used to
estimate the correct places to drill wells. Seismic surveys use echoes
from a vibration source at the earth’s surface (usually
a
vibrating
pad
under a truck built for this purpose) to generate information about
40
Chapter
2
Origin
and
Production
the subterranean formation.
If
required, dynamite may be used to
provide the necessary underground vibrations.
Natural gas reservoirs are also being found at locations that are deep
underwater. Offshore drilling operations used to be some of the most
risky and dangerous undertakings but improved offshore drilling rigs,
dynamic positioning devices, and modern navigation systems allow
efficient offshore drilling
in
waters more than
10,000
feet deep.
In either case (i.e., onshore or offshore), once the gas reservoir is
found and penetrated, gas flows up through the well to the surface of
the ground and into large pipelines. Some
of
the gases that are pro-
duced along with methane, such
as
ethane, propane, and butane
(gas
liquids)
are separated and cleaned at a gas processing plant. The gas
liquids, once removed, are employed individually or collectively for
various uses.
Technological innovation
in
the exploration and production sector is
necessary
if
the industry is to continually increase the production of
natural gas to meet rising demand. New technologies serve to make
the exploration and production of natural gas more efficient, safe,
and environmentally friendly. Despite the fact that natural gas
deposits are continually being found deeper
in
the ground,
in
remote,
inhospitable areas that provide a challenging environment
in
which
to produce natural gas, the exploration and production industry has
not only kept up its production pace, but
in
fact has improved the
general nature of its operations.
However, even though technology advances have allowed for phe-
nomenal increases in the success rate of locating natural gas reservoirs,
the process of exploring for natural gas reservoirs is still filled with
uncertainty, as well as trial-and-error. The search for a reservoir that
actually contains gas (there can be a reservoir rock formation that is
empty) and that is often thousands
of
feet below remains complex.
2.2.1
Geological
Survey
The exploration for natural gas typically begins with a
geological
exam-
ination
(geological
survey)
of the surface structure
of
the earth and
determination of the areas where there is a high probability that a
natural gas reservoir exists.
In
fact, as early as the mid
1800s,
it
was
realized that anticlinal slopes
had
a particularly increased chance
of
containing natural gas. These anticlinal slopes are areas where the
2.2
Exploration
41
earth has folded up on itself, forming the dome
shape that is
characteristic
of
many reservoirs.
By surveying and mapping the surface and subsurface characteristics
of
a certain area, the geologist can extrapolate which areas are most
likely to contain a natural gas reservoir. The geologist has many tools
to do
so,
from the outcroppings
of
rocks
on
the surface or in valleys
and gorges, to the geologic information attained from the rock cut-
tings and samples obtained from the digging
of
irrigation ditches,
water wells, and other oil and gas wells. This information is combined
to allow the geologist to make inferences as to the fluid content,
porosity, permeability, age, and formation sequence
of
the rocks
underneath the surface
of
a particular area.
Once the geologist has located an underground formation that is geo-
logically possible for a natural gas or petroleum formation to exist,
further tests can be performed to gain more detailed data about the
potential reservoir area. These tests (commonly performed by a
geu-
physicist)
allow
for
the more accurate mapping
of
underground
formations, most notably those formations that are commonly asso-
ciated with natural gas reservoirs.
2.2.2
Seismic
Survey
Perhaps the biggest breakthrough in the exploration for natural gas
exploration came through the use
of
seismuhgy,
which is the study
of
the movement
of
energy, in the form
of
seismic waves, through the
Earth’s crust and how
it
interacts differently with various types
of
underground formations. The
seismograph,
an instrument used to
detect and record earthquakes, is able
to
pick up and record the vibra-
tions
of
the earth that occur during an earthquake. When seismology
is applied
to
the search for natural gas, seismic waves, emitted from a
source, are sent into the earth, and they interact differently with the
underground formation (underground layers), each with its
own
properties. The waves are reflected back towards the source by each
formation. It is this reflection that allows
for
the use
of
seismology in
discovering the character and properties of underground formations
leading
to
conclusions about the potential for one or more
of
the for-
mations to contain natural gas.
In
the early days
of
seismic exploration, seismic waves were created
using
dynamite.
These
carefully planned, small explosions created the
requisite seismic waves, which were then picked up by the geophones,
42
ChaDter
2
Orizin
and
Production
generating data to be interpreted by geophysicists, geologists, and
petroleum engineers.
In
practice, using seismology for exploring onshore areas involves
artificially creating seismic waves, the reflection
of
which are then
picked up by sensitive pieces of equipment
(geophones)
embedded in
the ground. The data gathered up by these geophones are then trans-
mitted to a seismic recording truck, which records the data for further
interpretation
by
geophysicists and petroleum reservoir
engineers.
The source of seismic waves (usually
a
pre-set underground explosion)
creates vibrations that reflect
off
of
the different layers of the earth, to
be picked up by geophones
on
the surface and relayed to a seismic
recording truck to be interpreted and logged.
Furthermore, due
to
environmental concerns and improved tech-
nology,
it
is often
no
longer feasible to use explosive charges to gen-
erate the needed seismic waves. Instead, most seismic crews use
non-explosive seismic technology to generate the required data.
This non-explosive technology usually involves use
of
a large,
heavy, wheeled or tracked vehicle carrying special equipment
designed to create a large impact or series
of
vibrations. These
impacts or vibrations create seismic waves similar to those created
by dynamite. In the seismic truck shown, the large piston
in
the
middle is used to create vibrations
on
the surface
of
the earth,
sending seismic waves that are used to generate useful data.
The same sort
of
process is used in
offshore seismic expzoration.
When
exploring for natural gas that may exist thousands of feet below the
sea floor, which may itself be thousands
of
feet below sea level, a
slightly different method of seismic exploration is used. Instead
of
trucks and geophones, a ship is used to pick up the seismic data.
Instead of geophones, offshore exploration uses
hydrophones,
which
are designed to pick up seismic waves underwater. These hydro-
phones are towed behind the ship in various configurations
depending
on
the needs
of
the geophysicist. Instead
of
using dyna-
mite or impacts
on
the sea floor, the seismic ship uses a large air gun,
which releases bursts
of
compressed air under the water, creating
seismic waves that can travel through the Earth’s crust and generate
the seismic reflections that are necessary.
The development of seismic imaging
in
three dimensions greatly
changed the nature
of
natural gas exploration.
This
technology
uses
traditional seismic imaging techniques, combined with powerful
2.2
Exploration
43
computers and processors, to create a three-dimensional model of the
subsurface layers.
4-D
seismology expands on this, by adding time as
a dimension, allowing exploration teams to observe how subsurface
characteristics change over time. Exploration teams can now identify
natural gas prospects more easily, place wells more effectively, reduce
the number of dry holes drilled,
reduce drilling costs, and cut
exploration time.
One of the greatest innovations in the history
of
natural gas
and
petro-
leum exploration is the use
of
computers to compile and assemble geo-
logic data into a coherent map of the underground formations. In the
last
two
decades, it has become relatively easy to use computers to
assemble seismic data that is collected from the field. This allows for
the processing of much larger amounts
of
data, increasing the reli-
ability and informational content of the seismic model.
There are three main types
of
computer-assisted exploration models:
2-dimensional
(2-D),
three-dimensional
(3-D),
and most recently four
dimensional
(4-D).
These imaging techniques, while relying mainly
on seismic data acquired in the field, are becoming more and more
sophisticated. Computer technology has advanced
so
far that it is now
possible to incorporate the data obtained from different
types
of
tests,
such as logging, production information, and gravimetric testing,
which can all be combined to create a “visualization”
of
the under-
ground formation. By this means, geologists and geophysicists are able
to combine all
of
their sources
of
data to compile one clear, complete
image of subsurface geology. An example of this is the use of an inter-
active computer by a geologist to enable visualization of the seismic
data thereby allowing exploration of the subsurface layers.
Two-dimensional
(2-0)
seismic imaging
refers to using the data collected
from seismic exploration activities to develop a cross-sectional picture
of
the underground formations. The geophysicist interprets the
seismic data obtained from the field, taking the vibration recordings
of the seismograph and using them to develop a conceptual model of
the composition and thickness of the various layers
of
rock under-
ground. This process is normally used to map underground forma-
tions, and to make estimates based on the geologic structures to
determine where it is likely that deposits may exist.
There also exists
a
technique using basic seismic data known as
direct
detectim
in which white bands
(bright
spots)
that often appeared
on
seismic recording strips indicated hydrocarbon reservoirs. Porous rock
44
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2
Orixin and Production
containing natural gas could often reflect stronger seismic reflections
than normal, water-filled rock. Therefore, in these circumstances, the
actual natural gas reservoir could be detected directly from the
seismic data. However, this concept is not universally applicable
because many of the bright spots do not contain hydrocarbons, and
many deposits of hydrocarbons are not indicated by bright spots on
the seismic data. Therefore, although adding a new technique of
locating petroleum and natural gas reservoirs, direct detection is not a
completely reliable method.
One of the biggest breakthroughs in computer-aided exploration was
the development of
three-dimensional
(3-D)
seismic imaging.
3-D
imaging uses seismic field data to generate a three-dimensional pic-
ture of underground formations and geologic features. This, in
essence, allows the geophysicist and geologist to see a clear picture of
the composition of the earth’s crust in a particular area. Thus, in the
exploration for natural gas, an actual image could be used to estimate
the probability of formations existing in a particular area and the
characteristics of those potential formations. This technology has
been extremely effective in raising the success rate of exploration
efforts. In fact, using
3-D
seismic imaging has been estimated to
increase the likelihood of successful reservoir location by
50%!
Although this technology is very useful, it is also very costly. The gen-
eration of
3-D
images requires data to be collected from several thou-
sand locations, as opposed to
2-D
imaging, which only requires
several hundred data points.
As
such,
3-D
imaging is a much more
involved and prolonged process. Therefore, it is usually used in con-
junction with other exploration techniques. For example, a geophysi-
cist may use traditional
2-D
modeling and examination of geologic
features to determine
if
there is a probability of the presence of nat-
ural gas. Once these basic techniques are used,
3-D
seismic imaging
may be used only in those areas that have a high probability of
containing reservoirs.
In
addition to broadly locating petroleum reservoirs,
3-D
seismic
imaging allows for the more accurate placement of wells to be drilled.
This increases the productivity of successful wells, allowing for more
petroleum and natural gas to be extracted from the ground. In fact,
3-D
seismic can increase the recovery rates of productive wells.
One
of
the latest breakthroughs in seismic exploration and
the
mod-
eling of underground rock formations has been the introduction
of