Natural Gas (eBook)

A Basic Handbook

James G. Speight (Autor)

eBook Download: PDF | EPUB

2007 | 1. Auflage
239 Seiten
Elsevier Science (Verlag)
978-0-12-799984-5 (ISBN)

Infos

Natural gas represents nearly one-quarter of the world's energy resources. More than half of American homes rely on it as their main heating fuel. It serves as the raw material necessary in everyday paints, plastics, medicines and explosives. It produces the cleanest of all fossil fuels. It is natural gas-and everybody should acquire a basic understanding of it. This valuable easy-to-use reference supplies all the basics that every person should know about the natural gas industry. Introductory engineers, managers and analysts will benefit from this informative, practical handbook. Natural gas remains a vital component of all energy sources, and with an increasing demand for information on this useful energy source, Natural Gas: A Basic Handbook is an essential tool for anyone involved in the energy industry.

Dr. Speight has more than fifty years of experience in areas associated with the properties and processing of conventional and synthetic fuels. He has participated in, as well as led, significant research in defining the use of chemistry of tar sand bitumen, heavy oil, conventional petroleum, natural gas, coal, oil shale, and biomass as well as work related to corrosion and corrosion prevention. He has founded and/or edited several international journals, most recently the Proceedings of the Oil Gas Scientific Research Project Institute, Azerbaijan, and Petroleum Science and Technology (Taylor & Francis, until 2020). Dr. Speight is an author/editor of several databases and encyclopedic works. He has also authored more than 95 books as well as more than 400 publications, reports, and presentations detailing these research activities, and has taught more than eighty related courses.

Textauszug

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 (associated 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.

Figure 2–1 An anticlinal reservoir containing oil and (associated) gas.

The composition of natural gas varies (Table 2-1, Chapter 1 and Chapter 3), and it is not surprising that there are several general definitions 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.

Table 2–1

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

C5H12

0–10%

Carbon dioxide

CO2

0–8%

Oxygen

O2

0–0.2%

Nitrogen

N2

0–5%

Hydrogen sulfide, carbonyl sulfide

H2S, COS

0–5%

Rare gases: Argon, Helium, Neon, Xenon

A, He, Ne, Xe

trace

For example, lean gas is gas in which methane is the major constituent and the amount of higher molecular weight hydrocarbons is low (<0.1 gal/1,000 ft3). On the other hand, wet gas contains considerable amounts (>0.1 gal/1,000 ft3) of the higher molecular weight hydrocarbons, such as ethane, propane, and butane. 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 casinghead gas is derived from petroleum but is separated at the separation facility at the wellhead. Another product is gas condensate, which contains 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, specifically 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 compressed 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 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 0.009°C/ft of depth or 8 to 9°C/1,000 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 thermogenic methane (natural gas), because it is presumed that the 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 temperature is not known and, like the remainder of the theory, is at best speculative. Experimentalists will note that laboratory studies at specified 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 temperature 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 surface 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.

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 (CnH2n+2), aromatic hydrocarbons (derivatives of benzene), carbon dioxide (CO2), hydrogen sulfide (H2S), and mercaptans (thiols, R-SH), as well as trace amounts of gases such as carbonyl sulfide (COS)...

Produkt-Details

• Erscheint lt. Verlag: 1.10.2007
• Sprache: englisch
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 0-12-799984-1 / 0127999841
• ISBN-13: 978-0-12-799984-5 / 9780127999845