Natural gas is a gaseous mixture, which is predominantly methane but does contain other com-bustible hydrocarbon compounds as well as nonhydrocarbon compounds (Table 1.7) (Mokhatab et al., 2006; Speight, 2007). Natural gas is colorless, odorless, tasteless, shapeless, and lighter than air. In the natural state, it is not possible to see or smell natural gas. In addition to compo-sition and thermal content (Btu/scf, Btu/ft3), natural gas can also be characterized on the basis of the mode of the natural gas found in reservoirs where there is no or, at best only minimal amounts of, crude oil.
Other constituents are paraffinic hydrocarbons such as ethane (CH3CH3), propane (CH3CH2CH3), and the butanes (C4H10). Many natural gases contain nitrogen (N2) as well as carbon dioxide (CO2) and hydrogen sulfide (H2S). Trace quantities of argon, hydrogen, and helium may also be present.
Generally, the hydrocarbons having a higher molecular weight than methane, carbon dioxide, and hydrogen sulfide are removed from natural gas prior to its use as a fuel. However, since the compo-sition of natural gas and refinery gas is never constant, there are standard test methods that can be used to determine the suitability of natural gas (and refinery gas) for further use and indicate the processes by which the composition of natural gas can be prepared for use (Table 1.8) (Mokhatab et al., 2006; Speight, 2007, 2014a, 2015b).
TABLE 1.7
Range of Composition of Natural Gas
Category Component Amount (%)
Paraffinic Methane (CH4) 70–98
Ethane (C2H6) 1–10
Propane (C3H6) Trace–5
Butane (C4H10) Trace–2
Pentane (C5H12) Trace–1
Hexane (C6H14) Trace–0.5
Heptane and higher (C7+) None–trace
Cyclic Cyclopropane (C3H6) Traces
Cyclohexane (C6H12) Traces Aromatic Benzene (C6H6), others Traces
Nonhydrocarbon Nitrogen (N2) Trace–15
Carbon dioxide (CO2) Trace–1
Hydrogen sulfide (H2S) Trace occasionally
Helium (He) Trace–5
Other sulfur and nitrogen compounds Trace occasionally
Water (H2O) Trace–5
1.3.1 Petroleum-related gaS
The generic term “natural gas” applies to gas commonly associated with petroliferous ( petroleum-producing, petroleum-containing) geologic formations (Figure 1.3). Natural gas gener-ally contains high proportions of methane (CH4), and some of the higher-molecular-weight paraf-fins (CnH2n+2) generally containing up to six carbon atoms may also be present in small quantities.
TABLE 1.8
Standard Test Methods That Can Be Applied to Determining Gas Propertiesa
ASTM D1070 Standard Test Methods for Relative Density of Gaseous Fuels
ASTM D1071 Standard Test Methods for Volumetric Measurement of Gaseous Fuel Samples ASTM D1072 Standard Test Method for Total Sulfur in Fuel Gases
ASTM D1142 Standard Test Method for Water Vapor Content of Gaseous Fuels by Measurement of Dew-Point Temperature
ASTM D1826 Standard Test Method for Calorific (Heating) Value of Gases in Natural Gas ASTM D1945 Standard Test Method for Analysis of Natural Gas by Gas Chromatography
ASTM D1946 Standard Practice for Analysis of Reformed Gas by Gas Chromatography
ASTM D1988 Standard Test Method for Mercaptans in Natural Gas Using Length-of-Stain Detector Tubes ASTM D3588 Standard Practice for Calculating Heat Value, Compressibility Factor, and Relative Density
of Gaseous Fuels
ASTM D3956 Standard Specification for Methane Thermophysical Property Tables ASTM D3984 Standard Specification for Ethane Thermophysical Property Tables
ASTM D4084 Standard Test Method for Analysis of Hydrogen Sulfide in Gaseous Fuels (Lead Acetate Reaction Rate Method)
ASTM D4150 Standard Terminology Relating to Gaseous Fuels
ASTM D4362 Standard Specification for Propane Thermophysical Property Tables
ASTM D4468 Standard Test Method for Total Sulfur in Gaseous Fuels by Hydrogenolysis and Rateometric Colorimetry ASTM D4650 Standard Specification for Normal Butane Thermophysical Property Tables
ASTM D4651 Standard Specification for Iso-Butane Thermophysical Property Tables ASTM D4784 Standard for LNG Density Calculation Models
ASTM D4810 Standard Test Method for Hydrogen Sulfide in Natural Gas Using Length-of-Stain Detector Tubes ASTM D4888 Standard Test Method for Water Vapor in Natural Gas Using Length-of-Stain Detector Tubes ASTM D4891 Standard Test Method for Heating Value of Gases in Natural Gas Range by Stoichiometric Combustion ASTM D4984 Standard Test Method for Carbon Dioxide in Natural Gas Using Length-of-Stain Detector Tubes ASTM D5287 Standard Practice for Automatic Sampling of Gaseous Fuels
ASTM D5454 Standard Test Method for Water Vapor Content of Gaseous Fuels Using Electronic Moisture Analyzers ASTM D5503 Standard Practice for Natural Gas Sample-Handling and Conditioning Systems for Pipeline
Instrumentation
ASTM D5504 Standard Test Method for Determination of Sulfur Compounds in Natural Gas and Gaseous Fuels by Gas Chromatography and Chemiluminescence
ASTM D5954 Standard Test Method for Mercury Sampling and Measurement in Natural Gas by Atomic Absorption Spectroscopy
ASTM D6228 Standard Test Method for Determination of Sulfur Compounds in Natural Gas and Gaseous Fuels by Gas Chromatography and Flame Photometric Detection
ASTM D6273 Standard Test Methods for Natural Gas Odor Intensity
ASTM D6350 Standard Test Method for Mercury Sampling and Analysis in Natural Gas by Atomic Fluorescence Spectroscopy
Note: Listed numerically rather than by order of importance.
a ASTM D4, Test Method for Bitumen Content, Annual Book of Standards, ASTM International, West Conshohocken, PA, 2015.
The hydrocarbon constituents of natural gas are combustible, but nonflammable nonhydrocarbon components such as carbon dioxide, nitrogen, and helium are often present in the minority and are regarded as contaminants. In addition to the gas found associated with petroleum in reservoirs, there are also those reservoirs in which natural gas may be the sole occupant. And just as petroleum can vary in composition, so can natural gas.
Natural gas is often located in the same reservoir as with petroleum, but it can also be found trapped in gas reservoirs and within coal deposits. The occurrence of methane in coal seams is not a new discovery and methane (called “firedamp” by the miners because of its explosive nature) was known to coal miners for at least 150 years (or more) before it was rediscovered and developed as coalbed methane (Speight, 2013a). The natural gas can originate by thermogenic alteration of coal or by biogenic action of indigenous microbes on the coal. There are some horizontally drilled coal-bed methane wells and some that receive hydraulic fracturing treatments. However, some coalcoal-bed methane reservoirs are also underground sources of drinking water, and as such, there are restric-tions on hydraulic fracturing operarestric-tions. The coalbed methane wells are mostly shallow, as the coal matrix does not have the strength to maintain porosity under the pressure of significant overburden thickness.
In addition to defining natural gas as associated and nonassociated, the types of natural gas vary according to composition. There is dry gas or lean gas, which is mostly methane, and wet gas, which contains considerable amounts of higher-molecular-weight and higher-boiling hydrocar-bons (Table 1.9). Sour gas contains high proportions of much hydrogen sulfide, whereas sweet gas contains little or no hydrogen sulfide. Residue gas is the gas remaining (mostly methane) after the higher-molecular-weight paraffins have been extracted. Casinghead gas is the gas derived from an oil well by extraction at the surface. Natural gas has no distinct odor and the main use is for fuel, but it can also be used to make chemicals and liquefied petroleum gas (LPG).
Some natural gas wells also produce helium, which can occur in commercial quantities; nitrogen and carbon dioxide are also found in some natural gases. Gas is usually separated at as high a pres-sure as possible, reducing compression costs when the gas is to be used for gas lift or delivered to a pipeline. After gas removal, lighter hydrocarbons and hydrogen sulfide are removed as necessary to obtain a crude oil of suitable vapor pressure for transport yet retaining most of the natural gasoline constituents.
The nonhydrocarbon constituents of natural gas can be classified as two types of materials: (1) diluents, such as nitrogen, carbon dioxide, and water vapors, and (2) contaminants, such as hydro-gen sulfide and/or other sulfur compounds. The diluents are noncombustible gases that reduce the
Trap: Structural anticline
Impermeable cap rock, e.g., shale Reservoir, e.g., sandstone Source rock:
Organic-rich shale Gas cap Oil layer Water layer
FIGURE 1.3 Schematic of a petroleum reservoir showing the gas cap.
heating value of the gas and are on occasion used as fillers when it is necessary to reduce the heat content of the gas. On the other hand, the contaminants are detrimental to production and transpor-tation equipment in addition to being obnoxious pollutants.
Thus, the primary reason for gas processing is to remove the unwanted constituents of natural gas such as (1) acid gas, which is predominantly hydrogen sulfide although carbon dioxide does occur to a lesser extent; (2) water, which includes all entrained free water or water in condensed forms; (3) liquids in the gas, such as higher-boiling hydrocarbons as well as pump lubricating oil, scrubber oil, and, on occasion, methanol; and (4) any solid matter that may be present, such as fine silica (sand) and scaling from the pipe. As with petroleum, natural gas from different wells varies widely in composition and analyses (Table 1.10), and the proportion of nonhydrocarbon constituents can vary over a very wide range. Thus, a particular natural gas field could require production, pro-cessing, and handling protocols different from those used for gas from another field.
Just as petroleum was used in antiquity, natural gas was also known in antiquity, although the use of petroleum was relatively better documented because of its use as a mastic for walls and roads as well as for its use in warfare (Abraham, 1945; Pfeiffer, 1950; Van Nes and van Westen, 1951;
Forbes, 1958a,b, 1959, 1964; Hoiberg, 1964; Speight, 2014a; Cobb and Goldwhite, 1995). The use of natural gas in antiquity is somewhat less well documented, although historical records indicate that the use of natural gas (for other than religious purposes) dates back to approximately AD 250 when it was used as a fuel in China. The gas was obtained from shallow wells and was distributed through a piping system constructed from hollow bamboo stems. There is other fragmentary evidence for the use of natural gas in certain old texts, but the use is usually inferred since the gas is not named specifically. However, it is known that natural gas was used on a small scale for heating and light-ing in northern Italy durlight-ing the early seventeenth century. From this, it might be conjectured that natural gas found some use from the seventeenth century to the present day, recognizing that gas from coal would be a strong competitor.
TABLE 1.9
Range of Composition for Wet and Dry Natural Gas
Constituents
Composition (vol%)
Wet Dry Range
Hydrocarbons
Methane 84.6 96.0
Ethane 6.4 2.0
Propane 5.3 0.6
Iso-Butane 1.2 0.18
n-Butane 1.4 0.12
Isopentane 0.4 0.14
n-Pentane 0.2 0.06
Hexanes 0.4 0.10
Heptanes 0.1 0.80
Nonhydrocarbons
Carbon dioxide 0–5
Helium 0–0.5
Hydrogen sulfide 0–5
Nitrogen 0–10
Argon 0–0.05
Radon, krypton, xenon Traces
Differences in natural gas composition occur between different reservoirs, and two wells in the same field may also yield gaseous products that are different in composition. Indeed, there is no single composition of components that might be termed typical natural gas. Methane and ethane constitute the bulk of the combustible components; carbon dioxide (CO2) and nitrogen (N2) are the major noncombustible (inert) components. Other constituents, such as hydrogen sulfide (H2S), mer-captan derivatives (thiols; R-SH), as well as trace amounts of other sulfur-containing constituents may also be present.
Before the discovery of natural gas, the principal gaseous fuel source was the gas produced by the surface gasification of coal (Speight, 2013a). In fact, each town of any size had a plant for the gasification of coal (hence the use of the term “town gas”). Most of the natural gas produced at the petroleum fields was vented to the air or burned in a flare stack; only a small amount of the natural gas from the petroleum fields was pipelined to industrial areas for commercial use. It was only in the years after World War II that natural gas became a popular fuel commodity, leading to the recogni-tion that it has at the present time.
There are several general definitions that have been applied to natural gas. Thus, lean gas is gas in which methane is the major constituent. Wet gas contains considerable amounts of the higher-molecular-weight hydrocarbons. 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.
To further define the terms “dry” and “wet” in quantitative measures, the term “dry natural gas”
indicates that there is less than 0.1 gal (1 gal, U.S. = 264.2 m3) of gasoline vapor (higher-molecular-weight paraffins) per 1000 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 gal/1000 ft3. Associated or dissolved natural TABLE 1.10
Variation in Natural Gas Composition with Source
Component
Type of Gas Field
Natural Gas Separated from Crude Oil, Venturaa Dry Gas, Los
Medanosa (mol%)
Sour Gas, Jumping Poundb (mol%)
Gas Condensate, Palomaa (mol%)
400 lb (mol%)
50 lb (mol%)
Vapor (mol%)
Hydrogen sulfide 0 3.3 0 0 0 0
Carbon dioxide 0 6.7 0.7 0.3 0.7 0.8
Nitrogen and air 0.8 0 0 0 — 2.2
Methane 95.8 84.0 74.5 89.6 81.8 69.1
Ethane 2.9 3.6 8.3 4.7 5.8 5.1
Propane 0.4 1.0 4.7 3.6 6.5 8.8
Iso-butane 0.1 0.3 0.9 0.5 0.9 2.1
n-Butane Trace 0.4 1.9 0.9 2.3 5.0
Isopentane 0 0.8 0.2 0.5 1.4
n-Pentane 0 0.6 0.1 0.5 1.4
Hexane 0 0.7 1.3
Heptane 0 0.1 1.0 4.1
Octane 0 6.3
Nonane 0
100.0 100.0 100.0 100.0 100.0 100.0
a In California.
b In Canada.
gas occurs either as free gas or as gas in solution in the petroleum. Gas that occurs as a solution in the petroleum is dissolved gas, whereas the gas that exists in contact with the petroleum (gas cap) is associated gas.
1.3.2 gaS HydrateS
Methane hydrates, which consist of methane molecules trapped in a cage of water molecules, occur as crystalline solids in sediments in arctic regions and below the floor of the deep ocean. Although taking on the appearance of ice, methane hydrates will burn if ignited. Methane hydrates are the most abundant unconventional natural gas source and the most difficult to extract. Methane hydrates are conservatively estimated to hold twice the amount of energy found in all conventional fossil fuels, but the technical challenges of economically retrieving the resource are significant. There is also a significant risk that rising temperatures from global warming could destabilize the deposits, releasing the methane—a potent greenhouse gas—into the atmosphere and further exacerbating the problem.
Another product is gas condensate, which contains relatively high amounts of the higher-molecular-weight liquid hydrocarbons (up to and including octane, C8H18). These hydrocarbons may occur in the gas phase in the reservoir. On the other hand, natural gasoline (like refinery gasoline) consists mostly of pentane (C5H12) and higher-molecular-weight hydrocarbons. The term “natural gasoline” has also on occasion in the gas industry been applied to mixtures of liquefied petroleum gas, pentanes, and higher-molecular-weight hydrocarbons. Caution should be taken not to confuse natural gasoline with the term “straight-run gasoline” (often also incorrectly referred to as natural gasoline), which is the gasoline distilled unchanged from petroleum.
Liquefied petroleum gas (LPG) is composed of propane (C3H8), butanes (C4H10), and/or mixtures thereof; small amounts of ethane and pentane may also be present as impurities.
1.3.3 CoalBed metHane
In coalbeds (coal seams), methane (the primary component of natural gas) is generally adsorbed to the coal rather than contained in the pore space or structurally trapped in the formation.
Pumping the injected and native water out of the coalbeds after fracturing serves to depressur-ize the coal, thereby allowing the methane to desorb and flow into the well and to the surface.
Methane has traditionally posed a hazard to underground coal miners, as the highly flammable gas is released during mining activities. Otherwise inaccessible coal seams can also be tapped to collect this gas, known as coalbed methane, by employing similar well drilling and hydraulic fracturing techniques as are used in shale gas extraction.
Coalbed methane is a gas formed as part of the geological process of coal generation and is contained in varying quantities within all coal. Coalbed methane is exceptionally pure compared to conventional natural gas, containing only very small proportions of higher-molecular-weight hydro-carbons such as ethane and butane and other gases (such as hydrogen sulfide and carbon dioxide).
Coalbed gas is over 90% methane and, subject to gas composition, may be suitable for introduction into a commercial pipeline with little or no treatment (Levine, 1993; Rice, 1993; Mokhatab et al., 2006; Speight, 2013a). Methane within coalbeds is not structurally trapped by overlying geologic strata, as in the geologic environments typical of conventional gas deposits (Speight, 2013a, 2014a).
Only a small amount (on the order of 5%–10% v/v) of the coalbed methane is present as free gas within the joints and cleats of coalbeds. Most of the coalbed methane is contained within the coal itself (adsorbed to the sides of the small pores in the coal).
The primary (or natural) permeability of coal is very low, typically ranging from 0.1 to 30 mD, and because coal is a very weak (low modulus) material and cannot take much stress without fracturing, coal is almost always highly fractured and cleated. The resulting network of fractures commonly gives coalbeds a high secondary permeability (despite coal’s typically low primary permeability).
Groundwater, hydraulic fracturing fluids, and methane gas can more easily flow through the net-work of fractures. Because hydraulic fracturing generally enlarges preexisting fractures in addition to creating new fractures, this network of natural fractures is very important to the extraction of methane from the coal.
1.3.4 BiogeniC gaS
Biogenic gas (predominantly methane) is produced by certain types of bacteria (methanogens) dur-ing the process of breakdur-ing down organic matter in an oxygen-free environment (Speight, 2011b).
Thus, biogenic gas is created by methanogenic organisms in marches, bogs, landfills, and shallow sediments. On the other hand, as a point of differentiation, thermogenic gas is created from buried organic material deeper in the earth, at greater temperature and pressure. Livestock manure, food waste, and sewage are all potential sources of biogenic gas, or biogas, which is usually considered a form of renewable energy. Small-scale biogas production is a well-established technology in parts of the developing world, particularly Asia, where farmers collect animal manure in vats and capture the methane given off while it decays.
Landfills offer another underutilized source of biogas (Speight, 2011b). When municipal waste is buried in a landfill, bacteria break down the organic material contained in garbage such as newspa-pers, cardboard, and food waste, producing gases such as carbon dioxide and methane. Rather than allowing these gases to go into the atmosphere, where they contribute to global warming, landfill gas facilities can capture them, separate the methane, and combust it to generate electricity, heat, or both.