72
The Chemistry and Technology of Petroleum
The most preferred type of natural gas is the nonassociated 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 increased expenditure for compression. Thus it is
not surprising that such gas (under conditions that are not economically favorable) is often flared
or vented.
The nonhydrocarbon constituents of natural gas can be classified as two types of materials:
1. Diluents, such as nitrogen, carbon dioxide, and water vapors
2. Contaminants, such as hydrogen sulfide and/or other sulfur compounds
The diluents are noncombustible gases that reduce the 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 transportation equipment in addition to being
obnoxious pollutants. Thus, the primary reason for gas processing is to remove the unwanted constituents of natural gas.
The following are the major diluents or contaminants of natural gas:
1. Acid gas that is predominantly hydrogen sulfide although carbon dioxide does occur to a
lesser extent
2. Water that 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
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
(Speight, 1993) and the proportion of nonhydrocarbon constituents can vary over a very wide range.
Thus a particular natural gas field could require production, processing, and handling protocols different from those used for gas from another field.
Liquefied petroleum gas (LPG) is composed of propane (C 3 H 8 ), butanes (C 4 H 10 ), and/or mixtures
thereof, small amounts of ethane (C 2 H 6 ) and pentane (C 5 H 12 ) may also be present as impurities. On
the other hand, natural gasoline (such as refinery gasoline) consists mostly of pentane (C 5 H 12 ) and
higher molecular weight hydrocarbons.
The term natural gasoline has also on occasion in the gas industry been applied to mixtures
of LPG, 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.
3.3.4 HeAvy oIl
When petroleum occurs in a reservoir that allows the crude material to be recovered by pumping
operations as a free-flowing dark to light colored liquid, it is often referred to as conventional petroleum. Heavy oils are the other types of petroleum that are different from conventional petroleum
insofar as they are much more difficult to recover from the subsurface reservoir. These materials
have a much higher viscosity (and lower API gravity) than conventional petroleum, and primary
recovery of these petroleum types usually requires thermal stimulation of the reservoir.
The definition of heavy oil is usually based on the API gravity or viscosity, and the definition is
quite arbitrary although there have been attempts to rationalize the definition based upon viscosity,
API gravity, and density (Chapter 2). Thus, the generic term heavy oil is often applied to petroleum
that has an API gravity of less than 20° and usually, but not always, a sulfur content higher than 2%
by weight. Furthermore, in contrast to conventional crude oils, heavy oils are darker in color and
may even be black. The term heavy oil has also been arbitrarily used to describe both the heavy oils
The Chemistry and Technology of Petroleum
The most preferred type of natural gas is the nonassociated 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 increased expenditure for compression. Thus it is
not surprising that such gas (under conditions that are not economically favorable) is often flared
or vented.
The nonhydrocarbon constituents of natural gas can be classified as two types of materials:
1. Diluents, such as nitrogen, carbon dioxide, and water vapors
2. Contaminants, such as hydrogen sulfide and/or other sulfur compounds
The diluents are noncombustible gases that reduce the 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 transportation equipment in addition to being
obnoxious pollutants. Thus, the primary reason for gas processing is to remove the unwanted constituents of natural gas.
The following are the major diluents or contaminants of natural gas:
1. Acid gas that is predominantly hydrogen sulfide although carbon dioxide does occur to a
lesser extent
2. Water that 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
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
(Speight, 1993) and the proportion of nonhydrocarbon constituents can vary over a very wide range.
Thus a particular natural gas field could require production, processing, and handling protocols different from those used for gas from another field.
Liquefied petroleum gas (LPG) is composed of propane (C 3 H 8 ), butanes (C 4 H 10 ), and/or mixtures
thereof, small amounts of ethane (C 2 H 6 ) and pentane (C 5 H 12 ) may also be present as impurities. On
the other hand, natural gasoline (such as refinery gasoline) consists mostly of pentane (C 5 H 12 ) and
higher molecular weight hydrocarbons.
The term natural gasoline has also on occasion in the gas industry been applied to mixtures
of LPG, 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.
3.3.4 HeAvy oIl
When petroleum occurs in a reservoir that allows the crude material to be recovered by pumping
operations as a free-flowing dark to light colored liquid, it is often referred to as conventional petroleum. Heavy oils are the other types of petroleum that are different from conventional petroleum
insofar as they are much more difficult to recover from the subsurface reservoir. These materials
have a much higher viscosity (and lower API gravity) than conventional petroleum, and primary
recovery of these petroleum types usually requires thermal stimulation of the reservoir.
The definition of heavy oil is usually based on the API gravity or viscosity, and the definition is
quite arbitrary although there have been attempts to rationalize the definition based upon viscosity,
API gravity, and density (Chapter 2). Thus, the generic term heavy oil is often applied to petroleum
that has an API gravity of less than 20° and usually, but not always, a sulfur content higher than 2%
by weight. Furthermore, in contrast to conventional crude oils, heavy oils are darker in color and
may even be black. The term heavy oil has also been arbitrarily used to describe both the heavy oils