16
The Chemistry and Technology of Petroleum
1.4.3 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 oil is a type of petroleum that is different from the conventional petroleum insofar
as it is 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 (Chapter 7).
Heavy oils are more difficult to recover from the subsurface reservoir than light oils. The definition of heavy oils 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.
For many years, petroleum and heavy oil were very generally defined in terms of physical properties. For example, heavy oils were considered to be those crude oils that had gravity somewhat
less than 20°API with the heavy oils falling into the API gravity range 10°–15°. For example, Cold
Lake heavy crude oil has an API gravity equal to 12° and extra heavy oils, such as tar sand bitumen,
usually have an API gravity in the range 5°–10° (Athabasca bitumen = 8°API). Residua would vary
depending upon the temperature at which distillation was terminated but usually vacuum residua
are in the range 2°–8°API (Speight, 2000 and references cited therein; Speight and Ozum, 2002 and
references cited therein).
Heavy oils 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 generic term heavy oil is often applied to a crude oil that has a less than 20°API and
usually, but not always, a sulfur content higher than 2% by weight (Speight, 2000). 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 that require thermal stimulation of recovery from the reservoir and the bitumen in bituminous sand (tar sand, q.v.)
formations from which the heavy bituminous material is recovered by a mining operation.
1.4.4 FoAmy oIl
Foamy oil is an oil-continuous foam that contains dispersed gas bubbles produced at the
wellhead from heavy oil reservoirs under solution gas drive. The nature of the gas dispersions
in oil distinguishes foamy oil behavior from conventional heavy oil. The gas that comes out of
solution in the reservoir does not coalesce into large gas bubbles nor into a continuous flowing
gas phase. Instead, it remains as small bubbles entrained in the crude oil, keeping the effective oil viscosity low while providing expansive energy that helps drive the oil toward the
pro ducing. Foamy oil accounts for unusually high production in heavy oil reservoirs under
solution-gas drive.
Thus, foamy oil is formed in solution gas drive reservoirs when gas is released from solution as the reservoir pressure declines. It has been noted that the oil at the wellhead of these
heavy-oil reservoirs resembles the form of foam, hence the term foamy oil. The gas initially
exists in the form of small bubbles within individual pores in the rock. As time passes and
pressure continues to decline, the bubbles grow to fill the pores. With further declines in pressure, the bubbles created in different locations become large enough to coalesce into a continuous gas phase. Once the gas phase becomes continuous (i.e., when gas saturation exceeds
the critical level)—the minimum saturation at which a continuous gas phase exists in porous
media — traditional two-phase (oil and gas) flow with classical relative permeability occurs. As
a result, the production gas–oil ratio (GOR) increases rapidly after the critical gas saturation
has been exceeded.
The Chemistry and Technology of Petroleum
1.4.3 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 oil is a type of petroleum that is different from the conventional petroleum insofar
as it is 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 (Chapter 7).
Heavy oils are more difficult to recover from the subsurface reservoir than light oils. The definition of heavy oils 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.
For many years, petroleum and heavy oil were very generally defined in terms of physical properties. For example, heavy oils were considered to be those crude oils that had gravity somewhat
less than 20°API with the heavy oils falling into the API gravity range 10°–15°. For example, Cold
Lake heavy crude oil has an API gravity equal to 12° and extra heavy oils, such as tar sand bitumen,
usually have an API gravity in the range 5°–10° (Athabasca bitumen = 8°API). Residua would vary
depending upon the temperature at which distillation was terminated but usually vacuum residua
are in the range 2°–8°API (Speight, 2000 and references cited therein; Speight and Ozum, 2002 and
references cited therein).
Heavy oils 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 generic term heavy oil is often applied to a crude oil that has a less than 20°API and
usually, but not always, a sulfur content higher than 2% by weight (Speight, 2000). 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 that require thermal stimulation of recovery from the reservoir and the bitumen in bituminous sand (tar sand, q.v.)
formations from which the heavy bituminous material is recovered by a mining operation.
1.4.4 FoAmy oIl
Foamy oil is an oil-continuous foam that contains dispersed gas bubbles produced at the
wellhead from heavy oil reservoirs under solution gas drive. The nature of the gas dispersions
in oil distinguishes foamy oil behavior from conventional heavy oil. The gas that comes out of
solution in the reservoir does not coalesce into large gas bubbles nor into a continuous flowing
gas phase. Instead, it remains as small bubbles entrained in the crude oil, keeping the effective oil viscosity low while providing expansive energy that helps drive the oil toward the
pro ducing. Foamy oil accounts for unusually high production in heavy oil reservoirs under
solution-gas drive.
Thus, foamy oil is formed in solution gas drive reservoirs when gas is released from solution as the reservoir pressure declines. It has been noted that the oil at the wellhead of these
heavy-oil reservoirs resembles the form of foam, hence the term foamy oil. The gas initially
exists in the form of small bubbles within individual pores in the rock. As time passes and
pressure continues to decline, the bubbles grow to fill the pores. With further declines in pressure, the bubbles created in different locations become large enough to coalesce into a continuous gas phase. Once the gas phase becomes continuous (i.e., when gas saturation exceeds
the critical level)—the minimum saturation at which a continuous gas phase exists in porous
media — traditional two-phase (oil and gas) flow with classical relative permeability occurs. As
a result, the production gas–oil ratio (GOR) increases rapidly after the critical gas saturation
has been exceeded.