177
Recovery of Heavy Oil and Tar Sand Bitumen
7.4.2.1 Steam Distillation
Steam injection pressures are limited during recovery of heavy oil because most heavy oil deposits
are relatively shallow. The maximum steam temperature is limited by the ideal gas law. For example,
at a 1000 m depth, the formation pressure is approximately 1450 psi, which permits a steam temperature of approximately 300°C (570°F), which is too low to provide significant upgrading on a
short timescale. In situ combustion is capable of much higher temperatures (approximately 700°C,
1290°F), which has the potential for measurable upgrading. Electric heating (resistance, induction,
or RF) should also be able to achieve the high temperatures required for in situ upgrading (Mut,
2005). But steam distillation then becomes an option.
The principal mechanisms responsible for oil recovery during heavy oil recovery by steam-based
processes are (1) thermal expansion of the oil, (2) viscosity reduction, and (3) steam distillation.
Steam distillation, once an ignored phenomenon, deserves recognition as the main mechanism that
reduces the residual oil saturation behind the hot-water front during steam flood particularly for
light and medium oils. Furthermore, steam distillation can also play an important role during heavy
oil recovery by steam flooding.
The vaporizing effect of hydrocarbons is induced by increasing the system temperature, and it is
reflected by the increase in the system pressure. Steam will evolve from the aqueous phase and strip
some hydrocarbon component from the oleic phase. Since the pressure of the system is increased;
the apparent bubble point of the water is consequently increased, which causes the higher molecular
weight, boiling point hydrocarbons to vaporize at the elevated saturation temperatures causing displacement of the oil by steam displacement and steam distillation (Volek and Pryor, 1972; Wu and
Brown, 1975; Sarathi et al., 1988; Sharpe et al., 1995).
To reduce viscosity of heavy crude oil, solvents frequently are used for dilution—this is one of
the most efficient methods of pipeline transportation of heavy oil. Solvents are also injected into the
reservoir for well cleaning, stimulation, fracturing, and, less frequently, for miscible displacement.
Thus, one benefit resulting from the distillation of heavy oil during a steam flood is that the lower
boiling constituents can act as a solvent to lower the viscosity of the original oil. In the steam distillation process, the vaporized distillate mixes with the original oil ahead of the steam condensation
zone. Depending on the quantity of light hydrocarbon available to mix with the oil, viscosity can be
substantially decreased—provided the distillate has a sufficient amount of aromatic constituents or
naphthenic constituents to maintain the asphaltene constituents in solution (Mitchell and Speight,
1973; Shu, 1984; Speight, 2007).
It has also been shown that, during steam–propane distillation of oil, the steam/propane vapor
strips the more volatile components from the heavy oil and encourages a decrease of the boiling
point of these components. The steam enriched with the propane and light components, flow through
the steam zone to the condensation front where both steam and light hydrocarbons condense. The
condensed hydrocarbons are miscible with the oil, reducing overall viscosity of liquids hydrocarbon. The steam/propane distillation begins when the total vapor pressure (steam and propane) in the
presence of two immiscible liquids (water and oil exerting its own vapor pressure at the temperature
of the system) equals the total pressure on the system. It is also possible that the role of propane is
to reduce the boiling point of the lower molecular weight components. As a result, heavy oil will
begin distilling at temperatures much lower than the normal boiling points of the constituents and,
as a consequence, the distillation yield increases.
As promising as this may seem, the steam distillation yields are mainly dependent on the oil
composition and may not correlate with crude API gravity. Changes in steam saturation pressure
and temperature have insignificant effect on the yields; however, superheated steam significantly
increases the yields for some crude oils.
7.4.2.2 Mild Thermal Cracking
Heavy oil constituents can be cracked into lighter hydrocarbon molecules at high-enough temperatures and pressures. In the pyrolysis of heavy oil, carbon–carbon bonds in the hydrocarbon chain
Recovery of Heavy Oil and Tar Sand Bitumen
7.4.2.1 Steam Distillation
Steam injection pressures are limited during recovery of heavy oil because most heavy oil deposits
are relatively shallow. The maximum steam temperature is limited by the ideal gas law. For example,
at a 1000 m depth, the formation pressure is approximately 1450 psi, which permits a steam temperature of approximately 300°C (570°F), which is too low to provide significant upgrading on a
short timescale. In situ combustion is capable of much higher temperatures (approximately 700°C,
1290°F), which has the potential for measurable upgrading. Electric heating (resistance, induction,
or RF) should also be able to achieve the high temperatures required for in situ upgrading (Mut,
2005). But steam distillation then becomes an option.
The principal mechanisms responsible for oil recovery during heavy oil recovery by steam-based
processes are (1) thermal expansion of the oil, (2) viscosity reduction, and (3) steam distillation.
Steam distillation, once an ignored phenomenon, deserves recognition as the main mechanism that
reduces the residual oil saturation behind the hot-water front during steam flood particularly for
light and medium oils. Furthermore, steam distillation can also play an important role during heavy
oil recovery by steam flooding.
The vaporizing effect of hydrocarbons is induced by increasing the system temperature, and it is
reflected by the increase in the system pressure. Steam will evolve from the aqueous phase and strip
some hydrocarbon component from the oleic phase. Since the pressure of the system is increased;
the apparent bubble point of the water is consequently increased, which causes the higher molecular
weight, boiling point hydrocarbons to vaporize at the elevated saturation temperatures causing displacement of the oil by steam displacement and steam distillation (Volek and Pryor, 1972; Wu and
Brown, 1975; Sarathi et al., 1988; Sharpe et al., 1995).
To reduce viscosity of heavy crude oil, solvents frequently are used for dilution—this is one of
the most efficient methods of pipeline transportation of heavy oil. Solvents are also injected into the
reservoir for well cleaning, stimulation, fracturing, and, less frequently, for miscible displacement.
Thus, one benefit resulting from the distillation of heavy oil during a steam flood is that the lower
boiling constituents can act as a solvent to lower the viscosity of the original oil. In the steam distillation process, the vaporized distillate mixes with the original oil ahead of the steam condensation
zone. Depending on the quantity of light hydrocarbon available to mix with the oil, viscosity can be
substantially decreased—provided the distillate has a sufficient amount of aromatic constituents or
naphthenic constituents to maintain the asphaltene constituents in solution (Mitchell and Speight,
1973; Shu, 1984; Speight, 2007).
It has also been shown that, during steam–propane distillation of oil, the steam/propane vapor
strips the more volatile components from the heavy oil and encourages a decrease of the boiling
point of these components. The steam enriched with the propane and light components, flow through
the steam zone to the condensation front where both steam and light hydrocarbons condense. The
condensed hydrocarbons are miscible with the oil, reducing overall viscosity of liquids hydrocarbon. The steam/propane distillation begins when the total vapor pressure (steam and propane) in the
presence of two immiscible liquids (water and oil exerting its own vapor pressure at the temperature
of the system) equals the total pressure on the system. It is also possible that the role of propane is
to reduce the boiling point of the lower molecular weight components. As a result, heavy oil will
begin distilling at temperatures much lower than the normal boiling points of the constituents and,
as a consequence, the distillation yield increases.
As promising as this may seem, the steam distillation yields are mainly dependent on the oil
composition and may not correlate with crude API gravity. Changes in steam saturation pressure
and temperature have insignificant effect on the yields; however, superheated steam significantly
increases the yields for some crude oils.
7.4.2.2 Mild Thermal Cracking
Heavy oil constituents can be cracked into lighter hydrocarbon molecules at high-enough temperatures and pressures. In the pyrolysis of heavy oil, carbon–carbon bonds in the hydrocarbon chain