141
Exploration, Recovery, and Transportation
front in the same direction as the injected air. Reverse combustion involves movement of the hot
front opposite to the direction of the injected air.
The relatively small portion of the oil that remains after these displacement mechanisms have
acted becomes the fuel for the in situ combustion process. Production is obtained from wells offsetting the injection locations. In some applications, the efficiency of the total in situ combustion operation can be improved by alternating water and air injection. The injected water tends to improve
the utilization of heat by transferring heat from the rock behind the combustion zone to the rock
immediately ahead of the combustion zone.
The performance of in situ combustion is predominantly determined by the four following factors:
(1) the quantity of oil that initially resides in the rock to be burned, (2) the quantity of air required to
burn the portion of the oil that fuels the process, (3) he distance to which vigorous combustion can be
sustained against heat losses, and (4) the mobility of the air or combustion product gases.
In many field projects, the high gas mobility has limited recovery through its adverse effect on
the sweep efficiency of the burning front. Because of the density contrast between air and reservoir
liquids, the burning front tends to override the reservoir liquids. To date, combustion has been most
effective for the recovery of viscous oils in moderately thick reservoirs in which reservoir dip and
continuity provide effective gravity drainage or operational factors permit close well spacing.
Using combustion to stimulate oil production is regarded as attractive for deep reservoirs
(Terwilliger, 1975) and, in contrast to steam injection, usually involves no loss of heat. The duration
of the combustion may be short (<30 days) or more prolonged (∼90 days), depending upon requirements. In addition, backflow of the oil through the hot zone must be prevented or coking occurs.
Both forward and reverse combustion methods have been used with some degree of success
when applied to tar sand deposits. The forward combustion process has been applied to the Orinoco
deposits (Terwilliger et al., 1975) and in the Kentucky sands (Terwilliger, 1975). The reverse combustion process has been applied to the Orinoco deposit (Burger, 1978) and the Athabasca. In tests
such as these it is essential to control the airflow (Wilson et al., 1963) and to mitigate the potential
for spontaneous ignition (Wilson et al., 1963; Dietz and Weijdema, 1968; Burger, 1978). There has
also been some success in the application of the reverse combustion technique to the Missouri tar
sands (Trantham and Marx, 1966).
In modified in situ extraction processes, combinations of in situ and mining techniques are used
to access the reservoir. A portion of the reservoir rock must be removed to enable application of the
in situ extraction technology. The most common method is to enter the reservoir through a largediameter vertical shaft, excavate horizontal drifts from the bottom of the shaft, and drill injection
and production wells horizontally from the drifts. Thermal extraction processes are then applied
through the wells. When the horizontal wells are drilled at or near the base of the tar sand reservoir,
the injected heat rises from the injection wells through the reservoir, and drainage of produced fluids to the production wells is assisted by gravity.
6.6 PRODUCTS AND PRODUCT QUALITY
Fluids produced from a well are seldom pure crude oil: in fact, a variety of materials may be
produced by oil wells in addition to liquid and gaseous hydrocarbons. The natural gas itself may
contain as impurities one or more non-hydrocarbon substances. The most abundant of these
impurities is hydrogen sulfide, which imparts a noticeable odor to the gas. A small amount of
this compound is considered advantageous as it gives an indication of leaks and where they
occur. A larger amount, however, makes the gas obnoxious and difficult to market. Such gas is
referred to as sour gas (Chapter 1) and much of it is used in the manufacture of carbon black.
A few natural gases contain helium, and this element does in fact occur in commercial quantities in certain gas fields; nitrogen and carbon dioxide are also found in some natural gases. Gas
is usually separated at as high a pressure as possible, reducing compression costs when the gas is
to be used for gaslift or delivered to a pipeline. Lighter hydrocarbons and hydrogen sulfide are
Exploration, Recovery, and Transportation
front in the same direction as the injected air. Reverse combustion involves movement of the hot
front opposite to the direction of the injected air.
The relatively small portion of the oil that remains after these displacement mechanisms have
acted becomes the fuel for the in situ combustion process. Production is obtained from wells offsetting the injection locations. In some applications, the efficiency of the total in situ combustion operation can be improved by alternating water and air injection. The injected water tends to improve
the utilization of heat by transferring heat from the rock behind the combustion zone to the rock
immediately ahead of the combustion zone.
The performance of in situ combustion is predominantly determined by the four following factors:
(1) the quantity of oil that initially resides in the rock to be burned, (2) the quantity of air required to
burn the portion of the oil that fuels the process, (3) he distance to which vigorous combustion can be
sustained against heat losses, and (4) the mobility of the air or combustion product gases.
In many field projects, the high gas mobility has limited recovery through its adverse effect on
the sweep efficiency of the burning front. Because of the density contrast between air and reservoir
liquids, the burning front tends to override the reservoir liquids. To date, combustion has been most
effective for the recovery of viscous oils in moderately thick reservoirs in which reservoir dip and
continuity provide effective gravity drainage or operational factors permit close well spacing.
Using combustion to stimulate oil production is regarded as attractive for deep reservoirs
(Terwilliger, 1975) and, in contrast to steam injection, usually involves no loss of heat. The duration
of the combustion may be short (<30 days) or more prolonged (∼90 days), depending upon requirements. In addition, backflow of the oil through the hot zone must be prevented or coking occurs.
Both forward and reverse combustion methods have been used with some degree of success
when applied to tar sand deposits. The forward combustion process has been applied to the Orinoco
deposits (Terwilliger et al., 1975) and in the Kentucky sands (Terwilliger, 1975). The reverse combustion process has been applied to the Orinoco deposit (Burger, 1978) and the Athabasca. In tests
such as these it is essential to control the airflow (Wilson et al., 1963) and to mitigate the potential
for spontaneous ignition (Wilson et al., 1963; Dietz and Weijdema, 1968; Burger, 1978). There has
also been some success in the application of the reverse combustion technique to the Missouri tar
sands (Trantham and Marx, 1966).
In modified in situ extraction processes, combinations of in situ and mining techniques are used
to access the reservoir. A portion of the reservoir rock must be removed to enable application of the
in situ extraction technology. The most common method is to enter the reservoir through a largediameter vertical shaft, excavate horizontal drifts from the bottom of the shaft, and drill injection
and production wells horizontally from the drifts. Thermal extraction processes are then applied
through the wells. When the horizontal wells are drilled at or near the base of the tar sand reservoir,
the injected heat rises from the injection wells through the reservoir, and drainage of produced fluids to the production wells is assisted by gravity.
6.6 PRODUCTS AND PRODUCT QUALITY
Fluids produced from a well are seldom pure crude oil: in fact, a variety of materials may be
produced by oil wells in addition to liquid and gaseous hydrocarbons. The natural gas itself may
contain as impurities one or more non-hydrocarbon substances. The most abundant of these
impurities is hydrogen sulfide, which imparts a noticeable odor to the gas. A small amount of
this compound is considered advantageous as it gives an indication of leaks and where they
occur. A larger amount, however, makes the gas obnoxious and difficult to market. Such gas is
referred to as sour gas (Chapter 1) and much of it is used in the manufacture of carbon black.
A few natural gases contain helium, and this element does in fact occur in commercial quantities in certain gas fields; nitrogen and carbon dioxide are also found in some natural gases. Gas
is usually separated at as high a pressure as possible, reducing compression costs when the gas is
to be used for gaslift or delivered to a pipeline. Lighter hydrocarbons and hydrogen sulfide are