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Recovery of Heavy Oil and Tar Sand Bitumen
through the upper well creates a steam chamber along the walls of which the heated bitumen flows
and is produced in the lower well.
Several variations of this process have been developed. One variation uses a single horizontal
well, with steam injection through a central pipe and production along the annulus. Another variation involves steam injection through existing vertical wells and production through an underlying
horizontal well. The key benefits of the SAGD process are an improved steam–oil ratio and high
ultimate recovery (on the order of 60%–70%). The outstanding technical issues relate to low initial
oil rate, artificial lifting of bitumen to the surface, horizontal well operation, and the extrapolation
of the process to reservoirs having low permeability, low pressure, or bottom water.
In the process, a pair of horizontal wells, separated vertically by about 15–20 ft are drilled at
the bottom of a thick unconsolidated sandstone reservoir. Steam, perhaps along with a mixture of
hydrocarbons that dissolve into the oil and help reduce its viscosity, is injected into the upper well.
The heat reduces the oil viscosity to values as low as 1–10 cP (depending on temperature and initial
conditions) and develops a steam chamber that grows vertically and laterally. The steam and gases
rise because of their low density, and the oil and condensed water are removed through the lower
well. The gases produced during SAGD tend to be methane with some carbon dioxide and traces
of hydrogen sulfide.
To a small degree, the noncondensable gases tend to remain high in the structure, filling the void
space, and even acting as a partial insulating blanket that helps to reduce vertical heat losses as the
chamber grows laterally. At the pore scale and at larger scales as well, flow is through countercurrent, gravity-driven flow, and a thin and continuous oil film is sustained, giving high recoveries
estimated to be as large as 70%–80% in suitable reservoirs.
Operating the production and injection wells at approximately the same pressure as the reservoir eliminates viscous fingering and coning processes, and also suppresses water influx or oil loss
through permeable streaks. This keeps the steam chamber interface relatively sharp, and reduces
heat losses considerably. Injection pressures are much lower than the fracture gradient, which means
that the chances of breaking into a thief zone, an instability problem that plagues all high-pressure
steam injection processes, such as cyclic steam soak, are essentially zero.
Thus, the SAGD process, as for all gravity-driven processes, is extremely stable because the
process zone grows only by gravity segregation, and there are no pressure-driven instabilities
such as channeling, coning, and fracturing. It is vital in the SAGD process to maintain a volume
balance, replacing each unit volume withdrawn with a unit volume injected, to maintain the processes in the gravity dominated domain. If bottom-water influx develops, this indicates that the
pressure in the water is larger than the pressure in the steam chamber, and steps must be taken to
balance the pressures. Because it is not possible to reduce the pressure in the water zone, the pressure in the steam chamber and production well region must be increased. This can be achieved
by increasing the operating pressure of the steam chamber through the injection rate of steam or
through reduction of the production rate from the lower well. After some time, the pressures will
become more balanced and the water influx ceases. Thereafter, maintaining a volume balance
carefully is essential.
Clearly, a low pressure gradient between the bottom water and the production well must be sustained. If pressure starts to build up in the steam chamber zone, then loss of hot water can take place
as well. In such cases, the steam chamber pressure must be reduced and perhaps also the production
rate increased slightly to balance the pressures. In all these cases, the system tends to return to a
stable configuration because of the density differences between the phases.
SAGD seems to be relatively insensitive to shale streaks and similar horizontal barriers, even up
to several meters thick (3–6 ft), that otherwise would restrict vertical flow rates. This occurs because
as the rock is heated, differential thermal expansion causes the shale to be placed under a tensile
stress, and vertical fractures are created, which serve as conduits for steam (up) and liquids (down).
As high temperatures hit the shale, the kinetic energy in the water increases and adsorbed water on
clay particles is liberated. Thus, instead of expanding thermally, dehydration (loss of water) occurs
Recovery of Heavy Oil and Tar Sand Bitumen
through the upper well creates a steam chamber along the walls of which the heated bitumen flows
and is produced in the lower well.
Several variations of this process have been developed. One variation uses a single horizontal
well, with steam injection through a central pipe and production along the annulus. Another variation involves steam injection through existing vertical wells and production through an underlying
horizontal well. The key benefits of the SAGD process are an improved steam–oil ratio and high
ultimate recovery (on the order of 60%–70%). The outstanding technical issues relate to low initial
oil rate, artificial lifting of bitumen to the surface, horizontal well operation, and the extrapolation
of the process to reservoirs having low permeability, low pressure, or bottom water.
In the process, a pair of horizontal wells, separated vertically by about 15–20 ft are drilled at
the bottom of a thick unconsolidated sandstone reservoir. Steam, perhaps along with a mixture of
hydrocarbons that dissolve into the oil and help reduce its viscosity, is injected into the upper well.
The heat reduces the oil viscosity to values as low as 1–10 cP (depending on temperature and initial
conditions) and develops a steam chamber that grows vertically and laterally. The steam and gases
rise because of their low density, and the oil and condensed water are removed through the lower
well. The gases produced during SAGD tend to be methane with some carbon dioxide and traces
of hydrogen sulfide.
To a small degree, the noncondensable gases tend to remain high in the structure, filling the void
space, and even acting as a partial insulating blanket that helps to reduce vertical heat losses as the
chamber grows laterally. At the pore scale and at larger scales as well, flow is through countercurrent, gravity-driven flow, and a thin and continuous oil film is sustained, giving high recoveries
estimated to be as large as 70%–80% in suitable reservoirs.
Operating the production and injection wells at approximately the same pressure as the reservoir eliminates viscous fingering and coning processes, and also suppresses water influx or oil loss
through permeable streaks. This keeps the steam chamber interface relatively sharp, and reduces
heat losses considerably. Injection pressures are much lower than the fracture gradient, which means
that the chances of breaking into a thief zone, an instability problem that plagues all high-pressure
steam injection processes, such as cyclic steam soak, are essentially zero.
Thus, the SAGD process, as for all gravity-driven processes, is extremely stable because the
process zone grows only by gravity segregation, and there are no pressure-driven instabilities
such as channeling, coning, and fracturing. It is vital in the SAGD process to maintain a volume
balance, replacing each unit volume withdrawn with a unit volume injected, to maintain the processes in the gravity dominated domain. If bottom-water influx develops, this indicates that the
pressure in the water is larger than the pressure in the steam chamber, and steps must be taken to
balance the pressures. Because it is not possible to reduce the pressure in the water zone, the pressure in the steam chamber and production well region must be increased. This can be achieved
by increasing the operating pressure of the steam chamber through the injection rate of steam or
through reduction of the production rate from the lower well. After some time, the pressures will
become more balanced and the water influx ceases. Thereafter, maintaining a volume balance
carefully is essential.
Clearly, a low pressure gradient between the bottom water and the production well must be sustained. If pressure starts to build up in the steam chamber zone, then loss of hot water can take place
as well. In such cases, the steam chamber pressure must be reduced and perhaps also the production
rate increased slightly to balance the pressures. In all these cases, the system tends to return to a
stable configuration because of the density differences between the phases.
SAGD seems to be relatively insensitive to shale streaks and similar horizontal barriers, even up
to several meters thick (3–6 ft), that otherwise would restrict vertical flow rates. This occurs because
as the rock is heated, differential thermal expansion causes the shale to be placed under a tensile
stress, and vertical fractures are created, which serve as conduits for steam (up) and liquids (down).
As high temperatures hit the shale, the kinetic energy in the water increases and adsorbed water on
clay particles is liberated. Thus, instead of expanding thermally, dehydration (loss of water) occurs