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The Chemistry and Technology of Petroleum
and this leads to volumetric shrinkage of the shale barriers. As the shale shrink, the lateral stress
(fracture gradient) drops until the pore pressure exceeds the lateral stress, which causes vertical
fractures to open. Thus, the combined processes of gravity segregation and shale thermal fracturing
make SAGD so efficient that recovery ratios of 60%–70% are probably achievable even in cases
where there are many thin shale streaks, although there are limits on the thickness of shale bed that
can be traversed in a reasonable time.
Heat losses and deceleration of lateral growth mean that there is an economic limit to the lateral
growth of the steam chamber. This limit is thought to be a chamber width of four times (4×) the
vertical zone thickness. For thinner zones, horizontal well pairs would therefore have to be placed
close together, increasing costs as well as providing lower total resources per well pair. In summary,
the zone thickness limit (net pay thickness) must be defined for all reservoirs.
The cost of heat is a major economic constraint on all thermal processes. Currently, steam is
generated with natural gas, and when the cost of natural gas rises, operating costs rise considerably.
Thermally, SAGD is about twice as efficient as cyclic steam stimulation, with steam–oil ratios that
are now approaching two (instead of four for cyclic steam soak), for similar cases. Combined with
the high recovery ratios possible, SAGD will likely displace pressure-driven thermal process in all
cases where the reservoir is reasonably thick.
Finally, because of the lower pressures associated with SAGD, in comparison to high pressure
processes such as cyclic steam soak and steam drive, greater wellbore stability should be another
asset, reducing substantially the number of sheared wells that are common in cyclic steam soak
projects.
Recently, one of the new improved techniques in SAGD is FAST-SAGD, which combines conventional SAGD wells with cyclic steam stimulation wells (offset wells) (Shin and Polikar, 2004,
2005). These offset wells are drilled at the same depth as that of SAGD production well.
After short period of time of operation of the SAGD, which is started to operate similarly to the
conventional SAGD, steam is injected through cyclic steam stimulation wells that inject steam with
high injection rate under high operation pressure to make heat easily transfer to reservoir. After
several cycles of cyclic steam stimulation, the steam chambers around these wells will contact and
merge together. At this time, injection of steam into the cyclic steam stimulation wells is ceased, and
producing period will be commenced but SAGD well steam injection is continued and heat transfer
is continued remained, and the reservoir can be heated effectively. The cyclic steam stimulation
wells are supported from injectors of SAGD wells, and thus accelerate production rate and reduce
production time.
However, in the FAST SAGD operation, steam has a tendency to bypass other wells during the
injection period due to the operation of the cyclic steam stimulation wells. Hence, live steam will
be produced at the producer, which has an adverse effect on the thermal efficiency of the process
thereby causing the production rate to decrease significantly.
In order to offset this condition, there are several constrains that need to be considered: (1) the
location of the offset wells should be calculated carefully so that the combination of the steam
chambers at the expected time to get the effective thermal process, (2) the cyclic steam stimulation wells should be planned with consideration of reservoir parameters—permeability, oil saturation, formation heat conduction, and heterogeneity (such as the presence of a shale barrier or clay
lenses)—in order to achieve high efficiency.
The hybrid SAGD process (HSAGD process) uses a similar well configuration to the FAST
SAGD method. However, the wells are operated very differently in terms of the operating conditions. In FAST SAGD process, the SAGD wells are operated first and cyclic steam stimulation
wells (offset wells) start later and require higher injection pressure and injection rate. Therefore,
steam is easy bypass to other wells but the HSAGD process can improve this phenomenon. In
the HSAGD process, all cyclic steam stimulation wells are placed in a staggered pattern and
the wells are operated at the same pressure and placed in operation earlier than SAGD wells
(Coskuner, 2009).
The Chemistry and Technology of Petroleum
and this leads to volumetric shrinkage of the shale barriers. As the shale shrink, the lateral stress
(fracture gradient) drops until the pore pressure exceeds the lateral stress, which causes vertical
fractures to open. Thus, the combined processes of gravity segregation and shale thermal fracturing
make SAGD so efficient that recovery ratios of 60%–70% are probably achievable even in cases
where there are many thin shale streaks, although there are limits on the thickness of shale bed that
can be traversed in a reasonable time.
Heat losses and deceleration of lateral growth mean that there is an economic limit to the lateral
growth of the steam chamber. This limit is thought to be a chamber width of four times (4×) the
vertical zone thickness. For thinner zones, horizontal well pairs would therefore have to be placed
close together, increasing costs as well as providing lower total resources per well pair. In summary,
the zone thickness limit (net pay thickness) must be defined for all reservoirs.
The cost of heat is a major economic constraint on all thermal processes. Currently, steam is
generated with natural gas, and when the cost of natural gas rises, operating costs rise considerably.
Thermally, SAGD is about twice as efficient as cyclic steam stimulation, with steam–oil ratios that
are now approaching two (instead of four for cyclic steam soak), for similar cases. Combined with
the high recovery ratios possible, SAGD will likely displace pressure-driven thermal process in all
cases where the reservoir is reasonably thick.
Finally, because of the lower pressures associated with SAGD, in comparison to high pressure
processes such as cyclic steam soak and steam drive, greater wellbore stability should be another
asset, reducing substantially the number of sheared wells that are common in cyclic steam soak
projects.
Recently, one of the new improved techniques in SAGD is FAST-SAGD, which combines conventional SAGD wells with cyclic steam stimulation wells (offset wells) (Shin and Polikar, 2004,
2005). These offset wells are drilled at the same depth as that of SAGD production well.
After short period of time of operation of the SAGD, which is started to operate similarly to the
conventional SAGD, steam is injected through cyclic steam stimulation wells that inject steam with
high injection rate under high operation pressure to make heat easily transfer to reservoir. After
several cycles of cyclic steam stimulation, the steam chambers around these wells will contact and
merge together. At this time, injection of steam into the cyclic steam stimulation wells is ceased, and
producing period will be commenced but SAGD well steam injection is continued and heat transfer
is continued remained, and the reservoir can be heated effectively. The cyclic steam stimulation
wells are supported from injectors of SAGD wells, and thus accelerate production rate and reduce
production time.
However, in the FAST SAGD operation, steam has a tendency to bypass other wells during the
injection period due to the operation of the cyclic steam stimulation wells. Hence, live steam will
be produced at the producer, which has an adverse effect on the thermal efficiency of the process
thereby causing the production rate to decrease significantly.
In order to offset this condition, there are several constrains that need to be considered: (1) the
location of the offset wells should be calculated carefully so that the combination of the steam
chambers at the expected time to get the effective thermal process, (2) the cyclic steam stimulation wells should be planned with consideration of reservoir parameters—permeability, oil saturation, formation heat conduction, and heterogeneity (such as the presence of a shale barrier or clay
lenses)—in order to achieve high efficiency.
The hybrid SAGD process (HSAGD process) uses a similar well configuration to the FAST
SAGD method. However, the wells are operated very differently in terms of the operating conditions. In FAST SAGD process, the SAGD wells are operated first and cyclic steam stimulation
wells (offset wells) start later and require higher injection pressure and injection rate. Therefore,
steam is easy bypass to other wells but the HSAGD process can improve this phenomenon. In
the HSAGD process, all cyclic steam stimulation wells are placed in a staggered pattern and
the wells are operated at the same pressure and placed in operation earlier than SAGD wells
(Coskuner, 2009).