Page 132 - Synthetic Fuels Handbook

118 CHAPTER FOUR

volume balance, replacing each unit volume withdrawn with a unit volume injected, to main-
tain the processes in the gravity dominated domain. If bottom water influx develops, this indi-
cates 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.
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 and this leads to volumetric shrink-
age 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 to 70 percent 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 2 (instead of 4 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 com-
mon in cyclic steam soak projects.
The Canadian tar sand has several SAGD projects in progress, since this region is home of
one of the largest deposits of bitumen in the world (Canada and Venezuela have the world’s
largest deposits). However, in spite of the success reported for the Athabasca deposit, the
SAGD process is not entirely without drawbacks. The process requires amounts of makeup
fresh water and large water recycling facilities as well as a high energy demand to create the
steam. In addition, gravity drainage being the operative means of bitumen separation from
the reservoir rock, the process requires comparatively thick and homogeneous reservoirs; to
date the process has not been tested on a wide variety of reservoirs. Different processes are still
being developed and these include steam-and-gas push (SAGP) and expanding-solvent-SAGD