Summary
SAGD is one successful thermal recovery technique applied in the Athabasca
and Peace River reservoirs in central and northern Alberta, Canada. In SAGD,
steam is injected into a horizontal injection well and is forced outward,
losing its latent heat when it comes into contact with the cold bitumen at the
edge of a depletion chamber. As a consequence, the viscosity of the bitumen
falls several orders of magnitude, its mobility rises several orders of
magnitude, and then it flows under gravity toward a horizontal production well
located several meters below and parallel to the injection well. Heat-transfer
mechanisms are pivotal to the SAGD process. Though heat energy is transferred
from steam to reservoir by conduction and convection, heat transfer by
convection is not considered in the classic SAGD mathematical models such as
Butler?s. Researchers such as Butler and Stephens (1981), Reis (1992), Akin
(2005), Liang (2005), Nukhaev et al. (2006), and Azad and Chalaturnyk (2010)
considered conduction from steam to cold reservoir to be the only heat-transfer
component. However, because the heat capacity of water is typically two to five
times that of bitumen, convection caused by the mobile condensate flow in the
reservoir may contradict these studies. Farouq-Ali (1997) was the first to
criticize the assumption that there is only a thermal conduction mechanism in
the SAGD process. He pointed out that with so much condensate flowing,
convection would be expected to be the dominant heat-transfer mechanism, which
can be plausible at high temperatures. In response, Edmunds (1999a) stated that
on the basis of the associated change in enthalpy, the heat transfer into a
cold reservoir because of convection is probably less than 5% of that because
of conduction. Ito (1999) challenged Edmunds (1999a) statement, on the basis of
Ito and Suzuki (1996, 1999) and Ito et al. (1998), pointing out that "this
number, 5%; i.e., ratio between convection to conduction presented by Edmunds
(1999a) is unrealistically low, (and) it should be in the range of 50%." This
study examined the relative roles of convective and conductive heat transfer at
the edge of SAGD steam chambers. In summary, the mathematical model developed
in this study considered both conduction and convection, and the resultant
output from the model is reasonably consistent with published field data. This
study supports the idea that although convection can dominate near the chamber
edge in high-water-saturation reservoirs, in bitumen-rich reservoirs, its
contribution to heat transfer is less than 1% and can be neglected.
© 2013. Society of Petroleum Engineers
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History
- Original manuscript received:
23 January 2012
- Revised manuscript received:
13 August 2012
- Manuscript approved:
21 August 2012
- Published online:
23 January 2013
- Version of record:
27 February 2013
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