Summary
Steam-assisted gravity drainage (SAGD) is a robust thermal process that has
revolutionized the economic recovery of heavy oil and bitumen from the immense
oil-sands deposits in western Canada, which have 1.6 to 2.5 trillion bbl of oil
in place. With steam injection, reservoir pressures and temperatures are
raised. These elevated pressures and temperatures alter the rock stresses
sufficiently to cause shear failure within and beyond the growing steam
chamber. The associated increases in porosity, permeability, and water
transmissibility accelerate the process. Pressures ahead of the steam chamber
are substantially increased, promoting future growth of the steam chamber. A
methodology for determining the optimum injection pressure for geomechanical
enhancement is presented that allows operators to customize steam pressures to
their reservoirs.
In response, these geomechanical enhancements of porosity, permeability, and
mobility alter the growth pattern of the steam chamber. The stresses in the
rock will determine the directionality of the steam chamber growth; these are
largely a function of the reservoir depth and tectonic loading. By anticipating
the SAGD growth pattern, operators can optimize on the orientation and spacing
of their wells.
Core tests are essential for the determination of reservoir properties, yet
oil sand core disturbance is endemic. Most core results are invalid, given the
high core-disturbance results in test specimens. Discussion on the causes and
mitigation of core disturbance is presented.
Monitoring of the SAGD process is central to understanding where the process
has been successful. Methods of monitoring the steam chamber are presented,
including the use of satellite radar interferometry. Monitoring is particularly
important to ensure caprock integrity because it is paramount that SAGD
operations be contained within the reservoir.
There are several quarter-billion-dollar SAGD projects in western Canada
that are currently in the design stage. It is essential that these designs use
a fuller understanding of the SAGD process to optimize well placement and
facilities design. Only by including the interaction of SAGD and geomechanics
can we achieve a more complete understanding of the process.
Introduction
Geomechanics examines the engineering behavior of rock formations under
existing and imposed stress conditions. SAGD imposes elevated pressures and
temperatures on the reservoir, which then has a geomechanical response.
Typically, the SAGD process is used in unconsolidated sandstone reservoirs with
very heavy oil or bitumen. In-situ viscosities can exceed 5 000 000 mPa•s
[mPa•s º cp] under reservoir conditions.
These bituminous unconsolidated sandstones, or “oil sands,” are unique
engineering materials for two reasons. Firstly, the bitumen is essentially a
solid under virgin conditions, and secondly, the sands themselves are not
loosely packed beach sands. Instead, they have a dense, interlocked structure
that developed as a result of deeper burial and elevated temperatures over
geological time. In western Canada, the silica pressure dissolution and
redeposition over 120 million years developed numerous concave-convex grain
contacts (Dusseault 1980a; Touhidi-Baghini 1998) in response to the additional
rock overburden and elevated temperatures. As such, these oil sands are at a
density far in excess of that expected under current or previous overburden
stresses. Furthermore, once oil sands are disturbed, the grain rotations and
dislocations preclude any return to their undisturbed state.
Oil sands, by definition, have little to no cementation. As such, their
strength is entirely dependent upon grain-to-grain contacts, which are
considerable in their undisturbed state. These contacts are maintained by the
effective confining stress. Any reduction in the effective confining stress
will result in a reduction in strength. Because the SAGD process increases the
formation fluid pressure, it reduces the effective stresses and weakens the oil
sand.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
20 August 2005
- Meeting paper published:
1 November 2005
- Revised manuscript received:
21 April 2007
- Manuscript approved:
26 May 2007
- Version of record:
20 August 2007
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