Summary
The Steam-Assisted Gravity Drainage (SAGD) process has been successfully
implemented to produce ultra-viscous bitumen from the Athabasca oil sands in
the province of Alberta. In the Hangingstone area, 15 pairs of SAGD wells had
been drilled in the reservoir by 2006, each a maximum of 30 m in thickness and
approximately 300 m in depth. The production reached an average of 8,000 BOPD
in recent years. The reservoir is geologically characterized as a stacked
incised valley with fills in fluvial to upper-estuarine channels. Thin mudstone
layers and abrupt changes in facies caused by sedimentary deposits present
complexities and difficulties for SAGD implementation.
A 3D seismic survey was conducted in 2002 to obtain a clear view of geology
that was fully utilized for planning additional wells. In order to evaluate
SAGD efficiency and performance, a time-lapse 3D seismic survey was carried out
in 2006. In this paper, P-wave velocity (Vp) maps, transformed from the seismic
travel-time maps, were interpreted with a new methodology for evaluating the
areal extent of the steam chamber zone created by the SAGD process. In the
previous experimental study of seismic velocity measurements with oil sands
cores, Vp was found to steeply drop with an increase in temperature and to
gently decrease with an increase in pore-pressure.
Based on the experimental results, a petrophysical model was formulated to
express Vp as a function of temperature, pressure and water saturation. The
high-pressure and high-temperature zone of the SAGD process should generate
differences between the first (2002) and second (2006) Vp maps from which we
can estimate the area of the reduced bitumen viscosity with a temperature
increase. As effects of pressure are probably more areally extensive than
effects of temperature, these two effects on the Vp maps need to be segregated.
As a new method, a scaling factor for the Vp reduction was first estimated to
adjust the laboratory-scale and field-scale. We then calculated a distribution
of Vp reduction corresponding to steam chamber conditions in order to decouple
composite effects of temperature and pressure, based on the petrophysical
model. Distinguishing the high-temperature and high pore-pressure zone from the
low-temperature and high pore-pressure zone, we could determine a steam chamber
distribution.
The bitumen volume in the steam chamber zone was estimated and compared with
the actual production. The methodology, interpretation procedures and results
obtained are presented in detail.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
25 March 2009
- Meeting paper published:
17 June 2009
- Revised manuscript received:
26 March 2010
- Manuscript approved:
30 March 2010
- Published online:
24 May 2010
- Version of record:
5 May 2010
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