Summary
A combination of microseismic and surface-deformation monitoring with an
array of tiltmeters was used to monitor the warm-up phase of a
steam-assisted-gravity-drainage (SAGD) well pair. A sequence of microseismic
events was recorded with signal characteristics that suggested deformation
associated with thermal expansion of the wellbore, in addition to events
apparently associated with induced fracturing in the reservoir. Integration of
the microseismic data with volumetric strain, inverted from the measured
surface deformation, indicates a discrete deforming region near the toe of the
well. The volumetric strain also shows another region near the heel of the
well, although the area is too far from the microseismic observation well for
any associated microseismicity to be recorded. The central portion of the well
pair did not have significant deformation, indicating poor steam conformance
during this warm-up phase. A comparison of the temporal response of the
microseismic deformation with the surface uplift suggests a lag between periods
of accelerated seismic deformation followed by an associated period of
accelerated uplift a few days later. This timing suggests the creation of a
fracture network and related seismic deformation, which then fills with steam
and starts to expand over a period of a few days. In a related paper (Du et al.
2007), stress changes associated with the volumetric strain are used to examine
potential geomechanical failure zones that match the observed locations of
microseisms. Together, the volumetric strain, computed stress changes, and
failure zones could be used to calibrate a geomechanically linked reservoir
simulator.
Introduction
Steam injection for reservoir stimulation is an important factor for the
economic development of heavy-oil reservoirs. Monitoring steam-chamber growth
is critical to optimize heavy-oil recovery, confine the stimulation to the
reservoir, and identify bypassed regions. Steam injection results in
geomechanical strains associated with increased pore pressure, thermal-stress
changes, and dramatic changes in material properties associated with heating
the reservoir sufficiently to mobilize the heavy oil/bitumen (Collins 2005;
McLellan 2006; Dusseault 2007). This geomechanical deformation may be expressed
through seismic deformation and the release of seismic energy as fractures
adjust to the strain field (Maxwell et al. 2003), and also may result in
surface expansion or subsidence (Davis et al. 2000). Monitoring the
microseismic activity and surface deformation with sensitive seismometers and
precise tiltmeters, respectively, could allow the steam injection to be tracked
with complementary technologies that respond to different expressions of
geomechanical deformation associated directly with steam injection. In some
fields, this geomechanical deformation also leads to casing deformations and
well-integrity problems, which may result in operational problems (Davis et al.
2000; Smith et al. 2006). The combined monitoring of passive seismic and
surface deformation provides insight into these mechanisms leading to casing
deformations and also potentially identifies circumstances that may lead to
casing failures. The combined monitoring also can track fluid movements in the
reservoir, allowing optimum well and pattern design and subsequent operational
improvements including as optimization of steam volumes, rates, and cycle
timing. Finally, the passive seismic and surface deformation monitoring can
also be used to track unwanted steam breakouts. Thus, combined monitoring of
passive seismic and surface deformations offers critical information for
several reservoir engineering and management issues during steam injection.
Many steam injections are at relatively low injection pressure, which may be
below the "frac" pressure required to create tensile hydraulic
fractures (Collins 2002). Nevertheless, fracture activation may still occur as
increased pore pressures induce shear movement along pre-existing fractures.
This potential mechanism for seismic deformation is further enhanced by thermal
stress changes and material property changes moving the rock mass closer to
shear failure. There are, therefore, a number of factors that lead to the
potential occurrence of microseisms/microearthquakes both for relatively high
pressure cyclic steam stimulation (CSS) or huff'n'puff injection and lower
pressure injections such as SAGD.
Previous studies have reported microseismic activity (Maxwell et al. 2003;
Smith et al. 2006; McGillivray 2004) and surface deformations for cyclic steam
injections (Davis et al. 2000). SAGD typically uses lower injection pressures
and rates compared to CSS (Collins 2002), and results in less seismic and
surface deformation. Furthermore, many of the heavy-oil reserves in western
Canada, where SAGD injections are commonplace, are relatively poorly
consolidated sands that are likely to be relatively weak and could reduce
seismic deformation further. Similarly, surface deformations have been
documented for CSS steam injections (Davis et al. 2000), although the amount of
surface deformation depends on the reservoir strain and depth. Surface
deformation can be monitored with various techniques, including Interferometric
Synthetic Aperture Radar (InSAR) and Global Positioning Systems (GPS)
monuments, although tiltmeters offer the highest precision for monitoring small
deformation changes. With SAGD applications, the slow injections of relatively
small steam volumes indicate the use of the most sensitive surface-deformation
measurements.
In this paper, we present a case study that demonstrates the monitoring of a
steam injection using both passive microseismic and surface-tiltmeter
deformation. We first describe the site and then the results of monitoring a
"warm-up" phase of a SAGD well pair. The microseismic and tiltmeter
results are then combined to provide an integrated interpretation of the
geomechanical response of the system. A related paper (Du et al. 2007)
describes an extended microseismic and deformation integration by computing
stress changes and geomechanical-failure conditions from reservoir volumetric
strains inverted from surface deformations (Du et al. 2008), and then comparing
the hypocentral locations of the microseisms with the predicted
geomechanical-failure zones.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
2 August 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
12 September 2008
- Manuscript approved:
22 September 2008
- Published online:
15 April 2009
- Version of record:
15 April 2009
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