Monitoring SAGD Steam Injection Using Microseismicity and Tiltmeters

  • Maxwell S
  • Du J
  • Shemeta J
 et al. 
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Abstract

A combination of microseismic and surface-deformation monitor-ing 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 char-acteristics that suggested deformation associated with thermal ex-pansion of the wellbore, in addition to events apparently associated with induced fracturing in the reservoir. Integration of the micro-seismic 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 dur-ing 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 asso-ciated 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 geomecha-nical 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 expan-sion or subsidence (Davis et al. 2000). Monitoring the microseis-mic activity and surface deformation with sensitive seismometers and precise tiltmeters, respectively, could allow the steam injec-tion 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 reser-voir, 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 unwant-ed steam breakouts. Thus, combined monitoring of passive seis-mic 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 activa-tion 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/microearth-quakes 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 com-pared 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 injec-tions (Davis et al. 2000), although the amount of surface deforma-tion 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 re-sponse of the system. A related paper (Du et al. 2007) describes an extended microseismic and deformation integration by comput-ing stress changes and geomechanical-failure conditions from res-ervoir 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.

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Authors

  • Shawn Maxwell

  • Jing Du

  • Julie Shemeta

  • Ulrich Zimmer

  • Neda Boroumand

  • Larry Griffin

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