Figure 2.8 Calculated vertical displacement based upon the distribution of aperture change plotted in Figure 2.7.
A key objective of the Aquistore project is to integrate numerous data sets in order to determine the subsurface fluid distribution, pressure changes, and ground deformation associated with the injection. For these purposes, a variety of surface‐based, continuous‐recording instruments were installed at the site including Global Navigation Satellite System (GNSS) receivers, tiltmeters, seismographs, and piezometers. Space‐borne Differential Interferometric Synthetic Aperture Radar (DInSAR) is used to measure the ground deformation over a large area surrounding the site. Ascending and descending RADARSAT‐2 Spotlight and Wide Ultra‐Fine SAR data are gathered in order to achieve a high temporal and spatial sampling of the deformation field. The footprints of the SAR images are shown in Figure 2.9. The Spotlight data consist of ascending Spotlight 18 and descending Spotlight 12 images (Fig. 2.9), acquired with the incidence angles of 44° and 40° and the range‐azimuth spatial resolution of 1.6x0.8 m. The Wide Ultra‐Fine data consist of both ascending and descending Wide 2 Ultra‐Fine images, acquired with an incidence angle of 37° and the range‐azimuth spatial resolution of 1.6x2.8 m. Combining the four independent data sets decreases the time between surveys from 24 days to 6 days. A LIDAR digital elevation map with 1 m resolution was used to remove the phase changes due to topography.
Figure 2.9 (a) Location of the Aquistore storage site in Saskatchewan, Canada. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) 30 m resolution Digital Elevation Model (DEM). The RADARSAT‐2 InSAR data frames are outlined in black. The region of interest is outlined in brown. Extent in top‐right corner shows location of study region in North America. (b) LIDAR 1 m resolution DEM plotted over ASTER DEM. The reference region “R” is assumed to be stable. The monitoring sites NE1, NE2, SE1, SE2, SE3, SITE, SW1, NW1, NW2 are plotted in black.
The SAR data are converted to individual interferograms, geocoded, and resampled to a common grid and then processed with the advanced Multidimensional Small Baseline (MSBAS) processor (Samsonov & d'Oreye, 2012). The MSBAS technique estimates two‐dimensional (vertical and east‐west) deformation time series, relative to a stable reference point, labeled R in Figures 2.9 and 2.10, by combing SAR data from various sensors and acquisition geometries. To further improve the precision of deformation measurements, especially during the winter months when the ground is covered by snow, a network of corner reflectors was installed with reflectors at nine monitoring sites marked in Figures 2.9 and 2.10. The corner reflectors were designed at the Canada Centre for Mapping and Earth Observation, Natural Resources Canada. They consist of two trihedral reflectors positioned for ascending and descending imaging geometry (Fig. 2.10) and mounted on a single monument. In order to more accurately capture ground deformation from a deep source (i.e., CO2 injection) and to reduce signals from the near surface processes caused by seasonal changes and postmining soil settlement, the corner reflector monuments were anchored at a depth of 20 m.
RADARSAT‐2 SAR data have been collected since June 2012, well before the start of injection in April 2015. The roughly three years of preinjection observations allowed for the determination of background surface deformation due to a variety of sources. The vertical and east‐west horizontal deformation maps are shown in Figure 2.11. During the 2012 – 2014 period, active ground deformation processes were observed across the region. Subsidence, exceeding 0.8 cm/year, was predominantly observed in areas undergoing mining reclamation and in regions where construction activities have influenced the surface. Uplift was observed at a number of man‐made sites, but other locations are affected by groundwater recharge and swelling clays. Horizontal motions of up to 0.4 cm/year were observed, mainly along the steep slopes of the diversion canal. Analysis of the preinjection deformation reveals that some sites are affected by near‐surface deformation processes, in particular sites located south of the injection well (SW1, SE2, SE3), emphasizing the importance of adequate preinjection monitoring. In Figure 2.12, we plot the displacement components associated with point P2 in Figure 2.11, a point close to the injection site that experienced some fast ground deformation.
Figure 2.10 (a) RADARSAT‐2 Spotlight 12 SAR intensity image acquired on 23 August 2014. Monitoring sites and stable reference region “R” are plotted in black. Points P1–P9 experiencing fast ground deformation are plotted in red
(RADARSAT‐2 Data and Products © MDA Geospatial Services Inc., 2014. All Rights Reserved. RADARSAT is an official mark of the Canadian Space Agency).
(b) Photograph of corner reflector designed at the Canada Centre for Mapping and Earth Observation, Natural Resources Canada by K. Murnaghan for ascending and descending imaging.
Figure 2.11 Observed (a) vertical and (b) east‐west horizontal deformation rates computed by applying Multidimensional Small Baseline (MSBAS) technique to RADARSAT‐2 data spanning a time interval, 15 June 2012 to 23 September 2014, prior to the start of injection. Monitoring sites and reference region “R” are plotted in black. Points P1–P9 experiencing fast ground deformation are plotted in red. SAR intensity image is exposed in background in regions of low coherence.
Elastic and poroelastic modeling were performed to simulate the ground deformation that would be caused by CO2 injection rates of 1,500 tonnes/day (i.e., the maximum expected rate) at a depth of 3,150 – 3,350 m (Samsonov et al., 2015). Elastic modeling with a point source model (Mogi, 1958) suggests a constant annual uplift rate of about 1.6 cm/year and east‐west motion of up to 0.6 cm/year. However, poroelastic modeling suggests that uplift will reach its maximum value of about 1.6 cm near the injection site almost immediately after the start of injection but its spatial extent will continue to increase during the entire modeled injection period of 25 yr. Horizontal motion calculated by the poroelastic model does not exceed 0.1 cm but forms a very broad pattern, extending 40 km from the injection well. The poroelastic model better represents the constitutive properties of the fluid‐filled porous rocks and the nature of the injection process, and is considered to be more realistic than the elastic model. Comparison of the poroelastic model predictions of ground deformation with the measurement precision (95% confidence interval) of the deformation maps, which does not exceed 0.3 and 0.2 cm/year, suggests that only the vertical motion will be detectable.
