11:15 〜 11:30
[HSC04-09] Geomechanical monitoring using Distributed fiber Optic Strain sensing for imaging of ground deformation in three dimensions at CO2 injection sites
キーワード：DSS, geomechanical monitoring, ground surface deformation, subsurface deformation, deformation source
Introduction: CO2 storage is one of the few technological solutions to make net zero emissions possible. Nevertheless, the CO2 injected into deep geological formations can leak or induce a significant pressure buildup that may reactivate the neighboring faults, reach the surface, and cause a ground surface deformation (uplift). Thus far, various subsurface and surface deformation monitoring methods have been used; however, these technologies do not provide a complete solution because of some limitations. In this study, we introduce Distributed optical fiber Strain Sensing (DSS) as an effective subsurface and surface geomechanical monitoring tool to track the dynamic responses at each spatial location along the cable due to the deformation; performing both procedures simultaneously could be ideal to ensure the safety of permanent CO2 storage sites and to overcome other method limitation regarding the continual measurement, easy installation and reducing the monitoring costs, we suggest installing DSS horizontally into the surface around the injection site and vertically in a shallow well to incorporate well-based strain sensing with surface monitoring, allowing ground deformation to be monitored in three dimensions by two pilot tests.
Method: We performed two field pilot tests using DSS, first for ground surface deformation monitoring by installing the flat fiber cable into the surface in a shallow trench; the DSS response was examined under continual deformation caused by a dynamic load produced by gradually filling a water tank placed above the cable covered by cement. While in the second test, the fiber cable was installed behind the casing in a borehole to a depth of 30 m during an observed rainy period of 15 days to detect the vertical strain profile of a groundwater deformation. In both tests, the strain was converted from Rayleigh frequency shifts recorded by a tunable-wavelength coherent optical time-domain reflectometer NBX-8000 interrogator produced by Neubrex Co., Ltd., Kobe, Japan.
3. Results and Discussion: In the First test, we could measure a clear strain profile using DSS, where subsidence appeared under the tank due to the dynamic water filling uploading. And uplift occurred on the two sides. The deformation magnitude increased over time as the water level rose, reaching a maximum when the filling stopped. This result suggests that DSS can accurately detect deformations and anomalies along the cable for long-term spatiotemporal surface deformation monitoring. In the second test, DSS responded to the deformation that occurred due to the rain in a shallow depth; the strain disappeared entirely after 13 days which corresponded perfectly to the groundwater level measurement; this result is significant to identify the deformation when we measure the ground surface deformation to understand the real reason of the ground surface's changes (the deformation source). Both results suggested that deploying both methods simultaneously in three dimensions at CO2 injection sites for DSS measurements is an ideal procedure for geomechanical monitoring for the safe implementation of CCS projects with high monitoring accuracy and minimal costs.
Method: We performed two field pilot tests using DSS, first for ground surface deformation monitoring by installing the flat fiber cable into the surface in a shallow trench; the DSS response was examined under continual deformation caused by a dynamic load produced by gradually filling a water tank placed above the cable covered by cement. While in the second test, the fiber cable was installed behind the casing in a borehole to a depth of 30 m during an observed rainy period of 15 days to detect the vertical strain profile of a groundwater deformation. In both tests, the strain was converted from Rayleigh frequency shifts recorded by a tunable-wavelength coherent optical time-domain reflectometer NBX-8000 interrogator produced by Neubrex Co., Ltd., Kobe, Japan.
3. Results and Discussion: In the First test, we could measure a clear strain profile using DSS, where subsidence appeared under the tank due to the dynamic water filling uploading. And uplift occurred on the two sides. The deformation magnitude increased over time as the water level rose, reaching a maximum when the filling stopped. This result suggests that DSS can accurately detect deformations and anomalies along the cable for long-term spatiotemporal surface deformation monitoring. In the second test, DSS responded to the deformation that occurred due to the rain in a shallow depth; the strain disappeared entirely after 13 days which corresponded perfectly to the groundwater level measurement; this result is significant to identify the deformation when we measure the ground surface deformation to understand the real reason of the ground surface's changes (the deformation source). Both results suggested that deploying both methods simultaneously in three dimensions at CO2 injection sites for DSS measurements is an ideal procedure for geomechanical monitoring for the safe implementation of CCS projects with high monitoring accuracy and minimal costs.