Distributed Fiber Optic Sensing System for Well‐Based Monitoring Water Injection Tests—A Geomechanical Responses Perspective

In this study, distributed fiber optic sensing (DFOS) based on hybrid Brillouin‐Rayleigh backscattering is examined for the first time in well‐based monitoring distributed profiles of geomechanical deformation induced by water injection. Field water injection tests are conducted under different injection scenarios between an injection well (230 m, IW #2) and 5.5 m away from a fibered monitoring well (300 m, MW #1) by deploying cables behind the casing at Mobara, Japan. Effects of injection rate, pressure, and lithological heterogeneity on the geomechanical deformation are quantitatively monitored via a DFOS approach and indicate that induced strains significantly depend on injection rate and pressure. The results also indicate that DFOS results are reasonably consistent with simultaneous geophysical well logging data and corresponding numerical simulation. The extent of impacted areas and magnitude of near‐wellbore strains are explored to evaluate formation heterogeneity and fluid migration behaviors. The field testing of hybrid DFOS technology is expected to definitely advance elaborate monitoring and wider applications, such as CO2 geosequestration sites. Plain Language Summary Geological CO2 storage (GCS) in deep saline aquifers is broadly recognized as possessing the potential to play a key role in mitigating anthropogenic climate change. Valid monitoring methods are important to characterize the spatial distribution of sequestered CO2 in underground reservoirs. In the study, hybrid Brillouin‐Rayleigh scattering based on high‐resolution distributed fiber optic sensing (DFOS) as an advanced monitoring tool to measure the distribution of temperature or strain is deployed behind casing to a depth of 300 m in an actual field of MW #1 to detect the vertical profile of strain changes at various locations along the entire cable length. Water injection tests are implemented to inject water from adjacent injection well, IW #2 (approximately 5.5 m distance) toward the fibered MW #1. Results indicate that the impacted zones induced by water injection are expanded to the vertical formations near the fibered well, thereby assessing overlying and overlying sealing. Additionally, the induced strain magnitude and state are largely associated with the injection rate, pressure, and formation heterogeneity compare well with the results of well logging and numerical modeling. The DFOS‐based downhole monitoring technology can capture continuous depth‐based geomechanical de