Optically quantifying spatiotemporal responses of water injection-induced strain via downhole distributed fiber optics sensing

Abstract

Harsh subsurface environments limit the robust workability of on-site instrumentation to be leveraged to track the Earth’s dynamics. Distributed fiber-optic sensing (DFOS) enables the long-period in-situ real-time detection of crustal geoenergy exploration-induced underground motions. Herein, we first deployed 300-m-long fiber-optic cables behind the casing of an actual injection well via a single-ended, hybrid Brillouin-Rayleigh backscatterings interrogator to monitor distributed water injection tests between two adjacent wells in onshore Mobara, Japan. Detailed DFOS recordings over the entire borehole visualized clear-cut spatiotemporal strain responses from two water injection tests. Potential injected water-transport footprints and impacted zones reasonably coincided with those of the analogy-based strain fronts. Therefore, our study further revealed that injection volume and injection pressure significantly dominated the water injection-driven strain magnitude and coverage. These findings open up a new avenue for finely optical geoenergy monitoring and fluid evolution imaging that boost the all-optical early-warning potential and quantitative environment risk decision-making for future quasi-geodetic strain-related geohazards.