Subsurface characterization using downhole passive Distributed Acoustic Sensing data in the Groningen gas field

This study presents passive downhole Distributed Acoustic Sensing (DAS) measurements conducted in the Groningen gas field, Netherlands, for subsurface and induced seismicity monitoring. The optical fiber installation, completed in Sept 2015, was partially cemented behind the inner casing along a deviated well (~3800 m), extending into the sandstone reservoir at temperatures ranging from 100-110 degrees Celsius. In Nov 2022, we interrogated the optical fiber utilizing a 10 m gauge length and 1 m sampling spacing. Within this setup, most DAS traces exhibit the lowest self-noise floor in the frequency range of 0.1 to 30 Hz. Noteworthy is the absence of visible differences between cemented and uncemented sections. Strong ambient seismic noise is observed in the near-surface unconsolidated sediment at approximately 800 m depth. Noise cross-correlation (CC) analysis is performed for DAS channels and DAS seismometer pairs. Surface wave signals in the 0.1 to 1 Hz range are identified in DAS-seismometer CCs, displaying amplitude and polarization changes with depth, following Surface wave theory. Induced seismicity is also recorded, with wavefields of events exhibiting clear amplitude variations along the fiber, strongly correlating with sonic logging. Our findings suggest that downhole DAS has the potential to characterize the subsurface with high resolution.