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[HSC05-08] Advantages of Helically Wound optic-fiber Cables on VSP records
Keywords:Geological CO2 storage, Optic Fiber, DAS, HWC, VSP
Geological CO2 storage is a promising technology to mitigate the global warming. The injected carbon dioxide underground needs to be monitored properly during/after injection periods. Some CO2 geosequestration projects conducted the repeated 3D seismic surveys to visualize the plume of injected CO2. However, due to the high cost, number of surveys had to be limited. The fiber-optic distributed acoustic sensing (DAS) for vertical seismic profiles (VSP) has quite high potential for more frequent surveys with low cost. The DAS is now widely used in exploration community as a novel technology. The small dimension of the fiber cables enables installation behind the casing, which increases availability of space of injection wells. Some indicate that the DAS records have less sensitivity than the conventional geophones, which requires more source efforts to achieve high signal-to-noise ratio (SNR). Another disadvantage of DAS is no sensitivity for waves incident laterally. The Helically Wound fiber-optic Cables (HWC) are recently developed in order to capture these waves.
To evaluate performance of a HWC, we installed the straight optic-fiber cable (STC) and the HWC in the vertical well of 250 m and cemented together along the tubing. Another vertical well of 880 m deep with metal casing had the same STC cemented behind the casing. All fibers were spliced together in one stroke and were connected to DAS interogator at the site. Also the downhole 4 of 3C geophones were put down in the 880 m well from 100 m to 700 m with interval of 7.5 m, which needed 21 levels of measurement. The P-vibroseis truck swept at the traffic road along the survey line of 2 km beside the site. The excited elastic waves from the offset of 90 m to 1350 m were recorded by optic fibers and geophones simultaneously.
From our field measurements, we revealed that (1) DAS records were noisier than geophone records, but 5-20 times more sweeps were enough to obtain geophone equivalent records; (2) The HWC records showed about twice higher SNR than the STC records in some offset of sweeps; and (3) the amplitude decay in HWC records were moderated significantly.
To evaluate performance of a HWC, we installed the straight optic-fiber cable (STC) and the HWC in the vertical well of 250 m and cemented together along the tubing. Another vertical well of 880 m deep with metal casing had the same STC cemented behind the casing. All fibers were spliced together in one stroke and were connected to DAS interogator at the site. Also the downhole 4 of 3C geophones were put down in the 880 m well from 100 m to 700 m with interval of 7.5 m, which needed 21 levels of measurement. The P-vibroseis truck swept at the traffic road along the survey line of 2 km beside the site. The excited elastic waves from the offset of 90 m to 1350 m were recorded by optic fibers and geophones simultaneously.
From our field measurements, we revealed that (1) DAS records were noisier than geophone records, but 5-20 times more sweeps were enough to obtain geophone equivalent records; (2) The HWC records showed about twice higher SNR than the STC records in some offset of sweeps; and (3) the amplitude decay in HWC records were moderated significantly.