Introduction to June Focus: DAS/Borehole Geophysics

Welcome to this special edition of the RECORDER with a focus on Distributed Acoustic Sensing (DAS) and borehole geophysics. There are four papers on DAS, in particular with application to Vertical Seismic Profiles (VSPs) and one on DAS for microseismic. Since DAS was first used as a downhole optic sensing technology in 2009, developments in the acquisition equipment, cables and processing techniques have enabled demonstration of it as a stable and applicable technology. Although there are many applications of DAS, the use of it to record VSPs is particularly attractive since the entire well-bore can be recorded in one pass with a finer channel sampling than that normally afforded by geophones. This allows much easier acquisition as geophones will typically need to be moved, requiring longer rig downtime and the sources to be repeated for each move. In addition the fiber can easily be re-deployed or re-used in order to study time lapse effects.

In the first paper (The use of fiber-optic sensing to efficiently acquire vertical seismic profiles) Dean et al., discuss how the use of fiber-optic systems can dramatically improve the efficiency of VSP recordings. Recording systems and data are shown and an example is described where the recording time of a simple VSP is reduced from hours to minutes, whilst maintaining consistency with results recorded using conventional VSP tools.

The second article (Comparing DAS and Geophone Zero-Offset VSP Data Sets Side-By-Side) by Willis et al., compares and discusses VSPs recorded with retrievable (wireline) geophones and DAS on cemented- in fiber-optic cable. A comparison of zero offset DAS and geophones shows excellent agreement and quality, and a comparison of corridor stacks allows a more in-depth understanding of their similarities and differences. The DAS data has a smaller response to tube waves, allowing an image to be created with less filtering. The resulting corridor stacks for both methods are very comparable.

In the third article (Simultaneous Acquisition of Distributed Acoustic Sensing VSP with Multi-mode and Single-mode Fiber-optic Cables and 3C Geophones at the Aquistore CO₂ Storage Site), Miller et al. compare signal recorded on Single-mode and Multi-mode fiber to that recorded on a 60 level 3-component wireline geophone array, using both vibroseis and dynamite as sources. Traditionally we have come to associate the acquisition of DAS data with single-mode fibers. This is because it utilises Rayleigh backscattering for which a single-mode fiber is sufficient. However multi-mode fibers can also carry Rayleigh backscattering and Miller demonstrates that all three recordings (SM, MM and geophones) produce comparable SNR’s and sensitivity. Furthermore the signal is of good quality and repeatable, making DAS 3D VSP a viable candidate for time-lapse monitoring.

In the fourth article (Field trials of distributed acoustic sensing for reservoir delineation with VSP) Zhan et al., describe the use of pre-existing fiber-optic cables deployed for downhole gauges (pressure, temperature) in wells that are used to record DAS data for monitoring reservoirs. Three case studies are shown which collectively show that the imaging is comparable to that of surface or seabed acquisition. Future issues to be addressed by the DAS community include the optical noise floor of DAS, directional sensitivity and depth calibration issues.

Lastly, the fifth article (DAS Microseismic) Webster et al., shows a different application of DAS with regards to borehole geophysics; that of microseismic. It is shown that microseismic can be detected and located using DAS and gives accuracies superior to those of geophones. Although fewer events are detected with the DAS system, the sensitivities of the recorders are increasing and are expected to further increase in the near future, so that DAS may become a truly viable microseismic technology.