Experimental study of temperature change effect on distributed acoustic sensing continuous measurements

Abstract

Distributed fibre-optic sensing is being actively used in various exploration and monitoring applications. Distributed temperature sensing (DTS) is used for measurements and monitoring of the temperature and Distributed acoustic sensing (DAS) for recording of the seismic wavefield. However, DAS measurements are also sensitive to temperature changes. Understanding how exactly DAS measurements are affected by temperature variations is important in order to remove this effect from DAS seismic data. This can be particularly critical for the DAS time-lapse seismic and DAS passive monitoring focused on low-frequency signals. On the other hand, such understanding can help to adapt DAS as a temperature sensor, which can be used to track temperature changes in the absence of DTS.In order to analyze low-frequency DAS data recorded during borehole water injections in Otway site and to estimate temperature change response on DAS measurements, we conducted a series of separate experiments at the Curtin/NGL research facility and Curtin Rock-Physics Laboratory. We examined three various DAS cables. Two fibres were tested in the laboratory and one cable, which is installed behind the casing in the NGL well, was examined in the well. After that, we analysed passive DAS and DTS field data recorded in four different wells during water injections within the Otway project. The results demonstrate that DAS is sensitive to long-period temperature changes and its response is proportional to the time derivative of temperature. Our study shows that there is a linear dependency between strain and slow temperature change and this dependency can be estimated for a particular cable. Proportionality constants in these relationships show some dependency on the cable type/design and acquisition setup, but are all of the same order of magnitude. DAS measurements can also be affected with low-frequency noise possibly associated with the effect of temperature on the DAS acquisition unit itself.

Monitoring Injected CO2 Using Earthquake Waves Measured by Downhole Fibre-Optic Sensors: CO2CRC Otway Stage 3 Case Study

“…Note that CO 2 injection causes not only changes in the reservoir properties but also changes of the borehole environment such as temperature, which can also affect the DAS response [ 31 ]. However, these changes are spread over a large depth range, while changes in the DAS response in our study are limited to the reservoir interval.…”

Fiber‐Optic Observations of Internal Waves and Tides

“…The uncertainty in these parameters is challenging to quantify, since no calibration has been performed in situ, but a factor of two deviation in the strain‐to‐temperature relation is conceivable. Cable construction and burial can only thermally insulate the fiber, so the conversion used here should otherwise represent the minimum value of relative temperature (Sidenko et al., 2022).…”

High resolution seafloor thermometry for internal wave and upwelling monitoring using Distributed Acoustic Sensing

“…However, only in recent years there have been publications demonstrating the diverse geophysical applications of low frequency DAS (LF-DAS) signals that are dominated by temperature [34,35,36,37,38,39]. In particular, Ide et al, 2021 [37] found distinctive patterns with several-hours periodicity on underwater LF-DAS data from a cable offshore Japan.…”

“…The current lack of knowledge about the exact transfer function between the optical fiber response and the input ambient temperature hampers the exact estimation of the latter. Although this transfer function is reportedly linear [37,39], it is generally expected to be a function of the composition and structure of each cable [35,95] and its coupling and thermal insulation by the host medium, from which detailed information is often lacking. This limitation, however, could be overcome through unique, temporary or regular temperature calibrations at single or multiple cable locations with dedicated temperature sensors and/or with auxiliary DTS/DSTS systems [96], depending on the required precision and possible logistics.…”

“…Under these assumptions, a one nanostrain difference ∆ε is approximately equivalent to a temperature increase of ∆T ≈ 0.1 mK. In terms of relative optical phase variations ∆Φ/Φ, the same relationship can be expressed as [38,39,46]:…”