Active Distributed Temperature Sensing to determine flow velocities in boreholes based on a cylinder in cross-flow approach

Fiber optic measurement techniques such as Raman Distributed Temperature Sensing (DTS) are beneficial for applications in boreholes, as they provide continuous measurements over long distances with all measurement equipment outside of the borehole. Efforts have been made for at least a decade to utilize these temperature measurements for indirect velocity measurements. Active DTS measurements refer to a setup, where the fiber is embedded in a hybrid cable, which itself is heated by Joule heating. Such Active DTS processing takes advatange of known heat transfer phenomena to determine velocitiy information.

When the heated cable is placed in a saturated porous medium, groundwater fluxes perpendicular to the cable's axis were quantified with low uncertainties in a controlled lab experiment [1]. Heated fiber placed in the free flow of a borehole was applied to identify active zones of groundwater flow and highly fractured zones [2]. A very similar setup of a heated fiber in a borehole was applied to measure vertical flow exploiting the heat transfer law of a cylinder in flow parallel to its axis  [3]. Utilization of this heat-transfer law was shown to be difficult as a thermal boundary layer builds up in the flow direction, thus influencing the downstream sections. This effect may be modelled, but additionally, the boundary layer mixes behind every centralizer and therefore enhances the heat transfer locally. The latter cannot easily be modeled and was removed using a postprocessing filter [3]. Even though only limited quantitative comparability with reference flowmeter measurements was possible the results rendered the approach a promising strategy, since the correct order of magnitude and moreover similar trends have been identified.

Inspired by the heat transfer of a cylinder in cross flow as state of the art in aerodynamics velocimetry [5], it has been demonstrated that the convective heat transfer of a heated cable in free flow can be better utilized if the cable axis is positioned perpendicular to the flow to take advantage of the heat transfer law of a cylinder in cross flow [4].

The objective of our research is to build an active DTS-based free stream flowmeter to monitor pump flows in arbitrarily deep boreholes. The system shall be scalable with an arbitrary number of point measurement flowmeters, which are connected to a single glass fiber and one heating cable. At the current state of the research, the flowmeter consists of a point flowmeter, which is a helically wound, heated glass fiber. The sensitivity in the predicted measurement range was verified and the temperature distribution along the cable cross-section was investigated. Presently, the major challenge is a precise reproducible DTS temperature measurement. Therefore, water baths and a new prototype were built to achieve measurement uncertainties within the range of the water bath reference sensors (cp. [7]).

[1] Simon et al. https://doi.org/10.1016/j.jhydrol.2023.129755 

[2] Banks et al. https://doi.org/10.1111/gwat.12157

[3] Read et al. https://doi.org/10.1002/2014WR015273

[4] Rautenberg et al. https://doi.org/10.1007/s00348-023-03741-5

[5] Örlü, Vinuesa, Chapter 9 Thermal Anemometry https://doi.org/10.1201/9781315371733-12  

[6] Giesen et al. https://doi.org/10.3390/s120505471