PSI - Issue 64

Michael Iten et al. / Procedia Structural Integrity 64 (2024) 1642–1648 Iten et al. / Structural Integrity Procedia 00 (2019) 000 – 000

1644

3

1.2. Down-borehole temperature sensing technologies For the purpose of downhole temperature monitoring, a variety of different sensor types are available: fixed single point or multi-point sensors at predefined locations as well as floating/sinking dynamic location temperature sensors (e.g. consult Aranzabal et al. 2019). When it comes to permanent monitoring of deeper and more complex geothermal systems, distributed fiber-optic temperature sensing (DTS) systems have several advantages: installed along the casing, DTS provides temporally continuous and spatially distributed measurement data in near real-time that captures borehole temperature dynamics. The DTS system used in this project is based on the principles of Raman scattering that occurs inside the fiber core and enables to calculate the absolute temperature from the intensity ratio of the Anti Stokes and Stokes band of the back-scattered light (Ukil et al. 2012, Shiota & Wada 1991). This effect was first used in 1985 for temperature distribution assessment along a silica-based optical fiber (Dakin et al. 1985). Today’s commercially available Raman DTS interrogators allow for distributed measurements up to several tens of km, a spatial resolution of about 1 m and temperature resolution down to 0.1°C. The measurement time can range from a few seconds to a few minutes. Raman DTS systems have been applied in various projects over the last years, especially for fire detection in tunnels (e.g. Karamehmedovic & Glombitza 2004) and for downhole monitoring in oil & gas (e.g. Williams et al. 2000). Among many further applications, Raman systems also have been used for borehole heat exchangers (BHE) similar to the demonstrated project in this paper, however on a smaller scale (e.g. Giuseffi et al. 2010). One inclined and two directional boreholes were drilled down to a maximum of 500 meters true vertical depth (mTVD). In all three boreholes, the 2" annulus of both, the surface casing (diameter of 16") and the production casing (10 ¾"), were instrumented with an armored multi-fiber sensor cable that allows combined DTS among two other fiber optic sensing technologies: distributed acoustic sensing (DAS) and distributed strain sensing (DSS). Only DTS data will be shown and discussed in this paper. To account for the specific requirements of the project with temperatures up to 120 °C, the original cable design was adapted by the manufacturer accordingly. The cable was installed in a loop configuration for redundancy and to allow for temperature calibration using a temperature matching approach. To cope with the single valve available at the wellhead of the pressurized system, the fibers installed along the production casings were looped using a “Mini - Loop” solution at the borehole base. The “Mini - Loop” solution allows looping a multi-fiber cable at the borehole base where there is limited space available. Therefore, the fibers are looped in a narrow U-turn and spliced in a hermetically sealed housing with an outer diameter of approx. 2 cm only. By using specially designed centralizers, the sensor was held in place to avoid destruction during the perforation of the production casing. Unfortunately, the sensing cable was ruptured several times during the heavy installation of the casings: for one of the boreholes (GES-F2) it is assumed that the curvature of the directionally drilled production casing caused the rupture of the fiber-optics (see Fig. 2), however data is available for almost the entire borehole length except the deepest 23m; at GES-F-3 the cable was cut at the casing shoe of the outer casing, and thus, this borehole is not further presented in this short report . For the first borehole GES-F-1 the cable was installed successfully, however due to geological instabilities this well was only drilled to 228m TVD. 2.2. Distributed temperature measurements The technical specifications of the Raman device used for the monitoring in this project are as follows: spatial resolution 1.0 m, distance resolution 0.25 m, optical budget of 8 dB (4 km distance range), average time for a reading of the sensors 60 s. The sensors of fiber type MM 50/125 μm were measured in a double -ended configuration, a calibration bath was not applied since the accuracy of the absolute temperatures was not the main objective. Therefore, relative temperatures are sufficient. 2. Instrumentation and measurements 2.1. Technical details of instrumentation