In the framework of MEET project, in order to monitor the Soultz-sous-Forêts geothermal plant, FEBUS Optics, in coordination with ES Géothermie, deployed, in July 2020, a fiber optic cable in the observation well EPS-1. It enabled the monitoring of various physical parameters, such as temperature, strain and induced seismicity, using different Distributed Fiber Optic Sensing technologies (Distributed Strain and Temperature Sensing, DSTS, and Distributed Acoustic Sensing, DAS). This short article focuses on the study of the results of a DSTS acquisition campaign.
Seven months after the fiber optic cable deployment in EPS-1, an acquisition campaign has been led, in parallel with the restart of the Soultz-sous-Forêts geothermal plant after a phase of maintenance. When achieving this survey, Febus Optics emphasized, with the use of an Optical Time Domain Reflectometer, the presence of a progressive optical loss reaching 5 dB from a depth of 800 m to the bottom of EPS-1 well. This attenuation could be related to a water ingress along the wireline cable compressing the fiber proportionally to its depth. Thus, this could conduct, with the use of DSTS, to an estimation of the pressure gradient in EPS-1.

Figure 1: Temperature (blue) and non-thermal contributions (orange) profile. Non-thermal contributions are compared to the theoretical pressure gradient of the area (green).
Results of DSTS acquisition are presented in Figure 1. The non-thermal contribution curve (in orange) is compared to a theoretical pressure gradient (green curve) estimated from the density of the water in EPS-1 well:

With ρ the density, T the temperature, g the gravity acceleration, z the depth from the water level and P the pressure.
We observe that theory and survey results are in good correlation involving a good ability, in these specific conditions, for the DSTS to measure pressure. Additionally, it is highly probable that the initial slope (from 200 to 1,000 m) which appears higher than the one anticipated with geothermal brine (in green) reflects a higher density from a brine plug used to kill the well, whereas the bottom of the well (1,800 – 2,200 m) reflects a density closer to the geothermal brine. This non-thermal contribution of DSTS signal is thus a good indicator of pressure all along the well.
Finally, this DSTS acquisition also contains signal coming from static stretches along the fiber cable due to its position, with tensions or torsions, in the well. Indeed, we observe a strain event along the orange curve of Figure 1, at around 1,850 m depth which has been a point of vigilance when achieving the cable recovery the 15th of October 2021. No break has been observed, and DSTS acquisition proved the loss of strain at this location after recovery. This event was then most likely due to a tension in the cable at this location due to its installation in the well.
During this study, Febus Optics then demonstrated the possibility of using the DSTS solution for pressure monitoring. This led to the development of an innovative tool enabling both distributed temperature and pressure acquisitions in geothermal contexts.