Seven Years of Fiber Optic Monitoring in Highly Deviated Deep Geothermal Wells: Hard-Won Lessons from Deployment to Long-Term Data Integrity

In autumn 2019, fiber-optic cables with both single-mode and multi-mode fibers were permanently installed in an urban multi-well project in Munich, southern Germany, which was under construction at the time. One cable was cemented in place behind the casing to a depth of 700 meters, a practice commonly seen in the oil and gas industry but rare in geothermal energy. Meanwhile, a second cable was permanently suspended in a deviated, 3.7 km long (measured depth) and 2.9 km deep (true vertical depth) production well within the thermal water stream to bottom end. The newly installation was specifically designed and optimized for this purpose. Since then, DTS data, as well as pressure/temperature data from a Fabry-Pérot PT gauge spliced into the cable, have been continuously measured during shut-in, testing, and production phases of the well. DDSS/DAS data were also collected as part of various campaigns, including water injection tests and vertical seismic profiling. In 2023, a third well at the site, an injection well with up to 69° deviation with a more challenging geometry (no production casing in the reservoir “open hole” and large outbreaks), was reequipped with an additional cable including two Fabry-Pérot gauges and integrated into the underground monitoring infrastructure.

This contribution presents key learnings from seven years of continuous fiber-optic monitoring across all operational phases of a geothermal site. We discuss installation concepts and their practical trade-offs, including the challenges of data acquisition and interpretation in a complex urban geothermal setting: distinguishing dynamic strain from thermally induced signals, pump and flow signatures, and other operational states remains a central analytical challenge. The long-term performance of the permanently installed cables is evaluated with respect to fiber integrity, signal degradation, and the risks associated with well interventions as pump changes over a seven-year period. We further assess the potential and current limitations of single-mode DTS in our setting. On the application side, we examine which conventional downhole logging operations can be replaced or validated by the permanent monitoring system, and what benefit continuous monitoring provides for reservoir management and operational decision-making. Finally, we provide an outlook on the planned completion of the site's monitoring infrastructure and the scientific and operational objectives we aim to address with the expanded multi-well fiber-optic network.