Beyond Conventional Monitoring: Permanent Fibre Optic Sensing in a Geothermal Reservoir During Production and Injection

In deep geothermal wells, conventional downhole data is commonly very limited, especially during long-term plant operation. Since 2021, permanently installed fibre optic cables in one production and one injection well at a hydrogeothermal site in Munich have enabled continuous DTS (Distributed Temperature Sensing) measurements along the entire borehole length (~4 km) as well as pressure monitoring at reservoir depth at 3000 m MD (measured depth) using Fabry–Pérot gauges. In addition, several DDSS (Distributed Dynamic Strain Sensing)  campaigns were conducted during different operational stages of the geothermal plant. In this contribution, we present borehole and reservoir processes derived from this unique long-term fibre optic dataset and discuss their implications for reservoir characterization and geothermal field development.

Different approaches were applied to resolve production and injection zones within the reservoir at high spatial resolution. These include energy and mass balance modelling of temperature profiles during production, thermal slug tracking to derive fluid velocities and flow contributions, and analysis of borehole warmback during shut-in periods. DTS-derived results provide rapid and robust characterization of flow zones and extend beyond the spatial and temporal limitations of conventional flowmeter logging. In addition, low-frequency DDSS (LF-DDSS) measurements reveal highly detailed flow dynamics and previously unresolved flow processes within the wells and reservoir. In the injection well, 78 % of the injection happens in the upper 120 m MD of the 1000 m long reservoir section. In the lower half, free convection cells dominate in the wellbore during steady injection and a 40 m thick localized hydraulic anomaly even shows 1 l/s of inflow into the wellbore from the formation.

Furthermore, the temporal evolution of the production/injection temperature and flow zones indicates dynamic changes in reservoir properties during plant operation. Pressure data from fibre optic gauges enables repeated pressure transient analysis (PTA) of shut-in phases. These show that long-term operation has significantly increased the transmissivity of one of the wells and suggest an altered flow regime. The measurements further provide an important basis for calibration and validation of 3D thermo-hydraulic numerical models of the entire six-well system at the Munich plant.

Overall, the presented results show how the applied methodical approaches can improve the geological and reservoir understanding of deep geothermal systems in the Bavarian Molasse Basin and will support future reservoir engineering, field development, and forecasting of long-term well performance.