Fiber-optic sensing technologies for strain and temperature monitoring in shallow geothermal systems

An in-depth knowledge of local geology and temperature gradient around wellbores is essential in order to characterize and understand geothermal systems. Fiber-optic sensing allows for the measurement of temperature and strain with high spatial and temporal resolution. For downhole applications, deployment is relatively easy and does not interfere with production, making fiber-optic based technologies attractive for the monitoring of geothermal operations. We aim to demonstrate the potential of distributed strain sensing (DSS) and fiber Bragg grating sensors (FBGs) to monitor geothermal energy production in space and time using passive and noise-based seismic methods. Here we present preliminary results from DSS and quasi-distributed FBG measurements in a future shallow low-enthalpy urban heating system in Brussels, Belgium. Fourteen 120m deep geothermal probes were equipped with fiber optic cables, providing continuous downhole measurements of strain and precise temperature point measurements from specific target depths. Measurements obtained directly after wellbore completion are used to establish a baseline of the geothermal gradient and its variability in an urban context before heat production. These results are compared with data from temperature sensing FBG arrays deployed in a small scale heating system during operations. By calculating the root mean square amplitude of seismic noise on all channels of the DSS cables a purely noise-based borehole log is derived which is in agreement with known (hydro-)geological logs. Thus our findings demonstrate the promise of employing fiber-optic technologies in the monitoring of geothermal operations.