Towards SEE-MORE - A multi-use borehole for optimisation of Subsurface Energy Exploration and MOnitoring of low-enthalpy geothermal REsources

The use of low-enthalpy geothermal heat is rapidly expanding, especially in densely populated urban areas, to ensure energy security and sovereignty, achieve sustainability goals, and combat climate change. The TU Delft Campus in the Netherlands hosts a 2200-m deep geothermal doublet within a lower Cretaceous clastic, fluviodeltaic reservoir, complemented by heat storage in aquifers between 123 and 284 m depth.  In June 2024, the ICDP-sponsored UrbEnLab workshop brought together 75 scientists from 17 countries to plan a monitoring borehole between the cold-water injector and hot-water producer, highlighting a crucial knowledge gap: how does the subsurface respond to long-term cooled-water injection?

We therefore propose drilling a multi-use monitoring and exploration borehole of at least 3000 m depth to test the hypothesis that new monitoring and modelling techniques can measure, visualise, and forecast the long-term thermal, mechanical, and (bio)geochemical behaviour of an operating geothermal system when key state variables and rock and fluid properties are observed and constrained to the best possibilities. The project will combine monitoring, geological analysis, system optimisation, risk assessment, and societal engagement to advance geothermal science. Its primary goal is to image the cold front in an operational geothermal doublet, while there is the possibility to explore deeper targets.

With the borehole, we will perform time-lapse 3D geophysical monitoring focusing on surface-to-borehole electromagnetic and fibre-optic sensing. Geological and biogeochemical studies will further characterise the heterogeneity of Delft Sandstone and deeper formations up to 3000 m. Continuous seismic monitoring via fibre-optic sensing, a local network and a portable array, and in situ and laboratory microbial analyses will be performed to manage induced seismicity and biological risks, respectively. Integrated societal impact research will assess the perception of risk, uncertainty, and decision-making processes to ensure responsible deployment of urban geothermal infrastructure.

Feasibility tests show that using multiple surface transmitters in a surface‑to‑borehole electromagnetic setup provides sensitivity to 3D temperature variations within the reservoir. This is not the case for conventional surface-based measurements. New long‑term borehole EM sensors, fibre‑optic seismic monitoring approaches, and passive‑noise surface arrays are under development and evaluation. Incorporating geophysical constraints can improve forecasts of production temperature and cold‑plume migration, reducing uncertainty in geothermal reservoir modelling.

The multi-use borehole will supply high-resolution 3D monitoring data to image the geothermal cold front through time-lapse inversions and enhance long-term reservoir predictions of fluid flow, pressure, and temperature distribution. Combining petrophysical logs with geological insights will improve resolution and reduce uncertainty in reservoir forecasts. Consequently, through the proposed monitoring and exploration borehole in the Delft campus geothermal reservoir, it will be possible to assess a geothermal system’s evolution in a heterogeneous setting representative of many low-enthalpy systems worldwide. By integrating in-depth simulation and monitoring of dynamic reservoir processes with detailed characterisation, it will enhance understanding of subsurface behaviour for current and future energy operations and create a unique, open, field-scale research infrastructure to address emerging scientific questions.