The urgency of the question comes from the sheer crowding of the urban underground. Low-enthalpy geothermal energy, defined as geothermal resources below about 150 degrees Celsius, is becoming increasingly common as cities seek low-cost renewable heat. Sedimentary aquifers suitable for direct heat production can be found worldwide, and demand is highest precisely where space is scarcest: in densely populated areas. At shallow depths, aquifers are used to store heat, sometimes interfering with tunnels and subways; at greater depths, geothermal wells are being drilled into the same rock formations that oil and gas companies have exploited for decades. Each new use competes with the others, and the resulting interference is poorly understood. The Delft campus, sitting atop the West Netherlands Basin with its stack of water-bearing sandstone layers, offers a compressed version of this global problem, which is exactly why researchers chose it as their natural laboratory.

The campus is already one of the most intensively used pieces of subsurface in Europe. In late 2023, the developer Geothermie Delft completed a geothermal doublet, a pair of production and injection wells reaching 2.2 kilometers down into the Delft Sandstone, scheduled to supply heat to the campus and the city of Delft. Two other deep geothermal doublets targeting the same aquifer already operate in the nearby suburb of Pijnacker. Shallow aquifers beneath the campus hold low-temperature thermal energy storage systems operating below 25 degrees Celsius, and a high-temperature aquifer thermal energy storage system, or HT-ATES, with five wells is under development, with a pilot well drilled in 2024. A local seismic monitoring network was installed in 2022, complemented by an ultra-sensitive portable seismic array. Within a five-kilometer radius of the campus, thirty-six wells or sidetracks have been drilled to depths of up to about 2.5 kilometers, most of them documented in the publicly accessible Netherlands Oil and Gas Portal. All of this existing infrastructure and data makes Delft an ideal calibration site.

The planned deep borehole, expected to be drilled by 2028 with initial funding from the Dutch Science Foundation through the EPOS-eNLarge project, is designed to do what surface instruments cannot. The fundamental problem with subsurface processes is that they can only be observed indirectly from the surface, and cities are geophysically noisy environments that make signal detection even harder. Some changes are simply invisible from above: the researchers note that shifts in electrical resistivity caused by cold-water injection into a deep geothermal reservoir cannot be detected from the surface with current technology. The vertical borehole will pierce the geothermal reservoir between the production well DEL-GT-01 and the injection well DEL-GT-02, positioned several hundred meters away from the surface location of the doublet because of the wells’ deviation. From this vantage point, scientists intend to track the propagation of the geothermal cold front, the zone of cooled rock that grows around the injection well as spent water is pumped back down, and to monitor reservoir pressures in three dimensions with unprecedented precision.

The geological setting rewards this ambition with an unusually complete stratigraphic record. Every formation from the surface down to the Carboniferous Limburg Group is a potential investigation target. The shallow section, between roughly 100 and 300 meters, holds the aquifers and aquitards relevant to high-temperature heat storage. At about 2.2 kilometers lies the Nieuwerkerk Formation and its Delft Sandstone reservoir, the productive heart of the campus geothermal system, from which 86.5 meters of angled core were already retrieved during drilling of the doublet. Below that, the Jurassic Altena Group with its prominent Posidonian shale spans roughly 2,800 to 3,400 meters, followed by the Triassic Germanic Trias Group, a major potential geothermal play despite diagenetic cementation that has reduced its porosity, as demonstrated at the nearby Naaldwijk well that reaches 4,013 meters. The Permian Zechstein and Rotliegend deposits, and finally the coal-bearing Carboniferous Limburg Group expected between about 4,000 and 5,200 meters, complete the column. The Rotliegend in particular is exploited in the north of the Netherlands both for energy and for critical raw materials such as lithium.

Beyond energy, the borehole doubles as a climate archive. The workshop participants identified several key intervals in Earth’s environmental history that the drill will penetrate. The early Quaternary records the onset of extensive Northern Hemisphere glaciation, a critical comparison point for current warming. The Middle to Late Miocene transition captures the initiation of North Atlantic Deep Water formation. Most dramatically, the Paleocene-Eocene Thermal Maximum, a globally recognized greenhouse warming episode, is expected to appear as a 10-to-20-meter-thick interval based on nearby well data, offering a natural experiment in the rates and consequences of rapid climate change. The Triassic-Jurassic boundary, preserved in anoxic marine sediments, adds another window into a globally significant perturbation. Deeper still, the borehole will address paleotectonic questions, including the prominent unconformity between Carboniferous and Permian formations linked to the Variscan orogeny, and lateral equivalents of the metal-rich Kupferschiefer horizon that may hold clues to critical raw material distribution.

The workshop’s scientific agenda crystallized around a single broad aim: assessing the life cycle of a geothermal system situated in a complex, heterogeneous sedimentary basin, with revealing the detailed flow field as the top priority. Breakout groups tackled hazards, monitoring, system optimization, geology, societal impact, and drilling operations, then matched these questions to techniques. On the hazards side, participants weighed induced seismicity, which has never been recorded in the extensively exploited sandstone reservoirs of the West Netherlands Basin but remains a concern because the local stress field and fault properties are poorly known. They also considered wellbore and caprock integrity, contamination risks, corrosion and scaling from fluid chemistry, and microbial clogging. On the monitoring side, the group emphasized establishing comprehensive in situ measurements: temperature at critical positions, mass flow rates, wellhead pressure, chemical concentrations, seepage velocity, and electrical resistivity fields, many of which can be tracked continuously with durable probes in observation wells.

The technical toolkit planned for the borehole is correspondingly ambitious. A distributed temperature sensing fiber optic cable will measure temperature directly along the hole, while distributed acoustic and strain sensing cables will support time-lapse vertical seismic profiling over tens of years, using both ambient noise and active sources. For electromagnetic monitoring, the researchers plan a novel approach: capacitive electrodes integrated into non-metallic composite casing to track electrical resistivity changes in the reservoir as it cools. First tests in shallow boreholes were successful, and a 300-meter-deep test is planned. If the hole can remain open above the composite-cased section, repeated logging of density, gravity, resistivity, high-frequency electromagnetic response, and magnetic susceptibility becomes possible, revealing how reservoir properties evolve over time. Downhole pressure gauges and tracer tests will characterize flow, while fluid sampling will target chemistry, including lithium content, and microbial life at depth. Cores and cuttings will feed geomechanical testing and fluid-rock interaction experiments, and leak-off and mini-frac tests will pin down the in situ stress field that governs fault stability.

All of these data streams converge on a digital twin of the reservoir. Static data from logs, cuttings, and cores will be processed with AI-driven tools, while dynamic observations will be assimilated continuously using forward models of fluid flow and energy transfer. Unlike traditional reservoir simulation, geothermal modelling here can incorporate electromagnetic and seismic observations directly, which has been shown to improve the accuracy of temperature forecasts and therefore operational decision-making. The researchers acknowledge substantial challenges, including the geometric scale disparity between wellbore and formation that complicates coupling processes across their interface, and the need for more robust numerical simulators for multiphase flow in heterogeneous, fractured media. Their stated approach is a hybrid one, combining physics-based reservoir simulation with emerging data-driven modelling techniques to characterize and reduce uncertainty in forecasts of how the reservoir will respond to decades of cooling.

The project also carries an unusually public dimension. Because it sits on a university campus in a city, the infrastructure doubles as a demonstration site: more than a thousand people visited during drilling of the campus geothermal wells, and the team sees open data sharing, school visits, and transparent communication about risks such as induced seismicity and drinking water protection as integral to the monitoring programme. The full laboratory is embedded in the European Plate Observing System framework, guaranteeing open access for researchers worldwide, and the team envisions it as the anchor of an international community of geothermal test beds alongside facilities such as FORGE in the United States, the Bedretto Laboratory in Switzerland, and UKGEOS. If the plan holds, by the end of the decade a single vertical hole beneath a Dutch campus will transform an operating urban geothermal system into the best-instrumented natural laboratory for subsurface energy in the world, with lessons that transfer directly to the crowded undergrounds of cities everywhere.

Subject of Research: A deep urban scientific drilling project for monitoring low-enthalpy geothermal heat production and subsurface processes

Article Title: An urban energy laboratory for monitoring and better understanding of subsurface processes related to low-enthalpy geothermal heat production – UrbEnLab

Article References: Bruhn, D., Abels, H. A., Barnhoorn, A., Bossennec, C., Braiden, A. K., Brehme, M., Chassagne, R., Daniilidis, A., Darnet, M., Drijkoningen, G., Fulton, P., Harcouët-Menou, V., Huenges, E., Jansen, S., Koulidis, A., Laumann, S., Lei, H., Moore, J., Rulff, P., … Voskov, D. (2026). An urban energy laboratory for monitoring and better understanding of subsurface processes related to low-enthalpy geothermal heat production – UrbEnLab. Scientific Drilling, 35(1), 83-97. https://doi.org/10.5194/sd-35-83-2026

Image Credits: AI Generated

DOI: 10.5194/sd-35-83-2026

Keywords: geothermal energy, scientific drilling, TU Delft, subsurface monitoring, ICDP, urban energy, heat storage, reservoir monitoring, West Netherlands Basin, fiber optic sensing, induced seismicity, data assimilation