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

The study, published in Environmental Earth Sciences, focuses on the Upper Jurassic Malm aquifer, a karstified carbonate formation lying more than 1,400 meters below the surface of the South German Molasse Basin. The Malm is the workhorse of Bavarian geothermal energy: up to 600 meters thick, composed of limestones and dolomites riddled by multiple episodes of karstification, and cut by an extensive network of normal faults created as the Alpine orogeny bent the lithosphere. Its waters reach 80 to 85 degrees Celsius, hot enough to feed district heating grids across the region. But the same fractures and faults that make the reservoir productive also raise a persistent engineering question: how much can operators push fluid through this underground before the rock itself pushes back?

To answer that question, the researchers built a three-dimensional synthetic geological model inspired by the faulted Malm reservoir north of Munich, incorporating a real fault interpreted from the GRAME seismic survey, one of the largest urban geothermal seismic campaigns ever conducted in Europe, which imaged roughly 170 square kilometers of the city’s subsurface in 2015 and 2016. Into this model they placed two competing operational schemes. The first was the familiar hydrothermal doublet: a production well drawing ambient-temperature reservoir fluid continuously, paired with an injection well returning the cooled water year-round. The second was an HT-ATES triplet, a three-well arrangement in which a bidirectional hot well injects water heated to 140 degrees Celsius during six-month summer periods and produces it again in winter, a cold well reinjects the cooled fluid at 50 degrees Celsius, and a buffer well balances the mass by producing ambient-temperature water.

The simulations were run with the open-source PorousFlow module of the MOOSE framework, coupling single-phase saturated fluid flow, heat transport, and linear elastic rock mechanics into a fully coupled thermo-hydro-mechanical model. Water properties were calculated with the IAPWS-IF97 equation of state, and the model was subjected to the strike-slip stress regime of the Molasse Basin, with a maximum horizontal stress gradient of 34.6 kilopascals per meter oriented due north. Each simulation ran for ten years, spanning twenty alternating summer and winter cycles for the triplet and a decade of continuous operation for the doublet, with flow rates of 150 liters per second at every well. The team then evaluated the response of the reservoir and the fault using slip-tendency analysis, a standard geomechanical measure of how close a fault surface is to frictional failure.

The first striking result concerns pressure. Because the Malm aquifer is extraordinarily permeable, around 1.48 by 10 to the minus 12 square meters in the model, pressure perturbations from injection and production remained tiny, on the order of plus or minus 0.1 megapascal after ten years. That means poroelastic effects, the stress changes caused directly by fluid pressure, played almost no role in the mechanical response. Instead, the geomechanics of the reservoir turned out to be governed almost entirely by thermoelasticity: the expansion of rock where hot water is injected and its contraction where cold water is injected. Heating near the hot well increased compressive normal stress and even stabilized nearby fault sections, while cooling near the cold and injection wells reduced normal stress and pushed the fault closer to slipping.

The differences between the two operating styles were substantial. After ten years, the vertical displacement at the top of the reservoir reached 28 millimeters at the doublet’s injection well, compared with 19 millimeters at the triplet’s cold well and just 8 millimeters at its hot well. The hot plume and the cold plume also stratified by density: buoyant hot water accumulated near the top of the reservoir, where temperatures at the hot well reached 132 degrees Celsius, while denser cold water sank and spread along the base, where only 118 degrees was recovered. Because the doublet injects cold water continuously rather than cyclically, and injects twice the total volume over the decade, its cumulative thermal perturbation was larger, and its stress-field disturbance grew monotonically rather than partially recovering during rest periods.

The fault-stability comparison was even more decisive. The complexly oriented fault surface, with spatially varying strike and dip, showed initial normalized slip-tendency values ranging from 0 to 0.76, highest where the fault faces the direction of maximum horizontal stress at a favorable 30-degree angle. After ten years of operation, the hydrothermal doublet raised the maximum normalized slip tendency by 0.137 near its injection well, concentrated in the deeper fault sections where the cold plume pooled and the well sat closest to the fault. The HT-ATES triplet, by contrast, increased slip tendency by only 0.03, roughly four times less. Neither scenario crossed the critical threshold of 1.0, meaning no fault destabilization was predicted even under deliberately conservative assumptions, including critically stressed crust, zero cohesion, and wells placed a mere 178 meters from the most vulnerable fault segment, far closer than real projects would ever site them.

Heterogeneity proved to be the wild card. The Malm’s karst networks mean permeability can vary by orders of magnitude between layers, and the team tested moderately and extremely heterogeneous end-member scenarios spanning more than three orders of magnitude in permeability contrast. In the extreme case, a single 30-meter layer at the top of the reservoir, representing just 5 percent of its volume, carried 45 percent of the injected fluid. This concentration of flow reduced thermal recovery efficiency by roughly 10 percent compared with the homogeneous case, which achieved about 90 percent recovery after ten years, and it slightly increased fault slip-tendency changes to 0.04. Where the high-permeability layers sat deeper in the reservoir, closer to the slanting fault, the destabilizing cold-front stress changes shifted downward with them, a reminder that the three-dimensional geometry of karst zones matters as much as their existence.

To probe the long term, the team extended the extremely heterogeneous triplet scenario to fifty years. By then the cold front had finally reached the fault surface, and the maximum normalized slip-tendency change had grown from 0.04 to 0.2, with absolute slip tendency peaking at 0.9 in the most critical sections. Still below the failure threshold, but uncomfortably close, and a clear signal that decades of repeated cold-water cycling gradually erode the geomechanical safety margin. The authors are careful to note the caveats: their linear elastic framework cannot capture fatigue, damage accumulation, or plastic deformation across cycles; permeability was held constant; the model is synthetic rather than site-calibrated; and the two schemes serve different engineering purposes, so a rigorous comparison would need to normalize for delivered thermal energy.

Even with those limits, the implications for Munich and cities like it are concrete. Thermoelastic stress changes of this kind announce themselves in the field through falling injection pressures and rising injectivity near cold wells, exactly the locations where all major induced seismic events in the Munich area have historically clustered, making the vicinity of injection wells the natural place to focus monitoring with borehole strainmeters, extensometers, or distributed acoustic sensing. The predicted displacements, up to 28 millimeters at reservoir depth nearly 1.5 kilometers down, are expected to attenuate strongly toward the surface and are readily detectable with standard downhole instruments. As Munich pushes toward a climate-neutral heat supply and looks to store summer surpluses, whether from geothermal, industrial waste heat, or solar thermal, in its own bedrock, this study suggests that seasonal storage triplets can do the job while shaking the faults beneath the city less than the geothermal plants that have operated there for years.

Subject of Research: Comparative thermo-hydro-mechanical modeling of high-temperature aquifer thermal energy storage and hydrothermal doublet operation in the faulted Malm geothermal reservoir near Munich

Article Title: Thermo-hydro-mechanical response of the operation style in a faulted Malm reservoir: Storage triplet vs. hydrothermal doublet

Article References: Dashti, A., Stricker, K., Hörbrand, T., Habibi, R., Beichel, K., & Kohl, T. (2026). Thermo-hydro-mechanical response of the operation style in a faulted Malm reservoir: Storage triplet vs. hydrothermal doublet. Environmental Earth Sciences, 85(16), Article 404. https://doi.org/10.1007/s12665-026-13141-7

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13141-7

Keywords: geothermal energy, HT-ATES, aquifer thermal energy storage, hydrothermal doublet, Malm aquifer, Molasse Basin, Munich, fault stability, slip tendency, thermoelasticity, induced seismicity, district heating