Low-enthalpy geothermal heat production is becoming increasingly common, which leads to the potentially competitive use of the available subsurface space, especially in densely populated urban areas. A specific challenge presented by the high density of different geothermal systems is understanding the details of convective and conductive heat flow processes and detailed monitoring of properties and processes in the subsurface.

On the TU Delft campus, we aim to drill a borehole of around 4.5 km depth to be used for the exploration, observation, and monitoring of subsurface processes that will be part of a larger research infrastructure under development. This so-called urban energy laboratory includes – in addition to the deep multi-use borehole – a well-instrumented geothermal doublet drilled in 2023, reaching to a depth of 2.2 km; a local seismic monitoring system (installed in 2022); an ultra-sensitive portable seismic monitoring array; and a high-temperature aquifer heat storage system (HT-ATES), for which a pilot well was drilled in 2024. With this urban energy laboratory, we want to tackle problems and better understand processes related to multiple and/or competing subsurface uses in urban environments. The deep exploration and monitoring borehole is designed specifically to monitor fluid and/or flux movement in 3D with unprecedented precision, aiming to understand the propagation of the geothermal cold front and reservoir pressures.

During the 3 d International Continental Scientific Drilling Program (ICDP)-sponsored UrbEnLab workshop, 75 scientists from 17 countries met in Delft, the Netherlands, in June 2024 to prioritize the scientific ambitions of the deep exploration and monitoring borehole and to discuss potential techniques that could be applied to tackle them. Assessing the life cycle of a geothermal system situated in a complex heterogeneous sedimentary system was defined as the broad aim, with revealing the detailed flow field established being a key priority.

High-temperature aquifer thermal energy storage (HT-ATES) can play a key role in the energy transition. For well completion of conventional low-temperature ATES and groundwater wells, grout and/or clay pellets are typically utilised as annular materials to ensure the long-term well integrity. It is not yet known if such materials can also be used in HT-ATES working conditions. In this work, a novel approach to evaluate the sealing performance for such completion materials is proposed and tested over multiple thermal heating and cooling cycles representative of the conditions of HT-ATES operation. The experimental framework utilises a novel experimental design to test the apparent transmissivity of the annular material, followed by micro-CT scanning. During each test, up to 11 thermal cycles are applied, with temperature variations between 22oC and 90oC. For grouts after 7 days of curing, micro-CT scans reveal debonding and the occurrence of micro-annuli with an equivalent diameter of approximately 26% of the original cross-section. After 28 days of curing, the thermal cycles had a much reduced impact on micro-annulus formation. The corresponding apparent transmissivity decreased up to 80% for samples containing a high percentage of cementitious minerals and a low water-to-grout ratio. The clay pellets, saturated with fresh water, demonstrated effective sealing capacity and an impermeable behaviour. However, clay pellets saturated with 0.25 mol/L NaCl, showed up to an 85% decrease in swelling capacity yet still exhibited impermeable behaviour. The results indicated that thermal cycles affect the integrity of grouts, while clay pellets show resilience to them. Furthermore, longer curing periods and specific chemical compositions improve sealing performance and provide resilience to thermal cycles.

This document describes the drilling, completion and testing of the pilot borehole (DEL-HTO-P01), which will later be utilized as a monitoring well for High Temperature Aquifer Thermal Energy Storage (HT-ATES). The work includes all materials, components, tools, and services that are related to the preparation, delivery, installation, measurement, and completion of the components belonging to the test drilling and monitoring well. The following analysis combines various data sources to characterize the subsurface for the Delft Demonstration site (WP1). The location of the DEL-HTO-P01 is demonstrated in Figure 1, which is located at RD-coordinates X = 85,257 m, Y = 445,757 m.

All data presented in this report have been published via TU Delft institutional data repository 4TU.ResearchData under the CC BY 4.0 license. The initial borehole dataset and the CT scan data can be found via https://doi.org/10.4121/1694ba82-db41-4017-8d1c-9de3ce1a785e and https://doi.org/10.4121/9902ebc7-dbd6-43b1-a3c3-85ff9ab645a9.

Drilling wells in unconsolidated formations is commonly undertaken to extract drinking water and other applications, such as aquifer thermal energy storage (ATES). To increase the efficiency of an ATES system, the drilling campaigns are targeting greater depths and enlarging the wellbore diameter in the production section to enhance the flow rates. In these cases, wells are more susceptible to collapse. Drilling fluids for shallow formations often have little strengthening properties and, due to single-string well design, come into contact with both the aquifer and the overburden. Drilling fluids and additives are experimentally investigated to be used to improve wellbore stability in conditions simulating field conditions in unconsolidated aquifers with a hydraulic conductivity of around 10 m/d. The impact on wellbore stability is evaluated using a new experimental setup in which the filtration rate is measured, followed by the use of a fall cone penetrometer augmented with an accelerometer to directly test the wellbore strengthening, and imaging with a scanning electron microscope (SEM) to investigate the (micro)structure of the filter cakes produced. Twelve drilling fluids are investigated with different concentrations of bentonite, polyanionic cellulose (PAC), Xanthan Gum, calcium carbonate (CaCO₃), and aluminum chloride hexahydrate ([Al(H₂O)₆]Cl₃). The filtration results indicate that calcium carbonate, average d_p <20 μm, provides pore throat bridging and filter cake formation after approximately 2 min, compared to almost instantaneous discharge when using conventional drilling fluids. The drilling fluid containing 2% [Al(H₂O)₆]Cl₃ forms a thick (4 mm) yet permeable filter cake, resulting in high filtration losses. The fall cone results show a decrease of cone penetration depth up to 20.78%, and a 40.27% increase in deceleration time while penetrating the sample with CaCO₃ compared with conventional drilling fluid containing bentonite and PAC, indicating a significant strengthening effect. The drilling fluids that contain CaCO₃, therefore, show high promise for field implementation.

In horizontal well operations, the prevalence of stuck pipe incidents is largely attributed to inadequate hole cleaning, underscoring the critical need for a thorough understanding of this process to mitigate non-productive time and financial losses. Increasing fluid velocity, the major drilling parameter of hole cleaning, diminishes the formation of cuttings beds in a wellbore. This study primarily centered on the mechanical displacement and removal of solid particles, employing advanced image processing techniques to elucidate the dynamic behavior of solid particles in deviated wellbores. The core objective of this study was to experimentally scrutinize the effects of a downhole clamp-on tool on fluid velocity to improve hole cleaning practices. To address this challenge, a customized flow loop was designed and constructed to accurately replicate the conditions encountered in horizontal wells. Pure water was used to demonstrate the effects of the clamp-on tool on cuttings transport for lightweight drilling fluid conditions. Strategically deployed, the clamp-on tool played a pivotal role in agitating cuttings, mitigating their accumulation at the bottom of the borehole. The tool's agitation mechanism noticeably improved cuttings removal by increasing velocity, extending the perturbation of cuttings transport in the tool's downstream flow, and reducing bedding formation. At lower flow rates, the tool led to an over fourfold increase in average particle velocity within the tool and a twofold increase after the tool. Our results demonstrate the substantial potential of mechanical assistance to address hole cleaning challenges and significantly advance horizontal well operations.

At present, over half of all primary energy used in Europe is used for heating and cooling. Therefore, decarbonizing the heating supply is essential to achieve climate targets. Underground thermal energy storage is a key enabling technology for the energy transition to buffer the large seasonal mismatch between thermal energy demand and sustainable thermal energy production capabilities. In Delft, a High-Temperature Aquifer Thermal Energy Storage (HT-ATES) system will be installed at the campus of Delft University of Technology (TU Delft). It will be integrated in the wider heating system on and around the TU Delft campus, which itself is undergoing a transformation to optimally supply sustainable thermal energy. The district heating network will be extended and utilize the thermal energy from a geothermal doublet producing heat at around 75-80°C with a flow rate of ~350m3/hr. Excess energy produced by the geothermal well in summer will be stored in the HT-ATES system, and will be utilised when demand exceeds production throughout the winter. The HT-ATES system will comprise of 7 wells (3 hot wells of 80°C and 4 warm wells of 50°C) to a depth of approximately 200m, with storage in an unconsolidated sedimentary aquifer between 160-200m depth. It is designed so that the instantaneous excess power from the geothermal project can be stored and demand from the district heating network be extracted from the system.

The HT-ATES system at TU Delft is partially funded by local stakeholders and the European commission within the PUSH-IT project and has two primary goals: (i) to reduce carbon emissions on TU Delft campus , and (ii) to create a unique demonstration, education and research infrastructure. The complexity of a HT-ATES requires innovative solutions during the entire system life cycle. The scientific programme that is initially planned within the project is therefore focusing on various research fields and includes:

- Characterisation of the subsurface formations including mechanical, hydraulic, thermal, and chemical properties.
- Evaluation and monitoring of the biological conditions and microbial diversity, and potential impact on water quality.
- Innovations in drilling and completion, monitoring and performance.
- Quantification of the system performance and system impact during multiple storage cycles and the full lifecycle of the HT-ATES. This will include extensively monitoring temperature distribution and water quality in the subsurface to characterise behaviour and improve models.
- Demonstrate and develop the implementation of HT-ATES in an urban setting, including control of the system in the built-environment and transforming the conventional heat network to a future-proof heat network.
- To allow access to other universities or institutions with active programmes in the field of Geothermal Science and Engineering to jointly carry out research and perform experiments.
-Societal engagement and legal evaluation for improving the just energy transition.