Rock salt caverns offer a promising subsurface engineering solution for CO₂ storage, waste disposal, compressed air energy storage, and hydrogen storage, owing to their substantial extent, extremely low permeability, and minimal porosity. The long-term containment of injected gases, fluids or disposed materials relies on the integrity of cavern seals. However, fractures in the damaged zone formed during the wellbore drilling phase, storage operations, or the abandonment phase can compromise system integrity by acting as preferential flow paths and mechanically weak zones. This poses a challenge to the safety of subsurface storage throughout the operational lifespan of a salt cavern. Understanding the evolution of these fractures in terms of their physical and mechanical properties is therefore essential. In this thesis, we adopt an integrated experimental numerical approach to investigate both the dissolution behaviour and mechanical recovery of rock salt fractures.

During the leaching period, the walls of a rock salt cavern are exposed to undersaturated brine. Fractures in contact with undersaturated brine are expected to undergo dissolution, with the dissolved ions being transported and potentially precipitating at a further location. This process alters the geometry of the fractures over time. We develop a flow visualization experimental setup at the decimetre scale, combined with quantitative measurement techniques, to investigate how initial fracture geometry, salt concentration, and flow rate influence the evolution of rock salt fracture channels. The results show that dissolution within fractures is heterogeneous, with increased dissolution near the inlet and along fracture corners due to advection-dominated flow, leading to progressive smoothing of fracture surfaces. Fractures with larger initial surface areas exhibit higher dissolution rates and greater total mass loss compared to those with smaller surface areas, Moreover, the dissolution patterns observed in this study differ from those typically reported at the microscale. To further interpret the experimental observations, a coupled reactive transport model is developed using COMSOL Multiphysics to simulate fluid velocity distribution and solute concentration.

Following the leaching process, salt caverns are employed for energy storage and extraction, during which the surrounding rock salt, particularly near the upper and lower cavern walls, is subjected to alternating dry and wet conditions. Given rock salt’s inherent healing capabilities (i.e., mechanical recovery), we investigate the shear strength recovery of rock salt fractures through direct shear box experiments. The evolution of fracture surface geometry is further examined using digital microscopy and microcomputed tomography (micro-CT). Results reveal a log-linear relationship between healing and time, with wet conditions exhibiting a recovery rate approximately seven times greater than that under dry conditions, consistent with time-dependent frictional healing laws observed in gouge. Healing in dry samples can be attributed to plastic flow. Additionally, dynamic behaviour is characterised by an increase in the residual friction coefficient over time. Healing in wet samples can be attributed to pressure solution, recrystallisation, and possibly surface energy minimisation processes. A difference in stick-slip behaviour in terms of frequency and stress drop is observed between dry and wet samples. This difference may be due to the applicability of a distance-dependent model of stick-slip behaviour in dry conditions and a velocity-dependent model in wet conditions.

In addition, the eject of initial fracture surface roughness on healing behaviour is examined by preparing samples with smooth and rough surfaces. Under dry conditions, strength recovery is limited and observed only in smooth fractures. In wet conditions, smooth fractures exhibit substantial healing, whereas rough fractures show only minor improvement. These findings demonstrate a clear inhibitory eject of surface roughness on strength recovery in fully saturated environments. This eject is attributed to the larger aperture in rough surface samples, which hinders crystal growth across the fracture and impedes bridging between opposing surfaces.

Finally, an existing elasto-plastic constitutive law is shown to also be capable to simulate damage and healing behaviour of rock salt fractures, based on a cohesive zone model framework, enabling the prediction of strength recovery over time. Specifically, fracture initiation is governed by a hyperbolic yield surface in the Qn-Ts stress plane. The shrinkage (damage stage) and expansion (healing stage) of the yield surface are governed by two history-dependent variables: an internal variable (Wcr) represents the dissipated fracturing (plastic) work, as established in existing models. The expansion (healing stage), in contrast, is governed by an external variable (Ht), which we propose here to represent the progressive recovery of the mechanical properties of rock salt fractures over time. The key features of the existing formulation are demonstrated through single-element tests under various sequences of mechanical loading. Simulations with and without healing show good agreement with laboratory results for both non-healed and healed samples, confirming the model’s ability to capture the essential characteristics of fracture healing in rock salt. Additional simulations over healing periods from years to decades are performed to assess long term recovery behaviour.

This thesis examines the evolution of the physical properties of rock salt fractures during dissolution, the mechanical recovery of fractures, and the associated microstructural changes of fracture surfaces, using both experimental and numerical approaches. The findings provide insights into the design and performance of salt caverns for energy storage and radioactive waste disposal.
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The increasing global demand for heating and cooling is predominantly met by fossil-based energy sources, contributing to greenhouse gas emissions and placing growing pressure on energy infrastructure. Although greater use of renewable energy is crucial for decarbonisation, its generation frequently does not coincide with demand, creating a seasonal mismatch. Seasonal thermal energy storage offers a promising solution to balance this mismatch between energy supply and demand. This thesis investigates the technical, environmental, and economic feasibility of Mine Thermal Energy Storage (MTES) by coupling a virtual MTES model with the heating and cooling system of Ruhr-University Bochum (RUB).

To achieve this, separate Python and Modelica models of the MTES and campus heating and cooling systems were developed and linked through an orchestrator algorithm. The co-simulation was conducted to evaluate system performance. Sensitivity analyses were performed to assess the influence of system design parameters, operational parameters, and input data. In addition, the long-term effects were monitored and the optimal setup was determined.

Results demonstrate that MTES can reliably store excess heat in summer and supply it during winter while maintaining stable operational temperatures. The integrated system achieved a total COP which is lower than a stand-alone heat pump, but appropriate for a combined heating-cooling configuration with seasonal storage losses. The system reduced reliance on the gas boilers and cooling tower, and it also delivered the highest CO₂ reduction among comparable storage technologies. Economic assessment shows that the levelised cost of heat is competitive with similar storage systems, even under conservative operational assumptions.

Sensitivity analyses showed that the optimal heat pump size and operational variables such as the heat pump ΔT value and MTES temperature critically influenced performance, while the MTES size had a weaker but still noticeable effect. The optimal practical and theoretical setups further demonstrated the system’s potential, showing significant increases in recovered waste heat and CO₂ reduction.

Overall, this thesis demonstrates that MTES offers a technically, economically and environmentally viable solution for seasonal thermal energy storage in the heating and cooling system of the Ruhr- University Bochum. Ultimately, MTES can represent an important step toward achieving the EU’s long-term climate targets by enabling more efficient and resilient sustainable energy systems. ... Read more

Energy plays a fundamental role in societies impacting everything from basic human needs like lighting, cooling and heating to complex industrial processes that significantly influence social development, economic growth, and national security particularly in terms of access and affordability for all citizens. The world’s electricity demand grew by 2.2% in 2023 and is expected to rise at a faster rate by an average of 3.4% in 2026 (IEA, 2024). With the global climate warming, it is important to reduce our societal impact on Earth by using clean energy sources. One of the clean sources is geothermal energy which has a great potential to reduce dependency on the fossil fuels. Geothermal energy sources can deliver both, heat and electricity. Tanzania has a huge geothermal potential that has not yet been used and has only been explored to a limited extent. This thesis attempts to identify the geothermal potential of southwest Tanzania with appropriate drilling zones by assessing the main components of a geothermal system reservoir which are temperature, permeability and fluid flow at Songwe Tanzania. It is one of the identified potential development areas with very little information available.
The work flow begins with chapter 2 where a geological and thermal numerical model is set up to simulate the temperature at depth. The thermal model considers pure conductive heat flow to achieve a first idea of the temperature distribution in the different geological layers.
Chapter 3 describes the conducted field study at the thermal spring areas as well as the laboratory analysis of the samples in order to understand the geothermal fluid source by using geochemical modelling, obtain the reservoir temperature through geothermometer calculation and locate the up-flow and the outflow zones of the geothermal field.
In chapter 4 numerical simulations of fluid and convective heat flow are performed to allow understanding the heat transport by fluids and the hydrogeological behaviour of the geothermal reservoir by varying thermal and physical parameters.
Chapter 5 shows possible drilling locations for a first geothermal well in the study area, whereby areas of different productivity are distinguished.

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Reusing oil and gas reservoirs for geothermal energy production in the Dutch offshore could be an interesting energy resource for the energy transition. The Dutch offshore contains a large number of abandoned, and producing wells that are connected to hydrocarbon reservoirs. These reservoirs have undergone significant amounts of research, and large datasets are available on them. This reduces the need for expensive and extensive research. Additionally, developed reservoirs already have the necessary infrastructure used for production built around them, such as a platform, wells and technical equipment.

In this report a portfolio of reservoirs is constructed that have the highest probability of displaying an adequate geothermal potential for producing electricity. This is done by 1): assessing the reservoir wells on the temperature in the reservoir formation and 2): plotting the wells that show an adequate temperature over an interpolated permeability map. From this map the reservoirs with the highest formation temperatures and permeabilities are selected. The geothermal electricity production potential of the selected reservoirs is then evaluated by estimating their power output with DoubletCalc. To do so, the reservoirs are assumed to be laterally homogeneous on rock type, porosity, and permeability.

Of the 325 offshore reservoirs in the Netherlands, only 6 reservoirs met the required temperatures of over 120C and an expected permeability of over 40mD. These cut-off values were introduced based on the fact that below these values the economic potential of a reservoir becomes riskful. Using the available data on NLOG an estimate of the power output could be made, which unfortunately resulted in low expected thermal power outputs (P50 < 1MW). This results from the low permeabilities in the aquifer layer, and lead to a low flow rate in the production well. As the thermal energy needs to be converted to electricity, which only returns around 14% for a Binary conversion system, the electrical power estimates are not sufficient to make such projects economically viable. The reservoirs selected in this study are thus not suitable for development of geothermal electricity production. The output is too low and the costs to realize such a project would most likely be much larger than the potential of the project.

As these reservoirs were the best case scenario reservoirs from the reservoir selection process, the conclusion can be drawn that the hydrocarbon reservoirs in the dutch offshore are not good enough to reuse, as is. Further research should therefore be done on Enhanced Geothermal Systems (EGS). Here the permeability is enhanced through methods such as 'fracking', which could potentially increase the reservoir power output due to higher flow rates. ... Read more

With the help of the Dutch Seismic Acquisition Program for Geothermal Exploration (SCAN),
there is a growing nationwide interest in the exploration and development of Deep Geothermal
Energy (DGE) by the geothermal industry. One geological target in the Dutch subsurface
that could be suitable for harnessing DGE is the Upper Slochteren Formation of the Early Permian.
Previous studies have revealed the geological property distributions of the formation in
the densely populated Province of Utrecht. However, the facies distribution of the formation
across the area has not explored yet. The thesis is objective is to gather information on the
facies distribution of the Upper Slochteren Formation in the Province.
For this purpose, available core slabs from several wells in the region were studied and interpreted
for their facies groupings. Each classified facies group is characterized by a specific
range in porosity values. Additionally, seven well logs were analyzed for this thesis to evaluate
the porosity, net sand, and permeability distributions of the Upper Slochteren Formation. By
utilzing the resulting porosity borehole profile values and the 2D SCAN seismic sections, five
facies units (aeolian dune (AD), interdune (ID), sandy sheet (SS), fluvial channels (FS), and
flood sheet (SF)) were indentified and traced throughout the entire study area. The resulting
facies distribution and corresponding geological property distributions are discussed to gain
insight into the study area is paleogeographic setting and depositional environment during the
Early Permian.
Firstly, for the area north of Amersfoort, the formation is characterized by favorable geological
properties (average demonstrated permeability of 149.73 [mD]), making this area a highpotential
geothermal case. Secondly, the facies maps reveal that the area mainly consisted of
aeolian sandy sheet (SS) facies, ephemeral flood sheet (SF) facies, and ephemeral fluviatile
channel (FC) facies units. The results also indicate that their distribution was irregular across
the study area, with increased quantities of sandy sheet (SS) facies in the northeastern (northwestern
Gelderland) and southwestern (Southeast Province of Utrecht) parts of the research
area. Furthermore, the results show that at the beginning of the Early Permian period, the
quantities of ephemeral flood sheet (SF) and fluviatile channel (FC) facies units were higher,
while the amount of sandy sheet (SS) facies units was lower, in contrast to the end of the
Early Permian period. Similarly, aeolian dune (AD) and aeolian interdune (ID) facies units
were absent from the deposition area of the study region during the Early Permian period.
To better constrain the DGE potential for the Upper Slochteren Formation in the Province
of Utrecht, it is recommended to retreive a 3D seismic cube for the high-potential case area.
Additionally, it is advised to gather a large quantity of core plugs and core slabs from the
Upper Slochteren Formation during future geothermal exploration drilling.
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In this multidisciplinary project several aspects of geosciences are combined. The regional geology background was summarized and linked to the borehole data.
Multiple tests were conducted on the well to answer several questions. The slug test indicated that the fracture is still open and essentially confirmed that it is a shear fracture, however it is unclear to what extent that the fracture is open. The fracture seems to be hydraulically connected to a permeable unit or shallow aquifer. Unfortunately, the length of the fracture could not be determined with the data collected from the test.
Electrical resistivity tomography (ERT) and seismics were both applied to a location near the borehole to acquire lateral information of the subsurface. The ERT results showed that the layers were horizontally continuous and indicated layers with different compositions based on resistive properties.
Seismic refraction tomography conducted along a part of the same profile showed similar results as the ERT for that part of the profile. P-wave velocities indicate a horizontally layered subsurface in the upper 40m. Additionally surface wave analysis of the same setup utilizing active and passive measurements resulted in a vertical s-wave velocity profile that can be used for future implementation of the planned Borehole Thermal Energy Storage (BTES) system.
The last geophysical method was using gravity data on the region around the site. A map was made by using available data on changes in gravity in the region and plotting the results. On this map the location of remnants of volcanos and the Litoměřice deep fault can be recognised.
Thermal properties of cores were analyzed using a Hot Disk and an optical scanner. Unfortunately the drilling of a new well from which the cores were to be analyzed was delayed, and cores from an uranium mine were used. This way the advantages and disadvantages of both measuring devices could be argued and used for future research.
Past analysis of geothermal regions have shown that exploration of geothermal energy causes surface displacement. It can also be observed during the drilling phase. Interferometric Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) are valuable tools to monitor land surface changes. Measurement of surface deformation being one of its many applications. For this study, the above tools have been used to measure surface displacement in the region of Litoměřice.

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Borehole operations play a crucial role in managing various subsurface activities related to energy, including energy storage, geothermal energy production, CO2 sequestration, oil and gas extraction, wastewater disposal, and thermal recovery processes. In recent times, intelligent well technologies, such as long deviated multi-lateral wells equipped with advanced inflow control valves, have been employed to enhance field profitability and operational reliability. Additionally, different wellbore designs are utilized to efficiently extract heat from the subsurface in advanced geothermal approaches. To ensure accurate modeling, prediction, and optimization of all these energy production processes, it is essential to employ fully-coupled models that accurately capture thermal multi-phase compositional flow and transport in both the reservoir and the boreholes... ... Read more

Geothermal sources could provide society with clean and renewable energy nearly eternally. Geothermal district heating, in particular, could play a significant role in the energy transition. Although the application has been implemented to some extent, further upscaling is challenging due to various economic, technical and institutional conditions, resulting in barriers. This thesis presents a comparative, qualitative analysis of these barriers for three European countries (Germany, Hungary and The Netherlands). Through Energy statistics analysis, document analysis and interviews with experts in the field, these barriers are identified, decomposed, and their impact is analysed. It was found that barriers causing economic non-viability of geothermal district heating, poor insulation of buildings, and various regulatory and legal barriers are significant factors that prevent further upscaling. Finally, a set of policy recommendations is presented with the ambition to achieve a ten per cent share of geothermal district heating in the household, public and commercial services heat sector of these countries by 2030. To that end, European countries' governments are recommended: to restructure heat price regulation schemes; to reduce subsidies for conventional heating systems; to increase and reserve subsidies for geothermal district heating specifically; to provide attractive incentives for energy efficiency improvements in buildings; and to establish clear legal and regulatory arrangements for geothermal district heating systems. ... Read more

This thesis contributes to increasing the technology readiness level of hydrogen storage in gas reservoirs, which will be required when hydrogen has become a major energy carrier in the future Dutch energy system. It addresses the mixing processes with resident gases that occur during hydrogen storage operations in gas fields, and analyzes their implementation in a reservoir simulator. The CMG GEM reservoir simulator allows incorporation of mechanical dispersion and effective molecular diffusion into its simulations, while solving the advection dispersion transport equation fully implicitly, as well as gravitational segregation and other macro scale mixing phenomena. Mechanical dispersion is quantified by its main parameter dispersivity, for which a large uncertainty exists in the literature. Dispersivity represents pore scale fluid velocity differences caused by microscale heterogeneities in a porous medium. Due to the lack of adequate hydrogen dispersivity experiments, a range of dispersivity values is collected from literature on (groundwater) dispersivity experiments in sandstones. A sensitivity analysis is conducted, in which the influence of the different mixing processes on the mixing between working gas and cushion gas is analyzed. For this, a conceptual reservoir model (radial and homogeneous), with properties based on Dutch sandstone gas fields was built in CMG GEM. The effect of molecular diffusion on mixing proves to be negligible compared to mechanical dispersion at typical reservoir flow rates. Furthermore, the results of the simulations prove to be significantly influenced by numerical dispersion, which is a calculation error, dependent on grid size and time step. The effect of numerical dispersion compared to mechanical dispersion is quantitatively analyzed, after which various options are introduced to deal with numerical dispersion in a way that the physical processes are most realistically represented. The work in this thesis demonstrates the challenges of realistically implementing hydrogen storage mixing processes in a reservoir simulator, and should be regarded as a foundation for further research. ... Read more

The Netherlands has set the ambitious goal to be CO2 neutral by 2050 and signed the Paris Treaty in 2015. The contribution of geothermal energy to reaching this goal are outlined in the Masterplan in 2018 which attempts to reduce CO2 emissions. It is imperative to enhance geothermal participation in renewable energy resources, so this thesis proposes a consideration of large scale geothermal field development in order to meet these requirements. For these large scale projects, large scale geological heterogeneities must be taken into account in order to propose a development strategy that honours subsurface variability in properties like porosity or permeability of an aquifer. Furthermore, the nature of the large scale operations, inherently requires the consideration and application of well patterns typically used in oil and gas developments. Operating in these extensive domains, carries a lot of uncertainty in the final economical output of the project, so modelling the process could indicate the optimal conditions that would deliver the best possible operational outcome. Conceptual 2D model approaches were adopted to demonstrate the main ideas behind large-scale geothermal well pattern optimisation. The main objective of this project is to model, evaluate and optimise the performance of large scale geothermal field development. The proposed strategy is based on the use of well patterns as are frequently used in the oil industry. The heterogeneity in geological properties that may be expected to be encountered at larger spatial scales is addressed by the concept of a flexible well density function. This density function allows the well patterns to be resized (or ultimately, reshaped) and adapt to spatial variations in geological characteristics. The flexible well pattern is fed to an objective function created with embedded simulator. The output of the function is the net present value (NPV) of the project. Four test cases are created, starting with a homogeneous static model and building up more heterogeneous aquifer models, aiming to test the performance of the flexible well density function. The aquifer property models are representative of the West Netherlands Basin and specifically the Delft Sandstone Member. Per each aquifer model, line drive and 5-spot development strategies are assessed. Each development scenario is modeled in an objective function and optimised. The optimisation algorithm chosen is the Simplicial Homology Global Optimisation, suitable for black-box functions that show multiple local optimum solution and among them, a global optimum pattern size is found. The NPV of each project realisation is calculated based on the energy recovered and an economic model under Dutch fiscal conditions. The results suggest that, the flexible well placement is successfully aligning with the different aquifer geological properties. Pattern size is inversely correlated to porosity which corresponds to higher volume of pore fluid from which heat can be recovered. The optimisation algorithm managed to identify the global optimum solution of pattern size that delivers the highest possible positive NPV. The most efficient in terms of profitable strategy, is suggested to be the 5-spot pattern. The optimal pattern size ranges between 500 - 2160m depending on the aquifer model. The sweep efficiency, in terms of energy recovery, is also assessed per aquifer model and development strategy. The most efficient is the 5-spot pattern. The performance of the adopted optimisation algorithm, on the fully homogeneous aquifer, is tested with an exhaustive response curve of NPV. It is confirmed that the algorithm manages to identify the global optimum. The character of the NPV as a function of well pattern size/density proved the complexity of the system with respect to the amount and well types introduced. Different sensitivity analyses in the context of the uncertainty of the aquifer thickness, thermal conductivity, thermal heat capacity and full economic model are conducted in order to show the impact on the optimal patter size. The performance of the optimisation algorithm is assessed as well, indicating that further investigation on the tuning meta-parameters could potentially lead to better global optimum solutions in more heterogeneous aquifer models. ... Read more

This research is intended to create an insight into the heat exchange occurring in a well and to find ideal well conditions in which the performance of the heat exchange is maximised. For demonstrating and studying these aspects the well of Grouw-01 is analysed. The well of Grouw-01 is a depleted gas well attained from Vermilion Energy and is in shut-in state. Re-using these shut-in or abandoned wells can provide fossil fuel-free energy for the local community. Such wells have the potential to be used for geothermal energy generation, either by using them to directly access warm water from a reservoir or as borehole heat exchanger, without direct connection to a potential reservoir. In this study there is no contact with the reservoir. As there is no contact, a method is created in which the injected fluid can reach the surface within the same well. This is done by creating a circulating system of a working fluid in the well in which the heat is extracted from the subsurface and carried to the surface by the working fluid. For the fluid to be able to circulate, a tubing is installed inside the well which is an inner-tubing. Such a well with circulating fluid systems is designed like a Deep Coaxial Borehole Heat Exchanger (DCBHE). The software COMSOL Multiphysics 5.4 is used for the reconstruction of the well design in 3-D. The model is based on the former gas production well Grouw-01, which has been in the shut-in phase for over 5 years. For such systems, the design conditions need to be optimized as these conditions can help increase the heat extraction. The different properties that can influence this are the type of working fluid, the material and thickness (insulation material) of an inner-tubing for fluid circulation, flowrates, outer-well size and injection temperatures. Finding an ideal design condition is the main goal. The software allowed an intense sensitivity analysis on the properties which can influence the heat extraction. The outcomes of the different sensitivity analyses show that
-The heat capacity of the working fluid is critical for the amount of power that the fluid delivers. Working fluids with high heat capacity deliver a higher power output compared to the working fluids with low heat capacity. However, a working fluid with low heat capacity normally delivers a higher production temperature. -The flowrate in which the fluid circulates, has an optimum limit in which fluid cooling is minimum during upward flow and heat extraction during downward flow maximum. -Insulation material of VIT (Vacuum Insulated Tubing) is crucial for the system to function, the properties of the VIT play a prominent role in this. A material with similar thermal properties can function as a replacement for the VIT. -Increasing the thickness of the inner-tubing (VIT) delivers a higher power output. This power increase is small compared to the increase of inner-tubing thickness. -The outer-tubing has similar effects on the power output. If the size of the outer-tubing is doubled there is a small increase in power generation. -The simulated different well depths show a linear relation of the temperature, power output and depth of a well. -Injection temperature of the working fluid should remain low for higher efficiency. -Simulating a reversed working fluid cycle, in which the fluid is injected through the inner-well and produced through the outer-well can cool down the working fluid if the flowrate is low. Based on the amount of energy that the Grouw-01 well can provide to the energy requirements of households, a system of heat supply and demand is created for an economic analysis. In the economic analysis the value of the different materials, project costs and revenues from this geothermal project are analysed in three different cases. The costs involved are divided in capital costs, operational costs and a government take in the form of taxes. The results of the economic analysis showed that the project is financially viable, unless there is an increase in the OPEX or if the project load hours are drastically reduced. In this thesis, based on sensitivity studies and the literature review, an efficient DCBHE is introduced. In addition, the economics will give an insight into the financial viability of this efficient system. This thesis provides information for geothermal engineers, well engineers, researchers and whoever are interested in alternative energy methods. ... Read more

Geothermal energy is one of the more sustainable alternatives to fossil fuels that could facilitate the energy transition. The production of geothermal energy incorporates the use of corrosion inhibitors to protect the steel well-parts from the corrosive production fluid. The injection of inhibitors leads to a risk of exposing the reservoir rock to these solutions. Up until now no data has been published to describe the effects of the corrosion inhibitor on the mechanical reservoir rock properties. This thesis aims to remedy this situation by performing compressive triaxial experiments on rock samples which have been saturated with inhibitor solutions. Two rock types which are representative of geothermal reservoir rocks were tested using two different inhibitors. The results were compared to a set of control experiments performed on water-saturated samples. The sandstone experiments show no discernible difference between the different fluid-types. The limestone experiments do show a number of differences: The limestone samples that were saturated with the first inhibitor type saw an increase in rock cohesion and a decrease in internal angle of friction. In contrast limestone samples that were saturated with the second inhibitor saw a decrease in rock cohesion and an increase in internal angle of friction. The lack of difference between the sandstone samples could be attributed to the inert nature of the constituent minerals. Limestone minerals are chemically more reactive and even though the exact nature of the chemical processes which lead to the observed differences remains unclear, a number of potential explanations are provided. We suggest that the change in mechanical rock parameters in the limestone rock are a result of the adsorption of the inhibitor components onto the particle surface. This adsorption alters the particle surface charge which leads to a change of the electrostatic repulsive forces. Additionally the same adsorption may affect the interparticle friction of our rock samples. ... Read more

In a future scenario, the successful development of geothermal industry will result in the large-scale deployment of new deep geothermal projects. In highly populated areas, such as the Netherlands, such a development will lead to a dense grid of neighbouring licenses. In such a situation a new project requires careful design and planning as the available subsurface space can become scarce, leading to the potential thermal interference with the neighbouring licenses and competing usage of the resources. Interference can cause the reduction of lifetime, produced energy, and the profitability of neighbouring projects. Therefore, it becomes important to consider the thermal interference while designing these systems in such dense areas to be able to obtain maximum energy and profit without hindering neighbours. This research project assesses the question of thermal interferences in neighbouring geothermal systems using a numerical simulation approach regarding what parameters are ideal for optimisation in neighbouring systems and what parameters should be monitored. To generate a numerical model capable of describing and predicting the effect of thermal interference the software package COMSOL Multiphysics 5.4 was used. The physics of fluid flow and heat transfer in porous media is applied to the reservoir model, and the finite element method is used to approximate the solution of these equations. Sensitivity analyses are performed on all input parameters as a post-processed simulation to control their influence on the output performance. The input parameters are divided into two categories; first, operational-controlled parameters: start time, injection temperature, injection/production flow rate, well spacing, well distance to the license border, and second, natural-controlled parameters including permeability and its anisotropy as kx/ky ratio. At the end of the parametric sensitivity analysis, an economic model is used as an instrument to evaluate how these input parameters can affect the long-term project feasibility. In this study, we present the results of global comparisons between each parameter while incorporating all measurement control (lifetime, cumulative energy and NPV). The results from the 3D qualification, the correlation between measurement controls and the reference base case study are used as background for this comparison. Our results show that the injection temperature and well spacing are ideal designed parameters to optimise profitability because the negative effect on the neighbour is only 1% for every 10°C reduction injection temperature and increasing 300 m of well spacing. On the other hand, injection / production flow rate is the most influential parameter that must be monitored in neighbouring production areas. ... Read more

This study investigated the three possible main causes of injectivity reduction in geothermal wells (suspended solids, scaling, biological activity). That has been done by obtaining water samples at three operating geothermal doublet locations in the Netherlands. This paper contains new geothermal data and the interpretation of them. At the moment, injectivity reduction is the biggest and least understood issue in geothermal projects in the Netherlands. Investigating this problem is valuable for current and future geothermal injectors and essential for the upscaling of geothermal energy in the Netherlands.The three locations differ from each other in several aspects (reservoir, surface facilities, chemicals in use, injectivity reduction), which yields a good comparison. Water samples were taken at different moments in time, on different points in the geothermal systems and for different purposes: TSS, chemical composition and ATP concentrations. The data is obtained in the laboratory and used for scale analysis (using the Stream Analyzer software from OLI Studio®) and microbial kill tests to determine the influences on growth. Scientific quality and data reliability are guaranteed by multiple pre-experiments (e.g. to verify sterility of sample bottles) and extra measurements (e.g. duplo samples, oxygen concentrations). It has been found that TSS is reduced through the systems by filters (from 35-55 mg/L in production water to <8-16 mg/L in injection water). Also, scales can potentially form (calcite before- and barite after the heat exchanger). Furthermore, biological activity in the water is initially low, but can grow over time to high levels (>500.000 microbial equivalents). Microbial growth is site-specific and correlatable to injectivity reduction. When there is microbial growth, corrosion inhibitor (CI) enhances this growth. Scale formation is possible, but has a limited likelyhood to cause significant injectivity problems. Furthermore, there is no reason for TSS to be problematic, but it can also not be excluded as possible source for injectivity reduction. Mitigation of these issues can be done by using more injection filters in a geothermal system (for further TSS reduction). Scaling inhibitor can be used preventive and a CI without nutrients (in the solvent) should be used, or a biocide should be injected simultaneously with the CI. This report is interesting for geothermal operators, researchers and anybody else interested in geothermal operations in general or more specifically in injectivity reduction. ... Read more

As the demand for green energy is growing, geothermal energy will play an important part in the future energy mix. Geothermal energy is widely used for both direct-use and electricity generation. A viable geothermal reservoir must have a sufficiently high temperature, a fluid pathway through the rock (permeability) and a fluid that can collect and transport the heat to the surface. Deeper reservoirs often have high temperatures and a low permeability, whereas shallow reservoirs often have a high permeability but low temperatures. When the permeability is too low, the reservoir can be stimulated. This is conventionally done by fracking the reservoir rock; that is, creating fractures to enhance the flow. This is a very expensive and potentially hazardous operation. Another technique is Radial Jet Drilling (RJD), which uses a high pressure focused fluid jet in order to drill small diameter horizontal holes (laterals) from the vertical borehole. Laterals can be drilled in multiple directions of the borehole and can be as long as 100 metres, potentially increasing the production 3-8 times (Blöcher et al. 2016). However, the destruction mechanism of rocks during jetting is not yet fully understood. Experiments and simulations were conducted in order to investigate the destruction mechanism of a rock during jet impingement. First, experiments and simulations were carried out which had the aim to find a relation between stagnation pressure (exerted pressure on the rock by a water jet) and jetted cavity depth. The experiments provided information about the maximum jettable depth (with a static nozzle), while the simulations provided a continuous pressure depth relationship. Combining the simulation and experimental results leads to the conclusion that pressure fluctuations are key for the destruction process, which is consistent with the theorem of hydraulic fracturing. Furthermore, a Finite Difference Method (FDM) solver was developed in order to investigate the pore pressure inside a porous rock during jet impingement. The solver was used to investigate (1) the difference in pressure between the first layer of pores (adjacent to impinged area) and the fluid pressure of the jet and (2) the influence of an increased pore pressure in the whole sample (i.e., back pressure). It became clear that the pressure fluctuations in time between the pores and jet are significant and sufficient enough to induce hydraulic fracturing. Increasing the back pressure resulted in higher pressure fluctuations, but only when the back pressure is higher than the average jetting pressure. There are still some doubts in the literature in whether or not cavitation erosion is the governing destruction mechanism. Cavitation is the formation of vapour bubbles in a fluid due to fluid pressure drop below the vapour pressure. Once the bubbles travel to a higher pressure regime, they implode, creating either a shock wave or a mini jet, resulting in erosion of material. Hahn et al. (2019) and (Kumagaiet et al. (2011) evaluated the location of cavitation erosion on a flat surface and concluded that cavitation erosion is not the governing erosion mechanism as there was no damage at the impinged area. But those studies did not prove that cavitation erosion does not occur inside a hole. Experiments and simulations were therefore performed to investigate cavitation erosion inside a cavity. It is found that cavitation erosion does occur inside a cavity, even at cavity depths of >25mm. However, experiments on one of the rocks showed that the maximum jetting depth was 25mm. From these results, it was concluded that although cavitation erosion occurs inside a cavity, it cannot be the governing destruction mechanism. Based on the results in this study, it is concluded that hydraulic fracturing is the governing erosion mechanism, because of the proved dependency of destruction on pressure fluctuations and the disprove of surface erosion due to shear forces (in literature, Buset et al. (2001)) and cavitation erosion being the governing mechanism. Another, in literature, proposed mechanism is pore-elastic tensile failure, which is partly interconnected with the theorem of hydraulic fracturing and not further investigated in this study. Based on this conclusion, a 'jetting correlation' was developed which is able to predict whether a rock is jettable or not. Combined with the pressure-depth relationship, it can also predict the maximum jetting depth. The correlation matched with the experimental results, which is another indication that hydraulic fracturing is the governing destruction mechanism. ... Read more

For this thesis, a probabilistic techno-economic model is developed for deep, direct use geothermal projects in the Netherlands. As a case study, data from the future DAP doublet (TU Delft) is inserted wherein multiple scenarios are researched that model variations in well capacity, energy prices, project design, government policies, brine gas content and heat demand available. For the case study, the model considers technical and economic uncertainties over a period of 50 years. These uncertainties are incorporated in the form of probability distributions and 15000 Monte Carlo iterations. By modeling these scenarios, an overview is developed of the economic performance of a deep district heating project based on the Net Present Value (NPV), Levelized Cost of Heat (LCOH), Internal Rate of Return (IRR), Profitability Index (PI) and payback time. Additionally, an evaluation of the effectiveness of the Dutch subsidy scheme is researched combined with the main financial obstacles associated with geothermal exploitation. The reference scenario of the case study resulted in a 90% probability of a positive NPV with a median PI of 1.5. Higher returns are delivered from scenarios where the subsidy policies were altered in a way that they are more suitable for district heating purposes. A scenario where high constant demand was simulated combined with an extra initial investment for the purpose of heat storage, resulted in the best performing scenario overall. Considering the demand available for this case study, the fixed operational cost, SDE base sum and gas/water ratio are the most important parameters in terms of NPV sensitivity. In order to achieve the desired growth of the Dutch geothermal sector, focus should be aimed at the realization of sufficient heat network deployment, heat storage options and more project specific subsidies. ... Read more

Geothermal energy is a relatively sustainable energy source of which the essence is to extract heat from hot subsurface rocks. Circulating fluids serve as the transport agent of heat. The contact area between the fluids and the rocks is where the relevant heat transfer occurs, i.e., where the water is heated up. In some geothermal reservoirs, this circulation occurs naturally through porous matrix (mainly sediments) or through heavily fractured formations. Enhanced geothermal systems (EGS) are potentially favorable reservoirs where subsurface permeability is increased by means of artificial stimulation techniques. These stimulation techniques often involve hydraulic fracturing where a limited existing fracture network is expanded or “enhanced” by injecting fluids under high pressure conditions. While the geometry of the generated fractures influences permeability, the effect on heat exchange has received less attention. This thesis discusses the effects of fracture geometry on the heat transfer between solid and fluid. Along with laboratory experiments, numerical simulations were conducted. All investigations were performed on igneous granite rocks. Tensile fractures were generated to allow a fluid flow along the otherwise impermeable rock samples. Several parameters were varied throughout the experiments and simulations including volumetric flow rate, fracture aperture, rock temperature and fracture geometry and surface area in order to investigate their impact on heat transfer processes. Flow rate variations in the experiments have shown that higher flow rates cause the fluid to absorb less heat per unit volume and cause the rock to cool down more extensively, therefore thermal depletion of the reservoir is likely to occur within a shorter time frame. The dependency of exchanged heat on fracture aperture variations (in the range of 0.05 to 0.5 mm) did not yield a clear trend within the experiments, but does so in numerical simulations. Aperture variations in the numerical simulations did not cause notable differences in transferred heat as long as the volumetric flow rate is kept constant. However, as the fluid velocity is kept constant the amount of fluid flushed along the fracture per unit time is affected by varying apertures. This causes a difference in heat transfer as well. Increased fracture surface areas alone (more extensive topology/roughness) have shown a minimal impact on the heat production while a more extensive fracture network (additional branches) has shown notable enhancement in the amount of heat produced. Cooling behavior of the rock has shown correlations with Newton’s law of cooling and suggests a limitation of heat production by the heat conduction occurring within the rock.
Experimental findings cannot directly be compared with natural reservoir conditions. The reason for this is a thermal equilibrium that is achieved at each flow experiment, i.e., the heat withdrawn equals the heat resupplied by a heater. In natural reservoirs this is often not the case where a cold front propagates towards the production well and determines the lifetime of how long heat can efficiently be produced from a certain rock mass. This results in an unsteady heat conduction where the heat withdrawn does not equal the heat resupplied. ... Read more

To develop high enthalpy geothermal systems in the Netherlands, reservoirs with a depth greater than 4000 m need to be targeted. The Lower Carboniferous limestone is proposed as the main target due to its depth and wide occurrence. To investigate the potential of this formation, the physical rock properties are studied in a variety of laboratory experiments. This study consists of two parts: (1) the geomechanical and hydraulic characterization of the intact rock and (2) the permeability evolution of a fracture altered rock.
Analogous rock samples from a quarry were used to determine tensile, compressive and shear strength and the elastic moduli. Further rock properties such as porosity, microstructure and mineralogy were measured by a gas pycnometer, mercury intrusion porosimetry and electron microscope analyser. The permeability was determined by flow-through experiments at different effective pressures. Subsequently, shear fractures were generated by triaxial deformation and a punch-through shear test to investigate the effect on the permeability.
A high degree of heterogeneity between the samples and a clear transverse anisotropy was observed. The maximum intact rock permeability is 150 μD at low effective pressures (2 MPa) and at reservoir conditions the permeability is below 1 μD. This results in an insufficient matrix conductivity and enhancement of the reservoir is needed to improve the reservoir connectivity. Both shear experiments show an increase in permeability of one to four orders of magnitude. The fractures show to be sustainable in terms of time and pressure variations. In combination with its physical rock properties, it can be concluded that the application of hydraulic stimulation within the Lower Carboniferous limestone is a reliable technique to improve the reservoir permeability.
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This research identifies and provides a relative ranking of the parameters that control the thermal breakthrough time of a geothermal doublet. The ranking is based on simulations modelled by a three-dimensional model build in COMSOL Multiphysics with a simulation duration of 50 years. The model is a nonisothermal, isotropic, cuboid consisting of a sandstone aquifer surrounded by identical impermeable layers. The ranking is derived based on a solution space covering a minimum, maximum and mean value for each of the simulation parameters. The investigated simulation parameters are the density, porosity, permeability, thermal conductivity and specific heat capacity of both the surrounding rock formations and the aquifer, the depth and thickness of the aquifer, the flow rate, injection temperature and the well spacing. The mean values of the solution space form the base model, from which the parameters will divert separately to monitor the model’s behaviour on the varying parameters.

In this research, the breakthrough time is defined as the time at which the production temperature is decreased by 1% to 99% of its initial value. The ranking is based on comparing the proportional change of each parameter from the base model to the change in breakthrough time. The results show that the thickness, flow rate and the well spacing are the most crucial parameters influencing the thermal breakthrough time of a reservoir. Overall, the flow rate has the greatest impact, with a decrease of 16.7 years between a flow rate of 150 m3/hour and 250 m3/hour.

In addition, the surrounding rock parameters have a notably smaller impact on the thermal breakthrough time compared to the reservoir rock and process parameters. The surrounding rock parameter with the relatively largest impact on the breakthrough time is the specific heat capacity, which is only a change of 0.2 years between a specific heat capacity of 1150 and 1250 Jkg-1K-1. ... Read more

With an increased demand for renewable energy sources, geothermal developments have moved into the spotlight, including sedimentary structures. In the western part of the Netherlands, cretaceous sandstone bodies are already in use for the geothermal energy production. However, these reservoirs are generally producing water at temperatures less than 100°C from depths of 1500 to 3000 m. To reach higher temperatures, deeper reservoirs need to be targeted. Being more than 130 m thick in some parts and up to 4000 m deep, the Lower Triassic Buntsandstein would provide an ideal target reservoir for geothermal exploration in the West Netherlands Basin, given the high temperatures of over 140°C at this depth. However, the Buntsandstein is relatively heterogeneous in terms of porosity, as seen in numerous rock samples, and the overall permeability that was found in the Buntsandstein in various wells was generally low. Therefore a conventional development of a Buntsandstein reservoir, with a geothermal doublet in a natural hydrothermal reservoir, might not be applicable. To enhance the permeability and achieve economic flow rates, hydraulic stimulation can be performed. This technique has proven to be effective in many other reservoirs. To assess such effects on the Buntsandstein, this thesis focusses on the effect of fractures on the fluid flow in the Buntsandstein. Geomechanical characteristics and physical properties of the Buntsandstein at relevant pressure and temperature (PT) conditions are investigated in core experiments in uniaxial compression tests and triaxial tests. The laboratory experiments on cores provide reliable intrinsic information of permeability at relevant PT conditions and give a realistic estimation of fluid flow parameters. Triaxial deformation tests were conducted on different sandstones varying in porosity, clay content, degree of oxidization and initial permeability. Changes in permeability before and after deformation are compared. Moreover, the effect of increasing confining pressure on the permeability is measured on intact sandstones. Two principal modes of brittle fracture (extension fracture and shear fracture) were tested on samples under confining pressure. Experiments revealed that the permeability decreases by a factor of 17 in porous sandstones, from 85 mD to 5 mD, when inducing shear fractures at 30 MPa effective pressure. Buntsandstein samples with a low initial permeability (~1 mD) show a slight increase to 2 mD. For tensile fractures the permeability increased exponentially with the increase in apparent fracture aperture. Up to apertures of around 1000 µm, the permeability increased by factor of 2 for increments of 100 µm. Ultimately, effects of fractures and resulting changes in flow properties are analyzed for the impact on the production life of a geothermal injector-producer pair. Results from lab experiments are implemented in a field scale thermo-hydro-mechanical model, making it possible to predict the PT distribution in the reservoir in regard to a change in permeability. The laboratory experiments and the numerical modeling procedure indicate that the in-situ permeability is too low for a geothermal reservoir production and would require some sort of stimulation to enhance the permeability. ... Read more