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.

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.

Geothermal energy in the Netherlands is growing in general, however, medium deep geothermal (200 - 1500 m depth) is lagging behind while the potential at these depths could be very high. Development is held back by little geologic knowledge of this interval as well as legislative constraints. Better understanding of the reservoir characteristics and well design in this depth range is necessary in order to accelerate the heat transition away from fossil fuels. The Breda Formation is deemed as one of the most promising formations within the medium depth range. Therefore, this formation, particularly the part found in the Zuiderzee Low (ZZL), is the focus point of this thesis. The reservoir characteristics are determined using newly obtained cutting information, novel biostratigraphic analyses, old and new seismic interpretations and petrophysical log data, eventually leading to a static reservoir model. Consequently, the static model is upscaled and used as input for dynamic simulations in the Harderwijk case study area. The static model provides new thickness, depth, porosity, permeability and transmissivity maps of the Breda Formation in the ZZL. These maps were generated using more data (types) than the current ones present in the REGIS model, and should therefore contain less uncertainty. The dynamic model provides insights into doublet lifetimes and flow rates for three different well trajectories in two possible reservoir scenarios. The horizontal trajectory proved superior in terms of doublet lifetime and flow rate for both reservoir scenarios. Finally, the current Dutch legislative framework covering the medium depth is limiting medium deep geothermal development in the Netherlands. A revision of the 500 m depth boundary separating the Mining and Water Law jurisdictions should be considered if the full potential of the medium depth is to be accessed.

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.

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.

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.

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.

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.

Huge amounts of heat are stored in sedimentary aquifers in the Dutch subsurface. The amount of heat would be sufficient to provide our national heat demand for decades without any greenhouse gas emissions. Exploitation of this type of resource started some 10 years ago in the Netherlands. In 2016, 16 geothermal doublet systems had been installed that produce geothermal heat, and each year 2 to 3 new systems are realised. A doublet system consists of a production well that extracts hot formation water from kilometer deep aquifers. After the heat is extracted from the water in heat exchangers, the cooled water is reinjected into the same aquifer at approximately 1 to 1.5 km distance from the production well. Most of the current Dutch doublet systems provide heat for the horticulture sector. These systems have an average net energy production of approximately 10 MWth and therefore hundreds of additional systems are required to significant amounts of our heat consumption with geothermal energy. This PhD thesis investigated doublet system design and deployment strategies to optimise exploitation and increase the possible number of doublet systems exploiting the same aquifer. Based on detailed geological models, subsurface flow simulations are used to evaluate parameters such as required injector-producer distance, the preferred orientation of a well pair with respect to geological trends and required doublet distance to avoid negative interference. Based on the results, regional doublet deployment strategies can be developed to make optimal use of geothermal heat from sedimentary resources.@en

The Earth’s subsurface exhibits a high potential for generating and storing energy. Engineered fractured systems, for example geothermal or carbon storage reservoirs, highly depend on the capacity of rock to conduct and store fluids. Faults and fractures create the largest contrasts in flow in these reservoirs and can enhance the reservoir potential when being generated or engineered. While the scientific focus is mainly on the effectiveness of enhancements and the risks associated with them, the sustainability of these enhancements must be better understood. In this thesis, the dependence of fracture permeability on a variety of parameters is studied. The aim is to develop a better systematic understanding of the hydro-mechanical processes controlling the potential and sustainability of fractures to conduct fluids at a variety of conditions. Several parameters that are assumed to control fracture permeability are considered in laboratory experiments. These include the rock type (clastic vs. crystalline), the fracture type (shear vs. tensile), the fracture geometry (aperture and roughness) and effective stress changes (pore and external stress). Potential geothermal rocks are considered in order to directly relate the findings to potential geothermal exploration projects. The results demonstrate the complex dependency on a variety of parameters and highlights the different physical processes depending on mainly rock and fracture type. An attempt was made to assess the potential of fractures to act as fluid conduits in reservoirs, as well their hydraulic sustainability during effective pressure changes. From these results, general implications are made concerning the ability and sustainability of fractures to conduct fluids depending on rock and fracture type. The main controlling parameters are assessed and possible mitigation strategies are developed to reduce the risk of permeability losses. Generally, only reservoir enhancement strategies resulting in a sustainable productivity increase can guarantee the scientific and political breakthrough of geothermal energy supply. @en

In 2020, as the world Energy demand keeps on rising (International Energy Agency (IEA), 2019), and with the global climate warming a reality (The Organisation for Economic Co-operation and Development (OECD), 2020), reducing our societal impact on Earth is of utmost importance. Energy and Climate have always been intrinsically related. Therefore, solving the Energy-Climate problem is a challenge where not one but several solutions should come together. Part of this global solution is the potential of geothermal resources. Geothermal energy is a renewable energy resource which has large potential to reduce the dependency on fossil fuels. Within the several uses of geothermal resources, a promising technique is titled Enhanced Geothermal Systems. More than renewable, this method has the potential to be sustainable. EGS consists of an originally low permeability reservoir rock that is artificially enhanced. The enhancement can be achieved by different stimulation techniques, such as mechanical, chemical, thermal or a combination of all. This thesis focuses on the mechanical EGS stimulation, where opening of existing fractures and creation of new ones is achieved by injecting a pressurized fluid in the reservoir rock formation. Such a process results in propagating a hydraulic fracture. The complexity of the EGS technique stands in predicting the hydraulic fracture propagation phenomena. EGS research and development is part of the GEMex goals. The GEMex project is a collaboration between Mexican institutions and the European Commission, dedicated to the development of non-conventional geothermal techniques. The Acoculco geothermal field, located in Puebla, is foreseen as a potential EGS. Because this field has been explored with two geothermal wells, and because an analogue exhumed system is available nearby, in the LasMinas area, this systemconstitutes a great research site for developing knowledge on EGS. @en

The main objective of this thesis is to understand the environmental risks related to geothermal operations. The aim is to provide an integrated approach from both natural and social sciences and to perform this in three different country settings. These countries are Indonesia, Turkey and the Netherlands and they were selected because of their different geothermal system types. The integrated approach in this study results in a research process that requires a broad range of measurement and analysis techniques and a clear understanding of the natural and social disciplines. From the natural sciences approach this report studies the environmental risks through a geochemical characterization of the geothermal fluids, whereas the social sciences approach studies the risk perception on geothermal operations. The outline of the process followed is visualized in Figure 0.1. From the natural sciences approach, an extensive geochemical data set is used to characterize the geothermal fluids and their environmental risk. This contains approximately 750 sample measurements from three countries that were collected through partners and third parties. The samples are characterized with help of two ternary diagrams (Na-K-Mg and Cl-SO4-HCO3), their salinity (in TDS), pH and correlation coefficients between commonly occurring elements in fluids, such as Ca, Si and F. These fluid properties help to define the maturity, origin and characteristics of geothermal fluids at a broad range of locations in each of the three countries. Through this analysis we found a strong relation between the fluid classification and the sample type (well or spring) in Indonesia. Besides, a relative high influence of volcanic activity on the geochemistry was observed. The Turkish and Dutch samples are dominated by their geothermal system types that are respectively carbonatic and clastic sedimentary systems. The concentrations of nine toxic gases and elements dissolved in the geothermal fluids are analysed and compared to guideline values to determine the relative environmental risks in the three countries. The toxic gases are H2S, CO2 and CH4 and the toxic elements Al, As, Cd, F, Hg and Pb. Their effects on the environment differ but all of them affect the health of humans, flora and fauna when they contaminate groundwater or the atmosphere. From the risk analysis in the three countries it was found that the risk of excessive H2S pollution is highest in Indonesia, for CO2 in Turkey and for CH4 in the Netherlands. The contamination risk with toxic elements differs largely per element but often occurs in specific locations, for example with high volcanic impact. For the social sciences approach, a survey was distributed to measure how the public perceives the risks of geothermal energy. The population for this survey is people affiliated with the geothermal industry because of their prior knowledge on the subject. They are asked to indicate their (risk) perception through several statements and factors to which they indicate their level of agreement. The result indicates that the perceived risks were generally higher in countries with a higher risk, like Indonesia. However, there were several interesting exceptions to this rule in which the perceived risks were low whereas a relatively high environmental risk was identified, like for potential CH4 pollution in the Netherlands.

In a world transitioning towards renewable energies and lithium-battery powered cars, there has been an annual increase in demand of 6% for lithium and other rare metals between 2000 and 2008 (Stringfellow & Dobson, 2021a), and this increase in demand is forecast to continue through 2040 (Latnussa et al., 2020). Meanwhile, there is a concern that supply will not be able to keep up with demand. Most of the lithium in the European markets comes from other countries such as Chile and Australia, making the European lithium supply vulnerable to supply chain disruptions, as was observed during the COVID-19 pandemic (Latnussa et al., 2020). In addition, the economic value of lithium has increased more than threefold compared in 2022 compared to 2017 (Latnussa et al., 2022). The geological information of Northern-Western Europe at the latest stage, gives an explanation to the occurrence of lithium in the Netherlands. 50 water samples of 13 fields are considered for the chemical composition and origin of their brine. All geothermal brines contain dissolved lithium and other metals from the reservoir formation and surroundings, allowing for metal ‘mining’ in an environmentally friendly manner. The highest lithium concentrations in aquifer brines are in the province of Drenthe and Limburg, specifically in the Akkrum-13 gas field (47-48 ppm) and the Californie Geothermie (20-28 ppm). The correlation is positive only for lithium against rubidium and lithium against the vicinity of volcanic intrusions, from mineral mobilization through groundwater flow. There are no specific formations with high lithium concentrations. In Germany, construction has started on a project to extract lithium from geothermal process water, indicating that it is economically viable (Wedin, 2022). For the Netherlands there are 5 different surface options to extract the highest lithium concentrations. Reverse osmosis, nanofiltration, ion-exchange, sorbents and electrodialysis can be used as direct lithium extraction methodologies. One potential economical method to extract lithium from geothermal brines is by utilizing two centralized lithium extraction plants. A plant for the direct lithium extraction with ion-exchange resins, placed before injection wells, and one for the refinement of the material.