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.

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.

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 the energy transition from fossil fuels to less polluting renewable energy sources geothermal energy are considered as a promising technology. Gasses such as CO2 are often dissolved in geothermal waters. With the extraction of these fluids from the reservoir, a change in pressure will occur towards the extraction well which may cause dissolved gas to exsolve. The exsolved gas may clog the pores of the reservoir rock near the extraction well and therefore reduce the effective permeability, which can result in reduced production of geothermal waters. This project is aimed at experimentally investigating the conditions of the onset of the degassing process and what the influence of the degassing process is on the permeability. Therefore Bentheimer and Berea sandstone core flood experiments were performed, using either tap water or brine with different CO2 concentrations (0.2 1.3 mol/L) and at temperatures between 30 and 90°C. Flow rates were varied/kept constant, between 90 2 bar and between 5 and 22 bar respectively.

At 30 °C and up to 50 bar the onset of the degassing process is controlled by Henry’s law, i.e. it is governed by the solubility of gaseous CO2. The onset of the degassing process is not influenced by the pore size and initial permeability. At these conditions the effective permeability decreases by a factor 2 to 5 in the Bentheimer sandstone core and by about a factor 10 in the Berea sandstone core. This change in effective permeability is gradual in the Bentheimer sandstone while in the Berea sandstone the change is nearinstant. For rocks with small pore sizes and low initial permeability, the reduction in effective permeability is larger and the rate of permeability decrease is faster.

Experiments at temperatures between 30 and 90°C show that with increasing temperature Henry’s law becomes increasingly inaccurate to find the onset of the degassing process. The onset degassing pressure increases with temperature but are significantly lower than Henry’s law. This inaccuracy can partly be explained by the fluid in the core not reaching the right temperature or by the values used for extrapolating Henry’s law for different temperatures. In experiments performed with a 1M NaCl brine the pressure at which the degassing process starts is higher than for experiments performed with tap water. The increased salinity does not influence the change in effective permeability due to degassing. The reason for this is that the change in interfacial tension between CO2 does not change sufficiently between tap water and Brine to cause differences in the degassing process.

The results obtained from this research can be used for successful management of geothermal projects. It shows at what conditions the degassing process starts and what the effect of this process is. Once the conditions are known they can be avoided. The research also shows the degassing process and its effects can be reversed.

The main objective of this thesis is to assess the combined influences of specified reservoir conditions and operational parameters on the profitability of a geothermal project and on the potential for fault reactivation. The aim is to propose potential development strategies to maximize the profitability and minimize the potential for slip and reactivation of pre-existing critically stressed faults. The reservoir conditions of interest in this study include the fault permeability, the fault throw and the friction coefficient of the sandstone. The operational parameters of interest are the flowrate, the injection temperature of the re-injected water and the distance between the wells and the fault. As a case study a simplified homogeneous 3D box-shaped reservoir model is simulated based on the Delft Sandstone Member in the West Netherlands Basin, using the Delft Advanced Research Terra Simulator (DARTS). Reservoir production data and local pore pressure data are generated with DARTS and serve as the input data for the fault stability model and the economic model, which are both built in Python. The fault stability model is built based on the method of Mohr circles, regional stress values in the Delft area and the failure criterion of sandstone and allows to assess the fault slip tendency of a pre-existing fault. The economic model is based on the Dutch fiscal system and policies and includes the costs of the phases of a geothermal project and required energy calculations. The outputs of the model allow to assess the profitability of a geothermal project based on the Net Present Value (NPV). Outcomes of this study have shown that the profitability and the fault stability depend highly on the joint influences of specific reservoir conditions and operational options. Sealing faults generally have a negative influence on both the NPV outcomes and the fault stability as the presence leads to decreasing heat production, higher pumping costs and higher pressure build up near the fault. This influence is strengthened when the wells are placed close to the fault, while it is reduced by placing the wells far from the fault. The flowrate and the used injection temperature are found to be the most important operational options, regardless of the reservoir conditions. Combining the highest possible flowrates and the lowest possible injection temperature maximizes the NPV outcomes. As it is found that NPV outcomes increase by a factor 6 when increasing the flowrate 2.5 times and decreasing the injection temperature by 5 K may increase the NPV values up to 19\% to 43\% depending on the flowrate. With respect to the reservoir conditions the study has shown that the fault permeability and the sandstone friction coefficient are the most important influencing reservoir conditions, compared to the fault throw. The risk for fault instability increases with decreasing value of the friction coefficient and of the fault permeability. With respect to the operational options the potential for fault slip is minimized when the lowest possible flowrate is combined with the highest possible injection temperature. However, the use of a 5 K higher injection temperature allows the use of a 600 m3/day higher flowrate. Placing the wells minimally 200 m from a fault in the homogeneous reservoir and using a minimum flowrate of 7200 m3/day maximizes the NPV outcomes and fault reactivation is reduced as much as possible. The assessment of the fault stability and the profitability is however very sensitive to the reservoir conditions, which is explicitly found from the results comparing a homogeneous and a heterogeneous reservoir. This makes the potential development strategies extremely prone to heterogeneity effects and subsurface conditions which makes them highly dependent on locations specific properties. Though, the general influences of the reservoir conditions and operational options on a heterogeneous reservoir are similar to those found for the homogeneous reservoir.

Transition to cleaner energy while sustaining the heat demand necessity has always been an emphasized topic in the recent years. With the slowly declining utilization and production of natural gas in Netherlands, geothermal energy shows a promising future in providing sustainable power for heating and various applications. For majority of economically feasible geothermal project, either natural fractures or hydraulic fractures are present. These fractures served as a preferential fluid pathway between geothermal well couplets, which determines crucial parameters such as breakthrough time and project life. In this study, the main focus is the impact of stochastic distribution of fracture aperture with or without pressure dependence and gravity effect on geothermal energy production. The simulation conditions as well as reservoir physical model are collected from analogous enhanced geothermal systems (EGS)that are classified as deep geothermal energy projects. The test cases include base case, case with no pressure or stress dependence, case with half well doublet distance, and case with gravity that has well positioned in the dip or strike direction in relative to the fault orientation. The ensemble simulation of 100 realization is run for each scenario considered. The relevant parameters analyzed are pressure difference between well couplet, production temperature, NPV, and most importantly, net cumulative energy production. It is found out that the geothermal energy production greatly relies on the fracture aperture distribution with or without pressure dependence. For base case, the range of energy production is from 1.50E13 to 3.75E13 KJ with temperature variation of 35 K between the minimum and maximum value. Without pressure dependence, only pressure difference between injector and producer slightly changes, and production temperature as well net energy pro-duction remains nearly identical. For the case with half of original well-spacing, the spread of energy production distribution reduces from 1.50E13 to 3.25E13 KJ. The diminishing energy production results from less total enthalpy in the flow path with shorter distance, yet the outcome of energy production still demonstrates a considerable range. By taking account of gravity effect and populate the reservoir with non-uniform pressure and temperature distribution with corresponding gradient, the net energy production drops to approximately two times less than the base case. However, the ratio of maximum energy production to the minimum over100 realization is similar to that of the base case. Last but not least, two types of well orientation, along the strike and along the dip of fracture plane, are considered to observe the impact of gravity-assisted flow. It is concluded that with the current model setting, the low temperature gradient of injector in the dip configuration case overweighs the effect of gravity assistance, resulting in a smaller net energy production than the strike case. This project concludes that the fracture aperture heterogeneity is an important factor in predicting the outcome of deep geothermal production. The gravity is another crucial component to portray accurate flow behavior between the wells, but it does not result in additional increase in the range of energy production outcome in comparison to the base come with the simulation conditions applied.

The hot water produced from a geothermal doublet possesses energy, which once utilized, the water cools down and is re-introduced back into the same reservoir at a sufficient distance using an injector well. As cold water flows through the reservoir, it acquires thermal energy from surrounding in-situ rocks. This process recurs until a substantial drop in rock temperature occurs and the water is unable to be recharged adequately; as a result, cold water starts to "break-through" into the production well and the doublet soon needs to be abandoned.
This breakthrough time can be predicted using reservoir simulation. Significant work has been done in the past to determine the effect of different parameters on the breakthrough time and accuracy of models have been improved by incorporating real world physics. Despite being capable to predict breakthrough time, accurate high-fidelity 3D models require significant time in uncertainty quantification and data assimilation analysis due to CPU demanding simulations.
In this project, a physics-based proxy model is developed to predict flow and heat transport in low-enthalpy reservoirs. Streamlines that describe flow in a system and mostly controlled by steady-state pressure distribution are traced using Pollock's method (Pollock, 1988) and the reservoir is divided into streamtubes. Rock-heat depletion is modelled by semi-analytic model along streamlines. The objective is to predict geothermal doublet breakthrough time using only a limited number of streamtubes, thus minimizing simulation time and CPU resources.
Comparison with accurate high-fidelity model reveals that results for proxy model are optimistic; the error for pressure and temperature distributions, as well as the breakthrough curves is within the acceptable tolerance. The time required to simulate the proxy-model is less than the high-fidelity model. And as the number of streamtubes (to simulate the proxy model with) decrease, the time required for simulation further decreases but conversely the error between the breakthrough curves increases.