The presence of faults in the subsurface, crossing geothermal targeted aquifers introduces concerns related to induced seismicity. Improving the understanding of geomechanical processes due to geothermal operations is therefore important. Previous research has indicated that thermally induced stresses can have a significant effect on fault stability, especially in the long term. In depleting gas fields, the effect of fault offset has been studied and found to promote the onset of fault reactivation. However, little research has been conducted on this effect in geothermal reservoirs. This thesis will investigate the effect of juxtaposition of sandstone and claystone across faults on thermo-elastic stress changes and induced fault slip in geothermal projects targeting porous sandstone reservoirs.
This research question will be answered by constructing three different model geometries in a semi- analytical 2D plane-strain model assessing thermo-elastic stress changes and fault slip. The geology and properties of the Pijnacker Geothermie reservoir, located in the West Netherlands Basin and targeting the Delft Sandstone Member as its reservoir rock are used for the input of the model. The three geometries include a model scenario in which sandstone is fully juxtaposed against sandstone, a scenario in which sandstone is fully juxtaposed against claystone and lastly, a scenario with half-a-reservoir-thickness fault offset, juxtaposing sandstone against both sandstone and claystone. These geometries were chosen to represent a fault that experiences cooling on both sides, on a single side and on both sides, but with an offset. A comparison was made between the three model scenarios using base cases and evaluating the differences in stress changes and fault slip. A sensitivity analysis was performed in order to determine the key parameters influencing the results the most. Then, three metrics were extracted from the base cases and plotted against the varying key parameters. These metrics are: the temperature change needed to reactivate the fault, the maximum slip and the slip patch length. Lastly, heterogeneous friction was used in the model in order to assess its effect and make a step towards a slightly more realistic model. The results showed that the double sided cooling case with fault offset was the most destabilizing configuration, needing the least amount of cooling before reaching the onset of fault reactivation and also consistently experiencing the largest maximum slip. The single sided cooling model scenario is the most stable, however its slip patch length is the most sensitive to variations in key parameters, generating the largest slip length for destabilizing values of the key parameters. The inclusion of heterogeneous friction had a destabilizing effect on all scenarios. As the degree to which sandstone is juxtaposed against sandstone affects the amount of thermo-elastic stress changes and slip a fault experiences, it should be considered to include fault offset when assessing fault slip in geothermal projects. Not including fault offset will likely underestimate the thermo-elastic stress changes and subsequent induced fault slip. Panther can capture the effect of thermo-elastic stress changes in a first-order assessment on a simple reservoir geometry containing the essential parameters while avoiding extra model complexity. Further improvements to the model can be made to capture the effect of certain aspects that have been proven to affect thermo-elastic stress changes. These include property contrasts between the reservoir and surrounding rock as well as the addition of clay-rich layers in the sandstone formation and vice versa. Further geological work could be performed to decrease the uncertainty around the location of
the fault and its dip angle.

Addressing climate change and transitioning to renewable energy will involve subsurface activities like carbon storage, geothermal exploitation, and underground energy storage. However, fluid injection and extraction in the subsurface can alter the pressure, temperature, stress, and rock geochemistry, potentially leading to seismicity and subsidence. Understanding the mechanisms of fault reactivation and the geomechanical response of intact reservoir rock to variations in pore fluid pressure from injection and depletion operations is thus crucial. In this thesis, we integrate our findings from multiple studies to explore the impact of parameters related to injection and depletion, such as pattern (monotonic, cyclic) and rate, on the deformation of intact reservoir rock, slip behaviour in faulted reservoir rock, and the evolution of microseismicity, with a focus on how we can mitigate induced seismicity.

Our experimental investigations employ uniaxial compressive tests on intact Red Felser sandstone samples, subjecting them to cyclic recursive (CR), cyclic progressive (CP), and monotonic stress patterns at varying stress rates. The recording of Acoustic Emission (AE) waveforms revealed that cyclic stress patterns, especially CP, are characterized by lower maximum AE amplitudes compared to the monotonic pattern. By reducing the stress rate, the maximum AE energy and final mechanical strength both decrease significantly. Moreover, high-stress rates were found to alter the AE signature of events, suggesting that cyclic stress patterns combined with low-stress rates may mitigate induced seismicity in subsurface injection operations.

For underground energy storage, we investigate the geomechanical response of Red Felser sandstone to cyclic loading, crucial for safe and efficient underground porous reservoir operations. Experimental results, complemented by constitutive modeling, revealed various deformation mechanisms, including linear elastic, viscoelastic, and inelastic responses. Our study shows that the magnitude of inelastic deformations is influenced by mean stress, amplitude, and frequency of the stress waveform, with our models closely fitting the experimental data.

As part of our investigation into mitigating induced seismicity, we examine how stress and sliding patterns affect fault slip behaviour and seismicity evolution. To achieve this we carry out displacement-driven fault reactivation experiments on saw-cut Red Felser sandstones. Our results indicated that cyclic sliding, compared to continuous sliding, reduces seismicity but can accelerate slip velocity during the reloading phase due to the healing of gouge material on the fault plane. Additionally, under-threshold cycling effectively prevents seismicity and shear slip but poses a risk of increased seismicity if shear stress exceeds critical levels.

Furthermore, we explore the influence of injection pattern and rate on fault reactivation in porous Red Felser sandstone. High injection rates were linked to increased slip velocity and seismicity. Furthermore, our results from samples subjected to various injection patterns demonstrate that the cyclic recursive pattern exhibits a higher maximum slip velocity, more episodes of slow slip, and greater radiated AE energy than a monotonic pattern. A proper injection strategy must consider fault drainage, critical shear stress, injection rate, and injection pattern. Our results demonstrate that a monotonic injection pattern and low pressurization rate may mitigate seismicity on pre-existing faults in a highly permeable porous reservoir.

Finally, we investigate the fault slip nucleation within a displaced fault system. Our triaxial experiments on displaced faults reveal that differential compaction intensifies from the top of the sample towards the internal corner at the centre of the fault, indicating a variation in the stress field surrounding the fault plane. Our direct measurements near the displaced fault plane confirm the anomalies and peaks in stress observed in previous numerical and analytical studies.

This thesis offers new insights into the mechanical behaviour and seismicity evolution of intact and faulted reservoir rocks under variations in stress patterns and rates. These findings may contribute to mitigating injection-induced seismicity in intact and porous faulted rock settings. Furthermore, they enhance our understanding of the behaviour of deep geo-reservoirs subjected to diverse injection strategies, thereby expanding our knowledge of reservoir-related phenomena.@en

Induced seismicity refers to seismic events (earthquakes) triggered by human activities. Such events, even when characterized by relatively modest magnitudes, have the potential to jeopardize the safety of individuals, the surrounding environment, and infrastructure.

Production from hydrocarbon reservoirs can alter the in-situ stress state, leading to induced seismicity. This is reported in the Groningen field, where substantial gas production caused fault reactivation and subsequent earthquakes. Understanding events in the deep subsurface is crucial to proactively mitigate future seismic occurrences. To understand the causes of induced seismicity, the underlying physics are examined and defined in terms of relevant governing equations and models. This reveals the interconnected nature of fluid depletion, rock deformation, and fault slip. The goal of this study is to develop simulation techniques to solve these equations.

Towards this end, firstly, a finite volume embedded-numerical simulation method, called the Smoothed Enhanced Finite Volume method (sEFVM), is developed. This method is revealed to be computationally efficient for reservoir-scale modeling of heavily faulted systems and performed well in comparison to known solutions and other simulators.

However, in settings where analytical solutions indicated noncontinuous shear stress profiles, sEFVM accuracy suffers. Recognizing this limitation, a semi-analytical approach is developed, extending analytical expressions to be solved over the sEFVM mesh. This extension allows for more accurate solutions, accommodating complex reservoir and fault geometries. The semi-analytical method is successfully used to estimate the onset of fault nucleation and the magnitude of the seismic moment resulting from depletion.

The semi-analytical approach is limited to simulating fault slip up to the point of nucleation. To overcome this constraint, a hybrid method is developed. With appropriate assumptions regarding the post-nucleation state and the use of sEFVM to numerically calculate post-nucleation stresses, the hybrid method can effectively model multi-fault systems in the seismic stage assuming quasi-static behavior.

In summary, this research contributes by presenting novel computational frameworks for studying fault reactivation in faulted poroelastic media, offering insights into the complex interactions of the physics at play. The proposed embedded-numerical, semi analytical, and hybrid methods pave the way for further advancements in the field.
@en

A fundamental challenge in the successful implementation of large-scale energy storage (LSES) in the Netherlands is the required public support for underground activities of Dutch citizens. In this study the key considerations and perspectives regarding the (financial) participation, as a means to increase public support, were explored by conducting interviews among the key stakeholders of onshore underground energy storage projects. An empirically grounded, explanatory framework for understanding the role of perceptions of justice in the dynamics of controversy, introducing two assessment trajectories (the formal and the informal) was used to analyse the results. The study found that the definition and perception of technical risks of onshore underground energy storage projects vary considerably. In addition, four overarching themes (national coordination, the damage-handling process, the division of roles and responsibilities, and the distribution of benefits and burdens) were identified. Among the generic characterisation of the themes, there is agreement among the stakeholders. Concerning the conceptualisation of the values, the research findings show that the opinions and perspectives differ. Two processes were identified that are key to the existence of controversies and the effects these have on energy policies, ‘controversy spillover’ and ‘backflowing’. Perspectives and considerations, even if supported by years of experience, will differ from reality when drawing on physical experiences. Therefore, it is essential that in the future, when new procedures and policies regarding onshore underground energy storage projects are introduced, stakeholders’ considerations and perspectives are re-examined. Any new insights or themes identified will support the required adaptation for regulations and policies over time.

Injection and production of fluids into/from the subsurface has been known to trigger earthquakes, referred to as induced seismicity. This seismicity may occur when anthropogenically caused changes in the in-situ stress conditions result in reactivation of pre-existing faults in the subsurface causing slip accompanied by sudden release of energy. Several studies have numerically modelled the induced stresses due to production/injection in reservoirs of various geometries. In this report we present a simplified three-dimensional reservoir with a displaced fault and derive analytical expressions for induced stresses in and outside the reservoir due to production and injection of fluids. We use the calculated stresses for the three-dimensional model and analyse onset of slip across the fault. The research builds upon the analytical two-dimensional plane-strain analysis for induced stresses and slip initiation in \citet{Jansen2019} to which our work contributed. We reaffirm the findings from the plane-strain analysis in \citet{Jansen2019} and conclude that the effects of incorporating third dimension on induced stresses and slip behaviour are limited. We find infinite peaks in resultant shear stresses at the reservoir boundaries and observe a distinctly different pattern in induced stresses and slip behaviour between production and injection scenarios. In case of production, the slip patches are predicted to grow inwards into the reservoir initially until they merge, while for injection the slip patches grow separately into the overburden and underburden. The findings in this report are in agreement with the previous analytical and numerical studies on induced seismicity. In this research we also introduce geometrical complexity in the reservoir in the form of laterally varying height of the reservoir and we observe that the effects of variation in reservoir thickness are also minimal, however the induced stress patterns and slip initiation is significantly impacted by fault throw, initial stress conditions and fault frictional characteristics.

Jetted radial wells are a relatively new form of hydraulic stimulation for oil and gas wells, where small diameters holes extend radially from a wellbore. This technique for increasing well connectivity could also be applied to geothermal wells. Radials can be freely placed along a backbone well, making the design of an economically viable well plan time intensive. Numerical optimization has been extensively applied to well design and will be applied for the design of a radial well plan. A simple optimization case is used to compare gradient-based and gradient-free optimization for jetted radial optimization, the gradient-free CMA-ES algorithm is chosen for the workflow. In addition to CMA-ES, mixed integer CMA-ES and uncertainty-handling CMA-ES are incorporated in the workflow. Three reservoir cases are evaluated for optimization using these algorithms. The workflow is able to determine well plans with higher NPV in each of these cases though finding a global optimum remains difficult.

Industrial minerals are essential to human activity. The products derived from them make an integral part of a wide range of materials that are ubiquitously present in our daily lives. The performance and attributes of these materials depend significantly on the properties and quality of the industrial minerals and the products generated from them. These characteristics are ensured by the selection and mining of adequate ores, and by using various beneficiation and processing strategies to modify or enhance the original properties of the minerals.
One example of these strategies is calcination, in which the minerals are subject to thermal treatment. The success of the generation of high-quality products by using this technique partly depends on the capability of the plant to detect the factors that can degrade the quality of the raw ore, feed for calcination and final product. It also depends on its ability to inform and adapt the operations according to the presence of such factors. A possible approach for doing this is to characterise the minerals and materials with sensor technologies that can generate information on-site and in real-time, focusing on the identification of the degrading factors. Their timely detection can give operational feedback to the process and aid in the generation of high-quality products.
This Thesis aims to develop methods for the detection of factors that determine the quality of industrial mineral products by using data derived from infrared sensors, which have the potential to be implemented in mining and process control. For doing this, kaolin, perlite and diatomite have been selected as commodities that are relevant to the market and that represent different applications. This research shows the capacity of infrared sensor-based technologies to retrieve information, directly or indirectly, about the factors that affect the quality of industrial minerals at a lower cost and with comparable efficiency to other analytical methods.
@en

After decades of successful exploration and exploitation of gas fields in the Netherlands, the Dutch E&P industry has accumulated a substantial portfolio of gas fields that are considered economically unviable, the so-called ‘stranded fields’. There are various reasons for a gas field to be classified as economically unviable, one of them being the small size of the field. With the current challenges the E&P sector is facing it is valuable to re-evaluate the criteria often used in determining a fields economic viability. One of those criteria is the column height encountered in the well, which when it becomes too small can be very sensitive to water inflow. Gaining further understanding of the relation between gas column height and gas production can be valuable in determining the potential of a gas field with a limited gas column. Consequently, the focus of this study is twofold: 1) finding the minimum gas column height required for producing a sufficient amount of gas (0.1 – 0.2 BCM) and 2) investigating the effect of specific important reservoir heterogeneities (i.e. high-permeability streaks and clay layers) on the production results (total gas production and production time). These two research topics were approached through two separate methods: 1) a sensitivity study carried out using two types of models, a simple box model and a complex reservoir model and 2) an analysis of gas wells, and the corresponding gas reservoirs, that encounter small gas columns. Both research methods are focused on Rotliegend gas reservoirs. The results from this study show that the minimum gas column range for yielding economic gas production was found to be 20 – 40 m, depending on the field’s characteristics and configurations. Furthermore, in the models used for this study the presence of a highpermeability streak may either cause an increase or decrease in the total gas production, however they are found to significantly reduce the production time which is beneficial towards the economic analysis of a potential project. Additionally, they are found to be highly beneficial in gas reservoirs with a low average permeability. The clay layer on the other hand, significantly increases the total gas production of the models, although this is accompanied by an increase in the total production time. A potential downside of the clay layer is the restricted flow of gas which is particularly problematic in horizontal reservoirs with a significant portion of the gas column situated below the clay layer. Lastly, a possibly positive correlation between transition zone and the total gas production from thin gas columns reservoirs was found in the well analysis, on which further investigation is recommended in order to be able to draw more meaningful conclusions.

Simulation to predict the well and reservoir performance is a very important and established process in reservoir engineering. Integrated production modeling allows improving the understanding of the field’s performance and production network, fluid flow behavior and optimization at any stage and level during the life of the field through reservoir modeling processes.

This work forms an in depth study of the production data from the K5-F field in the Dutch sector of the Southern North Sea in which the field is analyzed through integrated production modeling to remove the uncertainties coupled with this field. Due to the absence of individual flow meters for the three wells producing from the K5F field, a first step was the determination of a production split per well. This production split is then used to quantifying the water gas ratio which forms the input data for material balance modeling. The effect of a changed production history per well on analysis and parameter estimation is also studied.

The well test analysis results and corrected production history incorporated in the material balance software allowed the estimation of other unknown reservoir parameters such as volumes, saturation, relative permeabilities and compartmentalization per well. Using material balance modelling as a tool for history matching, and taking into account the local geology the K5-F reservoir showed that the K5-F1 is a segmented reservoir behaving as separate compartments. Through interference testing it was found that is communication between K5-F1 and K5-F3 through a shared aquifer that is giving additional pressure support to the K5-F1 field. K5-F2 is not in communication with a different reservoir. For both K5-F1 and K5-F2 a reservoir pressure and water saturation match within a 2% range is found

Even though the K5-F field is one that has many production difficulties, unknowns and uncertainties, the workflow used and presented in this thesis allowed for the successful development of an integrated production model based on the most recent ideas and developments. The designed and simulated reservoir model resulted in cumulative production match around 3% and good reservoir pressure, wellhead pressure and gas rate match for K5-F1. K5-F2 found a cumulative production match of 4,8% which is above the acceptable 3%. With the assumptions and optimizations made throughout this study it was possible to create a reservoir model capable of forecasting the K5-F1 and K5-F2 well performance.

This research has shown that the amount and quality of production history available for a field and/or wells can have a huge impact on the analysis on reservoir parameters and model estimations. Besides the quality of the production data being used, the amount of production history plays a large role in well test analysis. Small amounts of production history for instance can influence in the late stage of the pressure build up derivative which can lead to an underestimation of the skin. Even something as simple as a new production split has led to a 16,76% lower cumulative production for K5-F1 and a 13,21% higher cumulative produced volume for K5-F2. Therefore it can be stated that the quality of a reservoir model and history match of a field has a direct correlation with the quality of the production and field data.

A petroleum production system is generally modelled based on the concept of nodal analysis, where the entire system is broken down into discrete elements such as near-well bore, tubing, surface choke and flow line. Operating flow rates and pressures can be estimated with a nodal analysis procedure by calculation of the intersection of performance curves. Input parameters in nodal analysis of production systems are considered deterministic, however, some of these parameters are better represented as distributions. In this report, the ensemble-based data assimilation methods “ensemble Kalman filter” (EnKF) and “particle filter” (PF) are applied to steady-state models of a production system for tuning of uncertain model parameters during the test separator phase. The performance of the EnKF and the PF is tested with the use of twin experiments. The calibrated model parameters of the choke, tubing and the near-well bore elements with EnKF and PF can be used to create an ensemble of performance curves leading to an ensemble of operating flow rates and pressures. The foreseen next step is to use the posterior distributions of model parameters as inputs for soft sensing of flow rates during semi-steady-state production for a single phase oil reservoir, where the oil rate and reservoir pressure are considered as unknown parameters. In the twin experiments as used in this thesis, a total number of three steady-state pressure drop measurements was used to estimate a total of six independent parameters which constitutes an ill-posed problem, resulting in non-unique parameter estimates. It is recommended to alleviate this issue by either reducing the number of parameters or by using multiple separator tests at different flow rates.

Modern geoscience challenges motivate the development of advanced simulation methods for large-scale geothermal fields, where single- or multi-phase flow is coupled with heat transfer equation in heterogeneous fractured formations. The state-of-the-art multiscale formulation for fractured media (F-AMS; Tene et al., JCP, 2016) develops an efficient approach for flow equation only.

Here, for the first time, the F-AMS formulation is extended to coupled flow-heat equations arising from single-phase flow in fractured geothermal reservoirs. To this end, the multiscale operator is obtained based on elliptic basis functions for both pressure and temperature, to preserve the simplicity and efficiency of the method. ILU(0) 2nd stage smoother is then used to guarantee convergence to any desired accuracy. Numerical results are presented to systematically analyse our multiscale approximate solutions compared with the fine scale ones for many challenging cases, including the outcrop-based geological fractured field. These results show that the developed multiscale formulation casts a promising framework for the real-field enhanced geothermal formations.