To mitigate renewable variability, large-scale energy storage systems are necessary to ensure a robust energy network. Subsurface hydrogen storage is considered a promising candidate for large-scale energy storage systems. In this study, corner-point geometry, a standard method for discretizing field-scale subsurface reservoirs, is first validated against Cartesian geometry. Then, the sensitivity of reservoir performance on various reservoir parameters is analyzed. For this purpose, the cyclic operation of a synthetic reservoir is simulated for a period of five years. Several reservoir parameters are then varied, and the resulting changes in recoverability rate are analyzed. From this analysis, it can be concluded that variations in reservoir parameters have the largest impact during the initial cycles of the simulations. However, the results show that reservoir permeability and anticline do have a lasting impact on performance in subsequent injection and production cycles. Finally, the concept of subsurface hydrogen storage in the Johansen formation, located off the coast of Norway, is demonstrated. The results from these simulations show that the injected hydrogen is wellrecoverable and underline the importance of careful geological site selection.

We investigate how creep-driven convergence after cavern abandonment compresses trapped brine and alters geomechanical risk. Using the open-source finite-element simulator SafeInCave, we implement a two-way coupling between cavern volume and hydrostatic brine pressure and run fully coupled simulations for a field-scale cylindrical cavern whose roof lies between 600 m and 2200 m. Two abandonment protocols are considered: hard shut-in, in which we permanently seal the well, and soft shut-in, in which we vent brine whenever its pressure reaches 70% of the overburden stress. Each depth-protocol pair is simulated with and without pressure-solution creep (PSC).

After 300 yr a hard-shut-in cavern loses only 0.30% of its initial volume at 600 m and 0.76% at 2200 m, yet shallow caverns approach the micro-fracturing threshold as brine pressure climbs to 95% of lithostatic pressure. Soft shut-in preserves σ ≥ 5 MPa safety margin against microfracturing but allows greater closure: volume loss rises from 1.6% at 800 m to 2.4% at 1600 m before declining in deeper settings. Convergence initiates faster in deep caverns but decelerates below shallow-cavern rates as deviatoric stresses relax over time. Even in the worst case, 600 m depth under soft shut-in, surface subsidence reaches only 3.1 cm. PSC accelerates early convergence for both protocols; under hard shut-in its influence fades within decades, whereas the constant pressure offset under soft shut-in sustains PSC for centuries, adding approximately 0.8 cm of subsidence at 800 m but slightly reducing it below 1000 m.

Depth therefore governs post-closure behaviour. Caverns shallower than approximately 1 km experience rapid pressure build-up that pushes brine pressure to within 1 MPa of lithostatic stress, while deeper caverns become self-limiting and converge slowly. Above 1 km the shut-in protocol dominates risk, whereas below 1 km brine-pressure feedback controls the response. The key findings are:

Soft shut-in is essential for caverns with roofs shallower than approximately 1 km.

Hard shut-in suffices at greater depth, as overburden filtering limits surface subsidence to the centimetre range.

Future models should incorporate a stress threshold for PSC and stratigraphic layering to avoid over-predicting far-field deformation and rebound.

The coupled-physics workflow developed here thus offers regulators and operators a transparent baseline tool to forecast post-closure deformation, tailor abandonment strategy to depth, and direct monitoring resources where they matter most.

The Hystream Battery is a modular, white-label product designed to produce green hydrogen using surplus photovoltaic and wind energy. The system comprises a battery, electrolyser, compressor, storage tank, and optional features such as a heat exchanger and dispensing unit, aiming to be easy to permit and install. This study investigates the technical and economic feasibility of the Hystream Battery through a make-and-adapt iterative methodology, addressing technological, economic, and regulatory aspects. Each iteration allows for further improvements to be researched.

The research shows that the Hystream Battery is technically feasible and can compete economically with conventional hydrogen production methods, such as steam methane reforming, especially when supported by subsidies like the OWE program. In terms of Levelized Cost of Hydrogen (LCOH), the Hystream Battery is also competitive with large-scale green hydrogen systems. For instance, while large-scale (>20 MW) systems have an LCOH of €13.69 (Eblé & Weeda, 2024b), the Hystream Battery, when evaluated using the same TNO model, achieves a lower LCOH of €12.36.

When evaluated under varying input conditions and grid configurations, the Hystream Battery proves to be economically viable within a defined operational range. In a stand-alone setup where the grid is used solely for standby, the system achieves a Net Present Value (NPV) above €1,000,000 and a Levelized Cost of Hydrogen (LCOH) around €11/kg when powered by a balanced input ratio of 1:1.6:1.6 (grid:PV:wind). A PV input between 1.3 and 2.5, and wind input between 1.3 and 1.7, define the viable operational window. Systems without wind input consistently return negative NPVs.

When the grid is used actively, up to a maximum input of 1500 kW, the system becomes significantly more favorable, with LCOHs dropping below €9/kg across all PV and wind input ratios. In this scenario, higher PV capacities correlate with increased NPVs, and electricity curtailment is reduced to below 5% at input ratios as low as 1:1.5:0. Additionally, system economics improve as wind input decreases. These results align closely with the energy profile of Dutch business parks, which often feature high rooftop PV potential, grid constraints, and multiple hydrogen-consuming stakeholders such as transport operators, thermal energy users, and industrial feedstock consumers.

Regulatory challenges, particularly those related to permitting, were identified as key obstacles to fast market deployment. The preparation of accurate and comprehensive documentation and studies for environmental authorities is essential to expedite the approval process, potentially reducing permitting timelines from 26 to 8 weeks. Furthermore, the inclusion of features like a heat exchange module enhances system efficiency and reduces CO2 emissions by replacing gas-based heating with recovered waste heat.

By analysing system configurations, market potential, and regulatory requirements, this thesis concludes that the Hystream Battery holds significant promise for advancing the hydrogen economy in the Netherlands. The system supports local hydrogen production, mobility applications, and renewable energy integration by reducing grid congestion, contributing to reduced energy dependence and the broader energy transition.

Underground hydrogen storage (UHS) in porous reservoirs offers a promising solution for addressing the temporal and spatial variability of renewable energy sources. To safely and efficiently utilize these reservoirs for UHS at an affordable cost, it is essential to understand the diverse disciplines involved at various scales. This study combines large-scale techno-economic evaluations with micro- scale laboratory research to assess the commercial and technical viability of UHS in porous media. A techno-economic case study of commercial-scale UHS in a depleted gas field in the Dutch nearshore is investigated, defining the integrated scope of infrastructure and facilities and estimating associated costs. Additionally, the multiphase flow behavior and redistribution of hydrogen in sandstone rock are analyzed through experimental assessment of relative permeability, capillary trapping, dissolution, and Ostwald ripening using CT imagery. The case study compares various surface facility design concepts, revealing that the Levelized Cost of Hydrogen Storage (LCOHS)
ranges from [cannot be disclosed for confidentiality reasons] , depending on the required purification scope of the production stream. Cushion gas cost significantly impacts the LCOHS. The multiphase flow behavior was examined on a micro-scale by co-injecting hydrogen and brine at varying fractional flows into a vertically placed 17 cm Berea sandstone rock sample at 25°C and 50 bar, while observing under CT imaging during both drainage and imbibition. High- resolution CT imaging visualized Ostwald ripening at the pore scale after several periods without flow. The experimental results indicate a hydrogen end-point relative permeability of 0.043 and a linear trapping coefficient of 0.725. No preferential flow paths were observed, however, dissolution was shown to have an impact the saturation profiles. Pore-scale image analysis demonstrated that Ostwald ripening leads to the fragmentation of mid-sized hydrogen ganglia and the growth of larger ganglia over time. These findings provide valuable insights into optimizing UHS and provide input for large-scale reservoir simulations, emphasizing the importance of integrating techno-economic assessments with detailed laboratory research for the commercial success of UHS.

This study investigates the technical feasibility and economic viability of utilizing depleted offshore gas reservoirs in the Dutch North Sea for underground hydrogen storage (UHS) to buffer intermittent hydrogen production from offshore wind farms in the Dutch North Sea. The research aims to address the existing knowledge gap in integrating an offshore hydrogen storage platform while considering the maximum re-useability of existing natural gas infrastructure.

The demand for sustainable and clean energy sources has become increasingly vital in addressing the challenges of climate change and energy security. Hydrogen, with its high energy density and potential for carbon-free energy conversion, has emerged as a promising candidate for future energy systems. Efficient storage and retrieval of hydrogen are crucial for its widespread utilization, for which a promising approach is underground hydrogen storage in geological porous media. This thesis aims to explore and advance the understanding of hydrogen storage in geological porous media, specifically focusing on pore-scale modeling and contact angle analysis.

This research aims to overcome the limitations of current hydrogen storagemethods and develop more efficient energy storage systems. Porous materials like sandstones have special characteristics that make them suitable for storing hydrogen underground. To design and operate underground hydrogen storage on a large scale, it is important to understand how fluids move through these materials. The way hydrogen is stored and released is influenced by complex processes happening at a very small scale (μm). To accurately simulate these processes, we need to study how fluids move in the pores, including factors like capillary pressure (the pressure difference between nonwetting and wetting phases, which is one of the main forces acting at pore scale transport) and relative permeability (how easily fluids flow through the pores where other fluids are also present).

Pore-scale modeling is a useful tool for simulating and understanding how hydrogen behaves in the tiny pore spaces of porous materials. These models help us see how hydrogen moves, spreads out, and interacts with the pore walls at a very small level. Another important aspect is studying the contact angles in the system of hydrogen, water, and porous material. These angles tell us about the way these substances interact at the interfaces between solids, liquids, and gases. By studying these processes and measuring contact angles, we can gain a better understanding of how hydrogen is stored and released, considering factors like pressure, temperature, the type of material, and how easily fluids flow through the pores. This knowledge will help us design better systems for storing hydrogen energy in porous materials on a larger scale.

The primary objectives of this thesis are as follows: To develop pore-scale models for simulating and understanding underground hydrogen storage in geological porousmedia. To investigate the contact angle between hydrogen, brine, and sandstone systems and their influence on storage and release mechanisms. To analyze the contact angle for a mixture of hydrogen-methane in the brine/sandstone system and assess its implications for hydrogen storage. To develop a dynamic pore network model to capture the dynamic behavior of hydrogen in geological porous media. To draw conclusions from the findings and propose future research directions in the field of hydrogen energy storage.

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.

Underground energy storage (UES) in porous and cavity reservoirs can be used to balance the mismatch between the production and demand of renewable energy, as well as for securing gas and oil supply during shortage or high demand periods. Understanding the geomechanical behavior of these reservoirs under different storage conditions, i.e., storage frequency and fluid pressure, is key in defining their capacity and effective lifetime. This thesis work presents a rigorous analysis performed on sandstones to unravel their geomechanical response under cyclic loading. This study includes, importantly, both experimental and numerical investigations under several conditions which are relevant to UES. The rock response was studied considering cyclic stress states above and below the onset of dilatant cracking, under different frequencies and amplitudes. Within the number of cycles studied, measurements of axial strains and acoustic emissions indicated that inelastic strains accumulated cycle after cycle following an exponentially decreasing rate. Five types of deformations were interpreted: elastic, plastic, viscoelastic, cyclic-plastic and brittle creep. Based on these novel experimental results and observations, Nishihara's constitutive model was used for simulating viscoelastic and brittle creep deformations, while dilatant plastic strains were modeled using a Hardening-Softening model. Finally, an extension of the Modified Cam-Clay model was proposed to account for cyclic-plastic compaction. This approach can be extended and improved to study cyclic sandstone deformation's implications on subsidence, fault reactivation and cap rock flexure, among other physical phenomena impacting a reservoir's storage capacity.

In this work, an efficient compositional framework is developed to simulate CO2 storage in saline aquifers with complex geological geometries during a lifelong injection and migration process. The novelty of the development is that essential physics for CO2 trapping are considered by a parametrization method.
The numerical framework considers essential hydrodynamic physics, including hysteresis, dissolution and capillarity, by means of parameterized space to improve the computation efficiency. Those essential trapping physics are translated into parameterized spaces during an offline stage before simulation starts. Among them, the hysteresis behavior of constitutive relations is captured by the surfaces created from bounding and scanning curves, on which relative permeability and capillarity pressure are determined directly with a pair of saturation and turning point values.On the other hand, the new development allows for simulation of realistic reservoir models with complex geological features by implementing in corner point gird. The extension to corner point grid is validated by comparing simulation results obtained from the cartesian box and the converted corner-point grid of the same geometry, and it is applied to a field scale reservoir eventually.
A set of sensitivity analysis reveals the roles of various physical effects and their interactions in CO2 trapping in a realistic reservoir model, apart from the investigation on the impact of migration path, linear trapping coefficient and depth. The results show the proposed compositional framework casts a promising approach to predict the migration of CO2 plume, and to assess the amount of CO2 trapped by different trapping mechanisms in realistic field-scale reservoirs.

This project is part of the larger REX-CO2 project, which assesses the re-usability of abandoned wells in reservoirs targeted for CCS operations. It comes as a necessity to also assess the abandoned wells that will not be re-purposed, by assessing the risk they pose in terms of allowing the stored CO2 to resurface across them, which is the focus of this work.

In order to assess these wells, a decision making framework was developed that will consider the main mechanisms that can lead to leakage across an abandoned wellbore. The objective is that this framework could be used by an operator of a prospective reservoir for a CCS project, prior to the start of the operations, in order to understand the risk associated with the abandoned wells present, and take decisions for remediation if necessary and possible. This framework considers three main aspects relevant for the formation of leakage paths. These are the effectiveness of the abandonment process itself, the chemical processes that can lead to the degradation of the isolation elements, and the mechanical processes that can lead to loss of integrity.

The framework consists of two main parts. The first is a qualitative analysis, based on a thorough literature review, assessment of experts and testing with case studies. This qualitative assessment consist of decision trees, formed by a series of questions which answers will dictate the final outcome that will reflect the state of the abandonment. The second part is a quantitative risk analysis. For this purpose "Bayesian Belief Networks" are used which is a probabilistic tool used for calculating the relative probability of a combination of factors. The BBNs are constructed and populated based on a thorough literature review including data on experimental results. In addition, geomechanical simulations were carried to populate these models. All this data was processed and converted into normal random distribution functions, which were used to infer the probabilities that would be included in the BBNs. The final outcome of the BBNs reflect the probability for leakage to occur.

Both the qualitative and quantitative parts of the framework are integrated together in order to obtain a complete analysis. The complete framework was tested with case studies based on real wells for which the abandonment states are known and reported by the operator, to observe how the outcome of the framework analysis matched the outcomes of the analysis performed by the operator.

With this study, a complete and systematic framework was developed that can be used to aid decision making for prospective CCS projects in the future. The framework is constructed such that it has the capacity to be improved and expanded in the future, to increase its consistency and broaden its applications. A series of potential improvements are also explained at the end of this report. Furthermore, the framework can be used to challenge the current abandonment standards and assess what will have to be changed to adapt the current abandonment techniques so that they include CCS projects.

Overcoming the intermittency problem of renewable energy is an issue that has to be addressed in order to achieve an efficient energy system. Hydrogen has been gaining interests, as green energy carrier, which is included in the energy transition plans of not only the Netherlands, but worldwide. Due to its physical characteristics, large scale storage of hydrogen requires volumes that can only be provided by porous media in the subsurface. Underground Hydrogen Storage (UHS) in depleted gas fields is a large scale storage possibility that is generating attention and increased research. However, it is currently in a Low Technology Readiness Level, meaning that more research, and especially field scale projects are necessary. There is very few literature that tries to use alternative gases as a cushion gas for a UHS. Therefore, this investigation is relevant for the future development of UHS. This investigation will use CMG GEM, a commercial compositional reservoir simulator to model the fluid interactions in the subsurface when hydrogen is stored in depleted gas fields. This will be done by means of a sensitivity analysis, where the hydrodynamic behaviour between hydrogen and possible alternative cushion gases, such as methane, carbon dioxide and nitrogen are studied. Apart from the technical analysis, a simplified economic evaluation is used to calculate a levelized cost to store hydrogen, which will allow for an economic optimized selection of cushion gases in an UHS. The two main constraints for an UHS system are the purity of the extracted hydrogen and the rate at which it is extracted from the subsurface. The results of this investigation show that the mixing of hydrogen with an alternative cushion gas will change drastically based on the degree of the reservoir heterogeneity and the location of the perforated interval. This ever-increasing mixing will have an effect on the capability of the system of delivering pure hydrogen for the expected time.

Hydrogen storage in porous media is a potential solution to the energy distribution problems we might face in the future. Intermittent energy sources such as solar and wind energy need to be accompanied with temporary energy storage to accomodate the times when production and demand do not match. Hydrogen is an energy carrier with a large mass energy density as it can be made completely green through electrolysis and it will not release greenhouse gasses when used to produce energy through combustion process. However it has to be stored in large enough volumes to reach the required energy demand. The storage can be done in the subsurface in either aquifers, depleted hydrocarbon reservoirs or salt caverns which are giant porous or cave volumes available to store compressed gases. At this moment, there is a lack of knowledge for safe and efficient hydrogen storage at reservoir (continuum cm and above) scale and pore scale (micrometer-scale). In a reservoir model the relative permeability and capillary pressure are key parameters to characterise flow behaviour and capacity. These are upscaled parameters we can find through pore-network modelling, among other approaches. For a pore-network model, the contact angle and accompanying wettability characteristics (i.e., interfacial tension) are important to obtain reliable upscaled functions and parameters.
Through the use of a captive bubble method, in this thesis work, we have been able, for the first time, to characterize the contact angle of a hydrogen/brine/rock system at different pressures, temperatures, salinity's and rock compositions. It was found that the contact angle of a hydrogen/brine/rock system is in the range of between 21.1 and 43 degrees. Moreover, for the studied range of varying parameters, the contact angle stayed remained relatively constant. The results of this study are published in the Journal of Advances In Water Resources (https://doi.org/10.1016/j.advwatres.2021.103964).

Underground Hydrogen storage (UHS) is an attractive technology for large-scale energy storage. The UHS safety and efficiency depends highly on accurate characterization of H2 interactions with reservoir fluids, specially wettability analyses for H2/brine/rock systems. This thesis reports experimental measurements of advancing and receding contact angles of H2/water, N2/water and CO2/water systems at P = 10 bar and T = 20 °C using a microfluidic chip (channel widths: 50 - 130 μm). The results indicate strong water-wet conditions with H2/water advancing and receding contact angles of respectively 13 - 39°, and 6 - 23°. It was found that the contact angles decrease with increasing channel widths. Little hysteresis was measured, and consequently, the results are not in line with Morrow’s curve. The receding contact angle measured in the smallest channel agrees well with the literature coreflood tests. The N2/water and CO2/water systems showed similar behaviours as the H2/water system.

A novel 2D finite element method (FEM) on unstructured grid for nonlinear time-dependent deformation of materials is developed. The objective is to model the complex deformation behavior of the rock salt, inside which caverns are mined to store green fuels (such as hydrogen). The analyses and the developments of the present work allow for quantification of the state of the stress of the cavern, and assess the safety and reliability of the storage structure over time. The novelty of the approach is in using minimization of the potential energy principle in conjunction with nonlinear creep deformation physics. While the available FEM-based simulators offer tools only for solving linear and non-linear elastic models, the developed simulator takes into account cyclic loading and material’s damage evolution in time with possibility to predict the material’s failure. Apart from that, the stored product (hydrogen) density is taken into account, which affects cavern’s pressure variation with depth. Impurities,causing heterogeneous rock salt properties, are also considered in the developed model. Multivariate Gaussian distribution is utilized to generate distribution of the heterogeneous mechanical properties. Eulerian strains are introduced in the model to take into account deformation of the mesh. As such,the computational grid changes its geometry according to the deformation. Several numerical test cases are studied. Firstly, a consistency (verification) study is performed, to validate the linear elastic model. Remark that the linear elastic deformation model casts the basis of the non-linear model with creep physics. Then, several studies have been performed to analyze, quantify and approximate the deformation of the salt cavern under the gas pressure change, including the time-dependent creep physics.

A transition towards more renewable energy sources such as wind- and solar energy is underway. These sources can be unpredictable with regard to energy production and therefore energy storage has become a major concern. A promising technique, Underground Hydrogen Storage (UHS), converts excess energy into hydrogen and stores it underground, after which, by reversing the process, the stored energy can be used when the imbalance between supply and demand is great. Hydrogen storage in salt caverns is particularly attractive because of their high sealing capacity, low amount of cushion required, the inert nature of rock salt and high possible injection and withdrawal rates. For energy storage purposes, these salt caverns are exposed to different cyclic loading conditions during operation, resulting in variations in stress in and around the cavern. Investigation of the mechanical response of rock salt under cyclic loading conditions is essential for the safety assessment and usability of the cavern. The response of the system to varying loading conditions can be mainly divided into two groups, either time-dependent or stress-dependent. The main focus of this thesis is to record the stress-dependent behaviour of rock salt under varying loading conditions. More specifically, it focuses on nonlinearity that occurs due to viscoplasticity. This phenomenon can be described as the rate dependent
behaviour of a material that occurs when a material exceeds a certain stress level, after which irreversible deformation occurs. This thesis describes the development of a 2D FEM (finite element method) simulator on an unstructured grid that captures the mechanical response. To model the stress-dependent behaviour, the viscoplastic model proposed by Desai is used. This model is
based on the non-associated flow rule and takes into account material dilatancy and compressibility. In addition, the model allows for hardening, the tensile strength of rock salt and variation in yield behaviour with pressure variation. Sonar data from cavern EPE S43 is utilised to test the simulator. This case showed that irreversible deformation occurs as a result of the stress-dependent behaviour of rock salt. For this specific cavern, the total working volume of the cavern decreased roughly by 0.0003% over three operating cycles, indicating that viscoplastic deformation itself does not pose significant risks to the cyclic storage of hydrogen in salt caverns.

The demand for accurate and efficient simulations in order to test the geomechanical effects is a reality for the entire geoscience community. The motivation that arises from that need is the development and the evolution of modelling methods to study these effects. Deep understanding of any problem in fine scale is crucial, especially when it extends to much coarser scales.
In this work the finite volume method (FVM) is used for mechanical modelling of deformation in elastic media. The momentum balance equation is solved as the governing equation for mechanics, assuming linear elasticity for the stress tensor. Here, displacement is mapped onto a vertex-centred grid in three dimensions (3D). A set of eight trilinear basis functions are used to locally interpolate the value of displacement within each grid cube. In the finite volume method, the discretized form of the equations are obtained by integrating the governing equation over control volume surfaces, since in 3D the control volume is a cube. Hence, discretized forms are obtained by considering 24 surfaces, which form between a displacement node and its neighbouring displacement cells. This required extensive derivation. The implementation of the numerical model was carried out by writing a code in MATLAB. Several numerical test cases are presented to demonstrate the capability of this model. In the first place, the consistency of the model is checked through comparison with synthetic analytical solutions, which are compared to the numerical solutions. Furthermore, the simple test case of uniaxial compression, has been carried out with this model, but also compared to the results with a 2D FVM model and a 3D finite element (3D FEM) one. In another test case, ground plain strain subsidence is studied in a real hydrocarbon field with a heterogeneous map for elasticity parameters. It is shown that 3D FVM is in close agreement with 3D FEM in predicting the subsidence due to field depletion. Last but not least, displacement and stresses, in a faulted reservoir in which fluids are injected, are modelled and the results are shown to coincide with a robust analytical solutions for the system. Finally, the aim of this work is to shed some more light on the finite volume method for mechanics and bring it closer to the audience of science.

Accurate and efficient predictions on the behavior of fluid flow and heat transport are required in the development of low-enthalpy geothermal reservoirs in fractured formations. Key challenges include the demand for high-resolution computational grids, the non-linear behavior of the system due to strong mass-heat coupling and the presence of fractures with large heterogeneity contrasts. In this work, a comparison is made between natural and molar variable formulation used to describe the coupled fluid-heat transport under non-isothermal conditions in low-enthalpy fractured porous media. The solutions and performance of the newly implemented molar formulation are compared to those of the existing natural variable formulation in the DARSim2 reservoir simulation framework. A fully implicit scheme (FIM) is applied to solve the coupled discrete system including mass and energy balance equations. Application of the Algebraic Dynamic Multilevel (ADM) method with projection-based Embedded Discrete Fracture Model (pEDFM) provides a scalable and efficient simulation framework for field-scale fractured reservoirs. The ADM method maps the fine-scale system onto a dynamically defined multilevel grid resolution system (Cusini et al., 2016, HosseiniMehr et al., 2020) based on the solution gradient and a series of restriction and prolongation operators, which ensure accurate capturing of fine-scale heterogeneities. Fractures are defined explicitly as either (highly) conductive passageways or flow barriers using the pEDFM formulation (Tene et al., 2017). Simulation results using the molar formulation are compared with an analytical solution as verification of the implementation. Results of various (un-)fractured test cases with homogeneous and heterogeneous permeability fields show that there is no clear difference between the solutions and performance of the different primary variable formulations, and the performance itself is largely dependent on the level of complexity embedded in the numerical model independent of the simulation strategy applied.

Regular Cartesian grid models provide satisfactory numeric results when a numerical scheme for reservoir flow simulation is applied. However, they cannot recreate complex geological features existing in realistic reservoir models such as faults and irregular reservoir boundaries. Corner point grids can represent these geological characteristics and can be adapted and represent any reservoir. In the subsurface reservoirs is usually typical to find fractures networks, and it is necessary to simulate the effect of them in reservoir models based on corner point grids. Although several works validate the precision of embedded Discrete Fracture Model (EDFM) for representing fractures in cartesian grids, very few studies have been presented to examine the accuracy of fracture modeling in geologically complex reservoir models. In this work, the novel discrete fracture model, the Projection-based Embedded Discrete Fracture Model (pEDFM), is implemented to represent fractures in reservoir models based on corner point grids. pEDFM provides additional features to the EDFM and is applied to explicitly and consistently define fractures. It implements independent grid sets for the fractures (described as lower-dimensional domains) and the rock matrix irrespective of the grid domains’ complex geometrical shapes. The suitability of the original pEDFM method has been expanded to a fully generic 3D geometry, and it lets on including fractures with any orientation on the corner point grid cells, an important development for the method’s viability in field-scale applications. Further to the geometrical flexibility of EDFM, matrix-matrix and fracture matrix connectivities are readapted to account for the projection of fracture plates on the interfaces. This allows for consistent modeling of fractures with generic conductivity values, from high conductive networks to impermeable flow barriers. A fully implicit scheme is used to get a discrete system with two main unknowns (i.e., pressure and phase saturation) on both matrix and fracture networks. Several 3D test cases of reservoirs models with complex corner point grids and fracture networks arbitrary designed in them are presented to demonstrate the devised method’s accuracy and applicability. The results show that the pEDFM implementation for two-phase flow is highly successful for modeling fractures with a broad range of conductivity on field-scale reservoir models.

It is a great challenge to accurately model the flow through subsurface reservoirs. The fact that they are located a few kilometers below the earth’s surface means that it is very difficult to locate flow affecting phenomena. Fractures are considered to be such phenomena. They can either enhance or block flow which has a substantial effect on the flow through the reservoir. This research addresses the flow properties of fractures and the location of fractures. This research introduces analogue fracture maps to get a grasp on possible fracture locations and introduces a classification of fractures based on their orientation with respect to subsurface stress. Using this classification, fractures are assigned different permeability values, a flow affecting property. Using this approach a fracture map is obtained that can be used to model flow through a reservoir. To model flow, the projection based Embedded Discrete Fracture Model (pEDFM) is implemented. This method uses independent grids for both the matrix and fracture and couples them using a transfer function. Even when using the pEDFM method, the computational costs for running simulations through a field scale model will be too large. Therefor the concept of multiscale is introduced and developed further in this research. This research uses algebraically calculated basis functions to prolong the coarse scale solution to finescale. Besides this, flexible coarsening is introduced for a static Multilevel Multiscale (MMs) modeling approach and for Adaptive Dynamic Modeling (ADM). This allows the user to independently choose coarsening ratios for matrix and fractures. This research shows that fractures can be represented accurately using only two vertices on a coarse grid. This can reduce computational costs. Besides the flexible coarsening, the research shows how the grid refinement scheme can be used in efficiently in multiphase simulations. Using a grid re-finement scheme enables the user to capture a saturation front and to use a multiscale grid at the same time. The grid refinement scheme works most efficiently when the cells belonging to high permeable fractures are coupled to the cells belonging to the matrix. The finescale solution This research finally shows the impact of geological uncertainty. The sensitivity analysis shows how geological uncertainty can have an influential impact on the problem that has to be solved. In the sensitivity analysis it is researched how a change in subsurface stress affects the finescale solutions.

The Netherlands will need to transform her traditionally fossil fueled energy system in the coming decades to achieve the goal of reducing CO2 emissions to net-zero in 2050. Therefore, an increase in renewable energy sources as solar and wind energy in the overall energy mix will be essential. However, due to the highly variable energy production patterns of these renewable energy sources, a primary need for energy storage is created. Since electricity can not be stored on a large enough scale to balance significant energy fluctuations, the need for a gaseous CO2 neutral energy carrier is created. In The Netherlands, this role could potentially be fulfilled by green hydrogen gas. Green hydrogen can be stored in geological formations such as depleted gas reservoirs. Due to the immense storage volumes and frequent occurrence in the Dutch subsurface, depleted gas reservoirs could be an excellent opportunity to serve as large scale energy storage sites. Moreover, underground natural gas storage in gas reservoirs is a proven and used technique in The Netherlands. Utilizing a natural gas reservoir as a hydrogen storage site comes with several challenges. This report provides a full overview of all the challenges with underground hydrogen storage in geological formations as aquifers, depleted gas reservoirs and salt caverns based on literature. In this way, the full potential and risks of using depleted gas reservoirs for this technique is clearly highlighted. Using this overview, a priority scheme for the usage of different geological formations as storage facilities for hydrogen is proposed. In combination with possible meteorological conditions combined with different Dutch policy scenarios, a minimum seasonal storage need of 16 TWh through the use of hydrogen is identified. Using the priority scheme, rock salt caverns are used as much as possible to fulfill the minimal need for seasonal hydrogen storage. By performing an analysis on the potential subsurface storage capacity in The Netherlands, it becomes clear that the Dutch subsurface can not realize more than 12.1 TWh of potential hydrogen storage capacity by only utilizing salt caverns. Since depleted gas reservoirs are identified as the best alternative for underground hydrogen storage, a minimum need for hydrogen storage from Dutch depleted gas reservoirs is estimated at 3.9 TWh in 2050. In this thesis, all physical and chemical aspects that are important during the subsurface storage of hydrogen in porous media are addressed. This leads to the identification of potential losses of hydrogen during the storage of the gas in the depleted gas reservoir. Analyzing all the possible methods leading to potential hydrogen loss shows that on the long term, bacterial conversion seems to be the biggest challenge if no measures against this conversion are taken. Using numerical reservoir simulation as a quantification and sensitivity analysis tool, the hydrodynamic behaviour of hydrogen in contact with other gasses is described. This is done by introducing a dimensionless gravity number. The interpretation of this number shows if the displacement process is either dominated by viscous or gravitational forces. Furthermore, the displacement efficiency of hydrogen towards other gasses is analyzed. Displacement of hydrogen towards residual gasses in porous media dominated by viscous forces proves to be more efficient than the displacement dominated by gravitational forces. By performing cyclic storage simulations, the overall efficiency of injecting and reproducing hydrogen from a depleted gas reservoir is examined. This is done for both homogeneous and heterogeneous reservoirs. By performing a sensitivity analysis on the input parameters of the simulator, an overview is given of the parameters that will have the most positive or negative impact on the cycle efficiency. The results of the simulation show a regular cycle efficiency that is estimated at roughly 70%. Applying a higher difference in injection and production pressure leads to a lower cycle efficiency whereas using a more elongate reservoir as storage site shows to have a positive effect on the overall cyclic efficiency.