Several Geological Disposal Facilities rely on clay-rich formations as the host geological formation or engineered barrier. During the lifetime of a GDF, gasses may be generated primarily due to the anaerobic corrosion of metal, triggering gas migration throughout the natural or engineered barrier. If significant gas pressures build up, preferential dilatant pathways may develop. Laboratory gas injection tests have shown that these pathways significantly affect gas migration by locally increasing the porosity and decreasing the air entry value. Material heterogeneity is one method that has been used to replicate these pathways in numerical modelling. The effect of modelling material heterogeneity on gas transport properties of clay-rich materials has been studied qualitatively, but no fully stochastic analyses have been performed thus far. As a result, uncertainties remain about the quantitative effects of heterogeneity on transport properties of clay. Furthermore, the role of the scale of fluctuation is unclear.
In this thesis, spatially correlated random fields of porosity were generated, and the Finite Element software LAGAMINE was used to model water permeability, gas diffusion and gas injection tests. Results indicate that the time necessary to reach a steady-state water or gas outflow is unaffected by heterogeneity. Monte Carlo analyses of permeability and gas diffusion tests showed that the variability of the effective permeability is higher than that of the apparent diffusion coefficient due to the nonlinear relation between the porosity and the permeability. Additionally, the variability of the effective permeability and apparent diffusion coefficient depends strongly on the ratio between the sample size and the scale of fluctuation. With larger samples, the effects of the random field are averaged, narrowing the distribution of results.
In gas injection tests, water saturation profiles suggest that the onset of desaturation occurs in zones of high porosity, reinforcing the hypothesis that preferential pathway development is related to material heterogeneity. A Monte Carlo analysis of a gas injection test shows that variability in the gas outflow and the degree of saturation not only depends on the scale of fluctuation but also the injection pressure due to the non-linear dependency of the air entry pressure on the porosity.
These results suggest that heterogeneity and the scale thereof are important aspects of gas migration in clay-rich materials. Specifically for laboratory testing of clay-rich materials, it appears that an understanding of the approximate scale of the heterogeneity is essential for estimating the minimum number of samples and the minimum size of those samples. Too small or too few samples will not allow for a reliable estimate of parameters of interest.
Some limitations apply to the findings of this research. First, the sensitivity of the results to factors such as the standard deviation of the distribution underlying the random field and anisotropy has not been addressed yet. Second, the dependency of the effects of the random field on the injection pressure in gas injection tests is not fully understood yet. Ideally, additional simulations are performed to separate these effects. Finally, hydromechanical coupling should be introduced to replicate experimental findings more accurately.

With increasing interest in developing green hydrogen infrastructure as a way to decarbonize the power, transportation and heating sector the storage of hydrogen becomes a crucial component. Although most present hydrogen storage techniques are of small-scale, the intermittent nature of renewable electricity and expected future green hydrogen production and consumption makes large-scale storage an important factor.
This study evaluates the techno-economic feasibility of storing hydrogen in an already-existing underground salt cavern, one of three large-scale gas storage systems. The work starts with an overview of the large-scale gas storage methods and the challenges associated with storing hydrogen. The current
existing salt cavern storage facility is discussed and is technically assessed for hydrogen storage. The investment and operating costs related to the hydrogen storage method were calculated using models developed, certain components of the model were modeled using Aspen simulation software, which was also used to calculate the cost associated with it. For various scenarios, the cost distribution and the Levelized Cost of Hydrogen Storage (LCHS) were estimated. Based on the outcomes, the LCHS ranged from a best case scenario of 0.34 e/kg to a worst case scenario of 1.94 e/kg. Sensitivity analysis was conducted for the various storage parameters, and based on the dominant component the Net Present Value(NPV) is assessed with an increased LCHS. To conclude the study an estimate for the electrolyser capacity required for the plant to operate continuously was determined.

The first part of the thesis explores the process of homogenization, particularly for the permeability properties of rock samples. The Hill-Mandel postulate of energy consistency throughout the transitioning of scales is revisited since traditional homogenization methods rely on applying specific boundary conditions to enforce energy consistency. However, it is shown that the applied boundary conditions influence the effective physical parameter and provide upper or lower bound estimations. Recently, it is shown that these boundary conditions influence a layer near the boundary of the sample and that homogenization applied on the subsample away from this boundary layer is not affected by the boundary conditions.

The research focuses on energy consistency by studying the evolution of the energy within the intrinsic subsample, away from the boundaries. With the help of Finite Element simulations of Stokes-flow through idealized structures, the energy of the fluid is traced without the influences of grain properties on the energy dissipation. By plotting the ratio of the energy dissipation of the macro- and micro-scale, it is shown that the energy consistency is not found within small subsamples. Yet, with a growing subsample, energy consistency is achieved naturally, without the enforcement of boundary conditions. As a result, it is concluded that the energy consistency is found at the Representative Elementary Volume (REV), which is a similar requirement as for traditional homogenization methods. The study of the natural energy consistency in idealized microstructures is extended to real microstructures, which include more natural heterogeneities, such as grain properties. It is shown that energy consistency is also found with the natural heterogeneities included, albeit with a slower convergence.

For the homogenization of the permeability of a sample, the energy ratio is now known to be unitary, which can be used as an accurate indicator to determine the size of the REV.

The second part of the thesis explores the process of determining the size of the REV, which is a common, yet essential practice in Digital Rock Physics. Currently, this is an extensive exercise, involving many and large-size simulations to trace the convergence of the physical property and requires a lot of computational resources and time. Numerical-statistical studies have shown that the convergence of the REV visualizes in a cone-like shape. By plotting the convergence for both the permeability and the energy dissipation ratio for idealized microstructures, this study analyses the shape and evolution of the cone of convergence. From this, the generic evolution law of the convergence is determined.

It is shown that the asymmetrical convergence cone is described with a log-normal distribution, with a stable mean throughout the evolution of the cone and a variance for each sample size. The evolution of the variance is described with the law of large numbers, taking into account a reference value. This makes it possible to determine the size of the REV. Since the statistical method applies, information about the error of the fit, the error of the determined homogenized property, and the error of the size of the REV is provided.

The study is extended to real microstructures to validate whether the determined evolution law applies when natural heterogeneities are included. It is shown that the evolution law still accurately describes the REV's convergence. Therefore, the REV's size of real rock samples can also be determined. Even when the REV is not included within the sample, the evolution law can provide an estimate of the size of the REV or the homogenized property.

By using the cone of convergence, it is not necessary to run simulations on the full sample to find the REV, which is computationally expensive, instead running a number of simulations on small subsamples is sufficient, which saves both time and computational resources. It also unlocks the possibility to find the REV for high-resolution samples, as splitting the sample into subsamples allows for smaller simulations.

This thesis aims to analyze the techno-economic feasibility of integrating a salt cavern storage system into a green hydrogen production system in the Dutch North Sea, with the objective to provide a constant green hydrogen supply to the industry sector. To this end, the potential for creating salt caverns in the North Sea was examined and the prospected theoretical storage capacity was evaluated and compared to the required storage capacity and the costs of the system were assessed.

The simulations reveal that a 19.8 GW wind park in combination with an 8.65 TWh salt cavern storage system would be sufficient to provide a constant hydrogen supply to a 10 GW pipeline. Four locations in the Dutch North Sea were identified where a group of adjacent salt caverns could be created with a collective storage capacity of at least the required 8.65 TWh. Although further research is required, from this analysis it is concluded that creating a salt cavern storage system of sufficient magnitude may indeed be technically feasible.

The incremental cost of adding to offshore hydrogen production an 8.65 TWh salt cavern storage system is estimated at €5.4 billion, which translates to an addition of 0.17 €/kg to the LCOH for offshore green hydrogen production. Optimization of system design and the financial state of affairs may reduce costs for the storage system and thereby the total LCOH for offshore hydrogen production and storage.

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.

Carbon dioxide (CO2) capture and sequestration (CCS) can play a significant role in reducing anthropogenic CO2 emissions while allowing society to slowly phase out traditional energy sources. One of the main challenge in CCS is the current absence of a clear revenue model. One idea is to use the sequestered supercritical CO2 as a working fluid for geothermal energy extraction from sedimentary reservoirs (Carbon-dioxide Plume Geothermal or CPG). Due to the large variability in density and mobility of CO2 under different temperatures, reservoir heterogeneities can give a rise to a combination of convective and conductive heat transfer. In this work, qualitative and quantitative descriptions
of the effect of reservoir properties on the performance of CPG in depleted
gas fields are provided using an example realistic depleted gas field. The primary focus is on the behaviour of the CO2 plume with regards to different reservoir properties such as porosity, permeability and thermal properties. The effect of large-scale reservoir structure, such as a presence of an aquifer, net-to-gross ratio and layering is also studied. In order to accurately model these effects, a thermal multi-component multiphase model based on a fugacity-activity Equation of State is built and validated for for pressures from 50-400 bar and temperatures from 35°C to 130°C. The developed thermodynamic model
is implemented into the Delft Advanced Research Terra Simulator. Numerous studies of 2D and 3D ensembles and sensitivity studies are carried out to examine the effects of isolated parameters on CPG performance. Results reveal that increased net-to-gross (N/G) ratio is associated with increased recovery factor. In addition, layering architecture becomes an important factor for the importance of conductive flux only at low N/G. Variations in the required pressure to sustain a production rate is associated with fluctuations
in production temperature and density due to expansive cooling. Varying reservoir properties and state also have a significant effect on brine upconing, which is detrimental to CPG performance. It appears that an increase in injection rate have a positive effect on the performance of CPG, but this should be studied in conjunction with a coupled wellbore and power plant model. Heterogeneous porosity-permeability realizations show a strong decrease in reservoir lifetime compared to their upscaled homogeneous counterparts, which is caused by a combination of preferential flow, reduced conductive flux and lower production BHP associated with the upscaled realizations. It was also found that reducing production rate delays the time of thermal breakthrough due to the combined effect of these factors.

In this paper the suitability of compressed hydrogen gas storage in salt caverns is analysed. The presence of microbial sulfate reducing bacteria create a contamination risk of H2S inside the cavern. Key cavern parameters that influence the production of H2S are highlighted by a chemical model. The model uses empirical data provided by Vattenfall, in order to predict what would happen inside an actual cavern. By doing so, it can be understood what cavern conditions are suitable for the storage of hydrogen. An analysis is done on the above ground process of a salt cavern storage plant to determine what extra separation steps are required to reach ISO limitations for hydrogen gas. The research is done by answering the following research questions: What are key cavern conditions that influence the suitability of hydrogen storage in salt caverns and for what purpose can the storage plant be implemented? The sub-research objectives are formulated as follows: 1. What are the potential process risks when storing hydrogen in salt caverns? 2. Are there substantial risks of contamination with subsurface hydrogen storage? What are the defining variables that contribute to said contamination? 3. How do these impurities build up when the salt cavern is used? 4. Is the equipment currently used in the gas storage facility in Epe useable for hydrogen gas storage? What changes should be made to the current storage process? When analysing future utilisation demands of hydrogen, one of the primary applications is energy con- version by fuel cells. There are severe limitations set by the International Organization for Standardization on the maximum amount of H2S in hydrogen gas used by fuel cells. This paper uses a chemical model based on PHREEQC to predict the chemical reactions in the cavern. In order to get close to actual results, the model input is constructed following empirical data of an existing salt cavern. With the chemical model different cases can be constructed each highlighting an important cavern constraint. What are the positive and negative forces on the production of H2S in salt caverns and what could be theoretically done to prevent H2S contamination? Primary aspects that positively contribute to H2S production, when the cavern is modelled as a batch reactor, sorted by significance are: • Bacterial growth and reduction rate • Brine volume and sulfate concentration. • Brine pH and ionic strength. • Cavern pressure and temperature. • Fe2+ and Fe3+ concentration. The chemical model only predicts what will happen in the cavern when there is no gas coming in or out, like a batch reactor. For this reason a dynamic model is constructed which predicts H2S outflow in the gas when applying different demand-cases to the cavern. The model results showed that with applying a maximum use case, which fluctuates between the maximum pressure and minimum pressure, there is still a minimum H2S output that is higher then the allowed limits. A demand curve is simulated where the cavern is used to power a hydrogen gas-turbine to profit from seasonal energy price changes. In this demand curve the cavern produces significantly less and less often, giving time for the H2S to build up. In two years, the H2S production reaches levels above the allowable limits set for hydrogen gas turbines. A gas process facility is required to eliminate H2S contamination, in order to size such a facility it was modelled in Aspen Hysys. The model is validated by using data from literature. After analyses of resulting process equipment and gas streams, the absorber tower’s efficiency is most dependant on its temperature its pressure and the absorbent flow rate. The water concentration is above ISO levels when withdrawn from the cavern. So to follow these limitations, an additional dehydration step is required. In conclusion the process is capable of accurately separating the H2S to below ISO limits. The process works using 6% of the potential chemical energy of hydrogen. The process is unable to purify the water concentrations. As a result of this research some conclusions can be made. When using salt caverns for long term hy- drogen storage can be a significant risk of H2S contamination as a result of microbial sulfate reduction. For the reference case the H2S concentrations reach above the levels set for fuel cell use and concentrations will increase in significance when utilisation of the cavern is decreased. For fuel cell application, a separation pro- cess based on MDEA gas sweetening can be used to get the hydrogen up to the demanded H2S purity. But a more economical solution would involve extensive testing of the cavern soil for any microbial activity. If there are no sulfate reducing bacteria there will be no problem. Other pre-process steps could involve increasing the pH of the brine in the cavern to avoid H2S production. As well as increasing the iron concentration in the brine.

Salt caverns formed by solution mining may cause soil subsidence, because the surrounding adapts to fill the void (cavity) created in the strata. The cavern volume is hence not only a function of salt extraction (estimated from mass balance) but also a function of salt creep. The cavern size can be measured by sonar, but a less costly way would be to correlate cavern volume to cavern compressibility. The size of the cavern can be obtained by a compressibility test. The compressibility test consists of injecting brine or fresh water into the cavern, causing a pressure build-up. The pressure increase depends on the cavern volume and compressibility. Bérest et al. (2006) [1] derived an equation that expresses the pressure response to injection of a volume V of fresh water. The equation includes creep that can be derived from a separate geometric creep model that relates the strain to the stress history using the Norton-Hoff law for describing an Arrhenius type of response model, and thermal expansion effects influenced by the geothermal temperature. It also considers Darcy flow and leakage through the permeable salt cavity. This thesis investigates the sensitivity related to the various mechanisms in the Bérest and Van Sambeek (2005) model. It also extends the model by introducing chemical dissolution of salt effects and taking into account the impact of the sump (insoluble precipitates at the bottom of the cavern). We state the model equations and solve them for an example of interest. We can also compare the calculated pressure response to the results of a pressure test. In this pressure test a given volume of fresh water is injected above the sump and produced at the top of the cavity. The produced salt concentration is also one of the parameters measured in the test. There are at least two widely used methods to obtain or predict cavern size of active salt solution mining caverns, namely sonar surveys and leaching simulators. However, these data can be flawed due to uncertainties of the methods and the accuracy of measuring devices like flow meters. The compressibility test can then be used as an additional survey method. The test execution is simple and inexpensive, making it possible to be performed more frequently than a sonar survey. The cavern volume is obtained from a compressibility test data by using a solution that includes not only the injection volume, but also volumes introduced by other phenomena. These other phenomena are caused by thermal, hydraulic, mechanical, and chemical influences. Previous work has attempted to introduced solutions incorporating these influences for idle caverns. The proposed solution incorporates these effects in an active leaching salt cavern. It also introduces the impact of sump (insoluble on cavern bottom) to be included in the model. The proposed solution is then tested against field data. The work here differs from previous work that it presents a comprehensive description of the various methods used in the literature and practice. Where current models predict cavity volumes with a mean absolute percentage error of 33% (Thiel’s method for BAS3O) to 127% (Bérest et al. for BAS4) for the studied caverns (BAS3O and BAS4), the model here, which includes the presence of insoluble particles, and a well-established creep model is able to predict cavity volumes to an accuracy of 33% (proposed model for BAS3O) to 12% (proposed model for BAS4).

A considerable contribution to the greenhouse effect originates from landfills. This contribution originates from the degeneration of waste in the landfill. The gas which is created during the degeneration consists of 40 to 60% methane. One mole of methane has a global warming potential which is 28 times higher than that of carbon dioxide. Therefore, emissions of methane should be reduced. One method which can be used to reduce this emission is the oxidization of methane to carbon dioxide. This can be done by the construction
of a cover soil with methanotrophic bacteria in the soil. These bacteria oxidize methane as their source of carbon and energy. This oxidation process discriminates against the heavier isotopes present in the gas, this shifts the isotope signature. A method to determine the oxidation efficiency of the methane is based on the evaluation of this change in isotope signature over the cover soil. Next to the fractionation of isotopes due to oxidation, fractionation also occurs due to diffusion. However, fractionation due to diffusion is generally
assumed to be negligible. Mostly, this assumption is valid, since the dominant part of transport is advective; which does not discriminate against an isotope.

In this thesis the effect of compaction and saturation on two sands are evaluated. Using this evaluation, the criteria during which diffusion becomes an important transport phenomena are determined. This is determined by a series of experiments, where the concentration of methane is monitored while the gas diffuses through a soil sample. In this thesis the effect of the soil matrix on the fractionation of methane is also evaluated. This is evaluated by performing isotope spectrometry on samples of gas, taken at different times during the experiments. These two aspects are combined to evaluate if the oxidation efficiency can still be determined using fractionation of stable methane isotopes when diffusion plays a considerable role in gas transport.

During the experiments the decrease of methane in the chamber was measured. Spread over two soils, for a total of 18 variations in compaction or saturation the decrease of methane over time was measured. The results of the experiments show that for diffusion no distinction between variation in compaction and saturation needs to be made. Compaction and saturation can be described together for both soils using the air-filled porosity. The relation between the air-filled porosity and the effective diffusion coefficient is linear for both soils. Due to the large sand fraction the share of coarse pores is high for the whole range of compaction, meaning that no significant increase of tortuosity is visible in the relation between air-filled porosity and the effective diffusion coefficient. When the effective diffusion coefficients are compared to effective permeability values of these soils, it showed that the diffusion is more important for drier and more compacted soils. Thus, diffusive transport becomes important for gas transport over a cover soil when the pressures are lower, the soil is drier, and the soil is more compacted.

The fractionation factors due to diffusion, which have been determined for a selection of experiments, showed no trend with any other measured parameters. The precision of the measured values from which the fractionation factors are determined is high. Thus, the confidence in the fractionation factors is high.
The lack of a trend present means that in sand the soil matrix has no effect on the fractionation factor. This means that the fractionation factor due to diffusion in soil is the same as in free air. For the calculation of the oxidation efficiency with a relevant diffusive flux the ratio of diffusive to advective flux needs to be
determined but the fractionation factor due to diffusion is constant. Before the oxidation efficiency can be calculated, first the load needs to be corrected for any loss through hot-spots. Next, the advective flux and diffusive flux are combined, this is done by adding the fluxes together. The fluxes can be added because there are no interdependent effects, each flux only increases the total flux. Thus, the oxidation efficiency can be calculated using the changes in isotope signatures, even when a significant diffusive flux is present.