Utilizing existing deep mining systems for geothermal extraction not only facilitates the development of geothermal systems but also helps meeting the cooling requirements for deep mining operations. In this study, a thermo-hydro-mechanical model of geothermal extraction in deep mines is developed to investigate the evolution of mine galleries stability and temperature, and the temperature changes in geothermal production wells. The uncertainty in system responses is predicted through the Bayesian Evidential Learning framework. Due to our limited understanding of the material properties and the scarcity of measurement data, uncertainties emerge in the forward simulations. Ideally, a comprehensive uncertainty analysis would be conducted to predict all possible outcomes and assess any risks. However, In light of the intractability of performing comprehensive uncertainty analyses in scenarios with vast unknown data, particularly due to the computational overhead of multiple inverse problemsolving, we employ the Bayesian Evidential Learning framework, which provides a feasible and rapid alternative for approximating prediction post-distributions and choosing the most informative data sets. Before implementing BEL, we employed Latin Hypercube Sampling to create 500 sets of realizations for forward simulations, and subsequently utilized global sensitivity analysis to evaluate the data's informational value, aiming to diminish the uncertainty in predictions. In this paper, the BEL framework is utilized to achieve two: firstly, to stochastically predict the responses of the system (stability and temperature) within the BEL framework, using machine learning to discover direct correlations between predictors (sensitive parameters) and targets (system responses). Subsequently, newly collected data can be utilized to predict the approximate posterior distributions of the corresponding gallery stability, temperature, and production well temperature, thus circumventing traditional data inversion steps. This framework can be adjusted to accommodate any predictions related to subsurface conditions; hence, our second goal involves predicting the system's long-term responses within the BEL based on shortterm data collection, forecasting posterior distributions from the acquired short-term data, and validating the efficacy of this approach. Our study indicates that in practical engineering, by (1) obtaining data of material properties and (2) key responses of short-term simulation, it is possible to predict the critical responses of the system in long-term geothermal extraction, thereby maximizing the information content of any measurement data while minimizing budget constraints and computational costs.@en

Bio-mediated methods, such as microbially induced carbonate precipitation, are promising techniques for soil stabilisation. However, uncertainty about the spatial distribution of the minerals formed and the mechanical improvements impedes bio-mediated methods from being translated widely into practice. To bolster confidence in bio-treatment, non-destructive characterisation is desired. Seismic methods offer the possibility to monitor the effectiveness and mechanical efficiency of bio-treatment both in the laboratory and in the field. To aid the interpretation of shear wave velocity measurements, this study uses the discrete element method to examine the small-strain stiffness of bio-cemented sands. Bio-cemented specimens with different characteristics, including properties of the host sand (void ratio, uniformity of particle size distribution) and properties of the precipitated minerals (distribution pattern, content, Young’s modulus), are modelled and subjected to static probing. The mechanisms affecting the small-strain properties of cemented soils are investigated from microscopic observations. The results identify two mechanisms controlling the mechanical reinforcement associated with bio-cementation, namely the number of effective bonds and the ability of a single bond to improve stiffness. The results show that the dominant mechanism varies with the properties of the host sand. These results support the use of seismic measurements to assess the mechanical efficiency and effectiveness of bio-mediated treatment.

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Bio-cemented soils can exhibit various types of microstructure depending on the relative position of the carbonate crystals with respect to the host granular skeleton. Different microstructures can have different effects on the mechanical and hydraulic responses of the material, hence it is important to develop the capacity to model these microstructures. The discrete element method (DEM) is a powerful numerical method for studying the mechanical behaviour of granular materials considering grain-scale features. This paper presents a toolbox that can be used to generate 3D DEM samples of bio-cemented soils with specific microstructures. It provides the flexibility of modelling bio-cemented soils with precipitates in the form of contact cementing, grain bridging and coating, and combinations of these distribution patterns. The algorithm is described in detail in this paper, and the impact of the precipitated carbonates on the soil microstructure is evaluated. The results indicate that carbonates precipitated in different distribution patterns affect the soil microstructure differently, suggesting the importance of modelling the microstructure of bio-cemented soils.

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Geothermal energy extraction through deep mine systems offers the potential to reduce the cost of geothermal systems while meeting the cooling needs of deep mines. However, the injection of cold water into the subsurface triggers strongly coupled thermo-hydro-mechanical (THM) processes that can affect the stability of underground excavations. This study evaluates the impact of geothermal energy extraction on the temperature and stability of a deep mine. By quantifying the sensitivity of the mine temperature and stability to various parameters, we propose a scheme to optimize geothermal energy production, while achieving rapid mine cooling and maintaining stability. We first evaluate the impact of geothermal operations on mine temperature and stability through THM numerical modeling. The simulations show that poro-elastic stress quickly affects mine stability, while thermal stress has a more significant impact on the long-term stability. We then use Distance-based Generalized Sensitivity Analysis (DGSA) to quantify parameter sensitivity. The analysis identifies the distance between the mine system and the geothermal system as the most influential factor. Other important parameters include the injection rate, injection temperature, well spacing, coefficient of thermal expansion, permeability, Young's modulus, and heat capacity. Finally, we propose a DGSA-based optimization framework that accounts for subsurface uncertainty and validate the optimized results. Our results indicate that, with favorable geological conditions, a rational selection of system design parameters can enhance geothermal energy production while ensuring rapid mine cooling and stability. This study provides essential insights for the optimization of deep mine geothermal systems and supports effective decision-making.

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Geological Disposal Facilities (GDF) for radioactive waste will generally rely on clay-rich materials as a host geological formation and/or engineered barrier. Gas will be produced within the GDF, which can build up significant gas pressure and will activate the migration of gas through the clay materials via different transport mechanisms. These transport mechanisms are usually investigated in laboratory tests on small clay samples of a few centimetres. In this paper, a new Pneumo-Hydro-Mechanical (PHM) Finite Element model to simulate gas migration in saturated clay samples of this scale is presented. In the proposed modelling approach, continuum elements are used to represent the mechanical and flow processes in the bulk clay material, while zero-thickness interface elements are used to represent existing or induced discontinuities (cracks). A new triple-node PHM interface element is presented to achieve this. The performance of model is illustrated with synthetic benchmark examples which show the ability of the model to reproduce observed PHM mechanisms leading to propagation of cracks due to the gas pressure (gas fracturing).

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Microbially induced carbonate precipitation (MICP) involves bacteria to drive calcite precipitation and naturally cement soils, thereby improving soils performance. Experimental studies have shown that bio-cemented specimen can suffer from severe spatial inhomogeneity of the calcite content, leading to large uncertainty in treatment efficiency prediction. To evaluate the effect of inhomogeneity on the mechanical behaviour of bio-cemented soils, the discrete element method (DEM) is used to model bio-cemented samples with a single carbonate distribution pattern (i.e. either bridging or contact cementing) but different characteristics of inhomogeneity. Both drained triaxial compression and triaxial extension simulations are carried out to evaluate the impact of inhomogeneity along different loading paths. The results indicate that inhomogeneity has different effects on bio-cemented samples depending on the carbonate distribution patterns and the loading path. Specifically, the shear strength in compression of samples exhibiting bridging cementation is largely affected by inhomogeneity, while the effect on shear strength in extension is negligible. On the other hand, samples with contact cementing show limited sensitivity to the variation of inhomogeneity under both triaxial compression and triaxial extension tests.

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The importance of physico-chemical processes at the particle scale for the engineering scale behaviour of fine-grained geomaterials is undisputed. Yet, despite great advances in the discipline, experimental evidence that fully resolves the clay micromechanics i.e. linking the evolving microstructure and interparticle actions under loading, is lacking. This paper will discuss the challenges ahead in quantifying the evolving kinematics and interparticle interactions of finegrained geomaterials. As such, the current limitations, and the potential opportunities of experimental methodologies for manipulating, monitoring and (post-mortem) analysing fine-grained materials at the particle scale will be discussed. In addition to the need of integrating multiple experimental techniques that span several length scales and modalities, the critical role of advanced data reduction and analysis is highlighted, as required for a measurement as opposed to qualitative observation. Throughout the paper, the link between experimental clay micromechanics and modelling will be discussed.@en

Soil-atmosphere interaction occurs frequently in most geotechnical situations. By including it in analyses, a high computational load can occur, and analyses are made more complex. This paper explores the key processes and demonstrates the impact and advantages of quantifying soil-atmosphere interactions. Possibilities of utilising the soil atmosphere interaction to accelerate soil processes and reduce environmental impact, and to generate energy are shown, highlighting benefits of consid- ering soil-atmosphere processes. It is also seen that it is not always necessary to directly include the processes in analyses, but that consideration of the processes can lead to approaches to directly monitor or guide monitoring. The soil-atmosphere interface is important, and whether it is necessary, useful or damaging to consider in analyses is situation dependent.@en

Microbially induced carbonate precipitation is a promising ground improvement technique which can enhance the mechanical properties of soils through the precipitation of calcium carbonate. Experimental evidences indicate that the precipitated carbonate can display different distribution patterns. Crystals can develop at grain–grain contacts (contact cementing), connect soil grains that were initially not in contact with each other (bridging), precipitate on the grain surface (coating), or fill in the void space (pore filling). This paper investigates the role of the aforementioned distribution patterns on the mechanical behaviour of lightly bio-cemented soil samples using discrete element modelling. Bio-cemented samples with different distribution patterns and carbonate contents are built, and a series of drained triaxial compression simulations are carried out at different confining pressures. The results show that cementation in the form of bridging and contact cementing leads to obvious improvement in stiffness, strength and dilatancy. In contrast, cementation in the form of coating contributes only slightly to mechanical improvement, and pore filling exhibits negligible influence on the mechanical response of the material. The findings suggest that, to gain strength improvement in the most effective way, treatments should be tailored to precipitate calcium carbonate crystals in the form of bridging.

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Nitrogen undergoes multiple biogeochemical transformations during waste degradation, which depend on speciation, prevailing geochemical boundary conditions, and waste surface properties. This study developed a waste biodegradation model with high flexibility in accommodating reaction pathways to assess different process dynamics. The model was applied to landfill simulator reactors operating anaerobically. Model results show that dilution with adsorption matches the experimental dissolved NH4+ concentration (C/N=25) at the early experimental stages. Also, NH4+ binding decreases due to competition with Ca2+, and the model better captures the dissolved NH4+ behavior when CaSO4 is present in solution. Mass removal due to sampling and posterior dilution are the main mechanisms to reduce NH4+ concentration in the leachate. The model highlights the role of nitrogen sorption as the main mechanism for nitrogen accumulation in the solid phase of municipal solid waste.@en

This paper presents a comprehensive investigation of the swelling behaviour of a compacted bentonite–sand mixture subjected to hydration under constant volume conditions. Contrary to previous studies, the tested sample was isotropically compacted before being hydrated under constant volume conditions until full saturation was reached. The total axial pressure, total radial pressures at four different heights of the sample, and injected water volume were recorded over time. The experimental data reveal a complex and non-uniform evolution of the axial and radial stresses over time, as well as anisotropy of the total stresses, which persist at the saturated equilibrated state. To gain further insights, a numerical analysis was performed using an advanced hydromechanical framework for partially saturated porous media, accounting for the evolving microstructure of the material. The complex evolution of the total axial and radial pressures with time is attributed to the advancing hydration and swelling front in the sample, along with the development of irreversible strains. The good agreement between the numerical results and the experimental data enables validation of the developed framework. Implications for engineered barriers in deep geological disposal of radioactive waste are discussed.

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With the increasing demand for mineral and alternative energy resources, as well as the gradual depletion of shallow resources, the exploitation and utilization of mineral resources and geothermal energy in deep strata is an effective way to solve the problem of resource shortage [1]. In recent years, as a new type of resource mining mode, the co-mining of deep mineral and geothermal energy has developed rapidly [2, 3]. This method can make use of the original equipment of the mine for geothermal exploitation. However, the deep co-mining system faces two significant challenges: the first is the significant uncertainty inherent in subsurface properties, while the second is the high levels of geostress and temperature associated with deep mining. These challenges are adding some constraints on the practicality of exploiting such systems and limit the feasibility of deep resource co-mining, so that modelling efforts are needed for actual risk assessment.
Consequently, we developed a Thermo-Hydro-Mechanical (THM) coupling framework for geothermal energy exploitation in deep mines using COMSOL to quantitatively characterize the temperature field of the geothermal system and predict the stress field of the mining system, considering the joint effects of large uncertainties and THM coupling. Through SGeMS, the uncertainty and spatial heterogeneity distribution of porosity are first generated. Then, the uncertainty of the hydraulic parameter [4] (permeability), mechanical parameter [5] (elastic modulus), and thermal parameter [6] (heat capacity and heat conductivity) was derived from the porosity. 500 samples were generated within a given uncertainty range, by means of Monte Carlo simulations. The spatial and temporal distributions of the temperature field of the geothermal system, and the stress field of the mining system were simulated, for each sample with COMSOL. Using the distance-based global sensitivity analysis [6], the most sensitive parameters for deep mining are identified, the heat storage capacity of the system and evolution of the maximum stress ratio are evaluated, including uncertainty.@en

In geothermal projects, reinjection of produced water has been widely applied for disposing wastewater, supplying heat exchange media and maintaining reservoir pressure. Accordingly, it is a key process for environmental and well performance assessment, which partly controls the success of projects. However, the injectivity, a measure of how easily fluids can be reinjected into reservoirs, is influenced by various processes throughout installation and operation. Both injectivity decline and enhancement have been reported during reinjection operations, while most current studies tend to only focus on one aspect. This review aims to provide a comprehensive discussion on how the injectivity can be influenced during reinjection, both positively and negatively. This includes a detailed overview of the different clogging mechanisms, in which decreasing reservoir temperature plays a major role, leading to injectivity decline. Strategies to avoid and recover from injectivity reduction are also introduced. Followed is an overview of mechanisms underlying injectivity enhancement during reinjection, wherein re-opening/shearing of pre-existing fractures and thermal cracking have been identified as the main contributors. In practice, nevertheless, mixed-mechanism processes play a key role during reinjection. Finally, this review provides an outlook on future research directions that can enhance the understanding of injectivity-related issues.

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Gas flow along localized dilatant pathways is considered as an important gas transfer mechanism in saturated plastic clays such as Boom clay. The process involves micro-fracturing of the pore space at rising gas pressures, gas flow and possibly sealing of the newly-created pathways. Modelling gas flow along localized dilatant pathways presents a number of challenges associated with: (1) the very strong coupling between the hydraulic and mechanical aspects of the problem, (2) the microscopic scale of the process and (3) the important influence of material heterogeneities on the process dynamics. This contribution presents a review of existing theoretical and numerical frameworks for damage, strain localisation and fracture of clay materials. In particular, it addresses the suitability of these approaches as a basis for the development of new modelling framework for gas flow along localized dilatant pathways. The research was carried out within the WP GAS of EURAD (2019 - 2024).@en

and boundary connected with aquifer respectively, are discussed. On the basis of these two boundary conditions, several factors that influence the fluid loss process are studied. For the thief zone with no-flow boundary, our key conclusions illustrate that once the pressure front reaches an opening in the sealing, fluid loss occurs. However, fluid loss is substantially limited (leakage rate lower than 0.02 m3/h) within one year of operation. The time at which leakage is limited, is controlled by the permeability shock between the thief zone and reservoir, transmissivity of fracture in sealing layer and injection pressure. In addition, only the injection pressure has a significant influence on the total amount of lost fluid, while the permeability shock and fracture conductivity do not influence it. However, more permeable thief zone and more conductive fracture could be helpful to quickly prevent the fluid loss, since the pressurisation up process can be achieved more quickly, and thus prevent the leakage in the early stage, ensuring a stable injection rate in the long term . In contrast, for the thief zone with fixed-pressure boundary, fluid loss does not stop. Instead, it keeps a constant leakage rate, indicating a continuous fluid loss. However, the increase in fluid pressure remains more limited. In this case, a more permeable thief zone, more conductive opening and higher injection pressure increase the leakage rate, yet reduce the elevated pressure. In conclusion, leakage can be prevented as a result of pressurization up of thief zones sealed by faults, and the only thing to consider in the planning stage is how fast it can be. While for thief zones linked with aquifers, injection plan should be made carefully to avoid induce or promote any fractures in sealing layer, or continuous leakage would occur. @en

The multi-purpose research borehole at the Delftse Hout is the third of four seismic monitoring locations of the seismic monitoring network for the geothermal research project on the TU Delft campus (Geothermal Delft GTD, also known as DAPwell, https://geothermiedelft.nl/). For the geothermal research project, two deep wells (“a doublet” consisting of an injector and a producer) for geothermal energy extraction will be installed on the TU Delft campus next to the combined heat and power plant (“warmtekrachtcentrale - WKC”). The system will produce geothermal heat to supply the campus of TU Delft and part of the city of Delft.
The herein presented borehole describes the installation of a multi-purpose research borehole (called DAPGEO-02), which was installed in the period February - May 2022. DAPGEO-02 is part of a seismic monitoring system for the shallow and deeper subsurface in the vicinity of the planned geothermal doublet. The locations of all four stations are given in Figure 1. The monitoring network and the related research gathers knowledge about the current status of the subsurface on the basis of periodic data measurements, and possible seasonal effects.
Within the seismic monitoring network, three seismic monitoring stations have already been installed, respectively DAPGEO-01 on the proposed location of the geothermal project near the Leeghwaterstraat in Delft, DAPGEO-03 on the Kerkpolderweg in Delft, and ZH03 in on the Ackersdijkseweg in Pijnacker-Nootdorp (installed and equipped by KNMI).@en