Indiana University-Purdue University Indianapolis (IUPUI) / The reaction between elemental sulfur and sulfide is a lynchpin in the biotic and abiotic cycling of sulfur. This dissertation is focused on the reactivity of elemental sulfur nanoparticles (S8weimarn, S8raffo) among other forms of elemental sulfur (S8aq, S8aq-surfactant, α-S8), and how the variation of their surface area, character and coatings reflect on the analytical, physical-chemical and geochemical processes involving sulfur cycling. A comprehensive electrochemical investigation utilizing mercury-surface electrodes showed that elemental sulfur compounds are represented by three main voltammetric signals, corresponding to potentials at -1.2V, -0.8V, and -0.6V in the absence of organics at circumneutral pH. Dissolved S8aq-surfactant signals can be found from -0.3V up to -1.0V, depending on the surfactant in the system. Variations in current response resulted from differences in electron transfer efficiency among the forms of S8, due to their molecular structural variability. Based on this observation a new reaction pathway between S8 and Hg-surface electrodes is proposed, involving an amalgam-forming intermediate step. The kinetics of the nucleophilic dissolution of S8nano by sulfide, forming polysulfides, were investigated under varying surface area, surface character and presence or absence of surfactant coatings on S8nano. Hydrophobic S8weimarn and hydrophilic S8raffo show kinetic rate laws of 𝑟𝑆8𝑤𝑒𝑖𝑚𝑎𝑟𝑛 = 10−11.33 (𝑒 −700.65 𝑅𝑇 ) (Molar(S8)/second/dm-1) and𝑟𝑆8𝑟𝑎𝑓𝑓𝑜 = 10−4.11 𝑖−0.35 (𝑒 −615.77 𝑅𝑇 ) (Molar(S8)/second), respectively. The presence of surfactant molecules can influence the reaction pathways by dissolving S8nano and releasing S8aqsurfactant, evolving the rate-limiting step as a function of the degree of the solubilization of S8nano. The reaction rate of S8biological can be compared with those of S8raffo and S8weimarn in circumneutral pH values and T=50oC, making the forms of S8nano successful abiotic analogue models of microbially produced S8biological. Field observations and geochemical kinetic modeling in the geothermal features of Yellowstone indicate that the nucleophilic dissolution reaction appears to be a key abiotic pathway for the cycling of sulfur species and the enhancement of elemental sulfur bioavailability. Furthermore, in situ and ex situ voltammetry in the same geothermal waters disclosed chaotic variability in chemical gradients of sulfide (observed over small temporal and spatial scales) which can be considered as an ecological stressor capable of influencing single cell physiology and microbial community adaptation.

The energy retrieval process in an Enhanced Geothermal System (EGS) depends on fracture networks to facilitate fluid movement, thereby enabling the extraction of heat from adjacent rocks matrix. Nonetheless, due to the inherent heterogeneity and intricate multi-physics characteristics of these systems, high-fidelity physics-based forward simulations ($f_h$) can be computationally demanding. This presents a considerable obstacle to the efficient management of these reservoirs. Therefore, creating an effective and robust optimization framework is essential, with the primary aim being to maximize the thermal extraction from Enhanced Geothermal Systems (EGS). A deep learning-assisted reservoir management framework incorporating a low-fidelity forward surrogate model ($f_l$) alongside gradient-based optimizers is developed to expedite reservoir management. A thermo-hydro-mechanical (THM) model for EGS is established by utilizing finite element-based reservoir simulation techniques. By parameterizing the fracture aperture and well controls, we carried out the THM simulation to produce 2500 datasets. Subsequently, we employed these datasets to train two distinct deep neural network (DNN) architectures to predict the variations in pressure and temperature distributions. Ultimately, these predictions from the forward model are used in calculating the total net energy. Instead of executing the optimization workflow with a large number of simulations from $f_h$, we directly optimize the well control parameters relative to the geological parameters using $f_l$. Since $f_l$ is efficient and fully differentiable, it could be combined with various gradient-based or gradient-free optimization algorithms to maximize the total net energy by determining the optimal decision parameters. Drawing from the simulation datasets, we analysed the effect of fracture aperture variation on temperature and pressure evolution. Our investigation revealed that the spatial distribution of the fracture aperture is a predominant factor in controlling the propagation of the thermal front. Variations of the fracture aperture exhibit a strong correlation with temperature fluctuations within the fracture, primarily due to thermal stress changes. When compared with a comprehensive physics simulator, our DNN-based forward surrogate model offers a significant computational acceleration, approximately 1500 times faster, without compromising predictive accuracy, achieving an $R^2$ value of 99%. The forward model $f_l$, when combined with gradient-based optimizers, enables optimization to proceed 10 to 68 times faster than when using derivative-free global optimizers. The proposed reservoir management framework exhibits both efficiency and scalability, facilitating the real-time execution of each optimization process.

Traditionally geothermal energy utilizes naturally occurring steam or hot water trapped in permeable rock formations through naturally occurring extraction wells or by implementing the hydraulic fracturing process by fracturing rock formations with water-based fracturing fluids. In contrast, in Enhanced Geothermal System (EGS) hydraulic fracturing process is utilized to create new or reopen existing fractures by injecting high-pressure fluid into deep Hot Dry Rocks (HDR) under carefully controlled conditions. Fracturing fluids are usually water-based that utilize an immense quantity of water. In EGS, they are essential for conducting hydraulic fracturing which bring the concern of technical approach and environmental impact. Thus, an alternative approach is to use waterless fracturing technologies, such as foam-based fracturing fluid. Foams are a complex mixture of the liquid and gaseous phases, where the liquid phase act as an ambient phase and gas is the dispersed phase. Foam fracturing fluids offer potential advantage over conventional water-based fracturing fluids, including reduced water consumption and environmental impact. Although foam-based fracturing has shown promising results in oil and gas industries, its feasibility has not been demonstrated in EGS conditions that usually involve high temperature and high pressures. One potential barrier to utilizing foam as fracturing fluid in EGS applications is that foams are thermodynamically unstable and will become more unstable with increasing temperature due to phenomena such as liquid drainage, bubble coarsening, and coalescence. Therefore, it is essential to stabilize foam fluids at high temperatures for EGS related applications such as fracking of HDRs. This project aims to evaluate the thermodynamic behavior of foams at high temperature and high pressure conditions closely resembling the geothermal environment. In this research, foam behavior was categorized as foam stability based on its half-life, i.e., the time taken by the foam to decrease to 50% of its original height. A laboratory apparatus was constructed to evaluate the foam half-life for a temperature range of room temperature to 200°C and a pressure range of ambient pressure to > 1000 psi. Two types of dispersed/gaseous phases, nitrogen gas (N2) and carbon dioxide gas (CO2), were investigated. Four different types of commercial foaming agents/surfactants with various concentrations were tested, including alfa olefin sulfonate (AOS), sodium dodecyl sulfonate (SDS), TergitolTM (NP – 40), and cetyltrimethylammonium chloride (CTAC). Moreover, five stabilizing agents, guar gum, bentonite clay, crosslinker, silicon dioxide nanoparticles (SiO2), and graphene oxide dispersions (GO), were also added to the surfactants to enhance foam stability. Experimental results showed that N2 foams were more stable than CO2 foams. It was observed that foam half-life decreased with the increase in temperature. Among all the surfactants, AOS foams showed the most promising thermal stability at high temperatures. Moreover, with the addition of stabilizing agents, foam's half-life was enhanced. Stabilizing agents such as crosslinker and GO dispersion showed the most stable foams with half-life recorded at 20 min and 17 min, respectively, at 200°C and 1000 psi. Finally, pressure also showed a positive effect on foam stability; with increased pressure, foam half-life was increased. Based on the experimental data, analytical models for the effect of temperature and pressure were developed, considering foam degradation is a first-order kinetic reaction that linearly depends on the foam drainage mechanism. The effect of temperature on foam half-life was studied as an exponential decay model. In this model, foam half-life is a function of drainage rate constant (DA) and activation energy (Ea) of the foam system. The effect of pressure on foam half-life was found to obey a power-law model where an increase in pressure showed an increase in foam half-life. Furthermore, a linear relation was studied for the effect of pressure on foam activation energy and drainage rate. Then the, combined effects of temperature and pressure were studied, which yielded an analytical model to predict the foam stabilities in terms of half-life for different foam compositions. This research indicates that with an appropriate selection of surfactants and stabilizing agents, it is possible to obtain stable foams, which could replace conventional water fracturing fluid under EGS conditions. / Mechanical Engineering

Les systèmes géothermiques de surface extraient l’énergie du sol via un fluide caloporteur circulant dans un échangeur pour une profondeur ne dépassant pas 200 m. Deux typologies d’échangeurs sont généralement utilisées : les systèmes avec échangeurs verticaux, principalement affectés par les conditions géologiques ; et les échangeurs horizontaux, plus proches de la surface du sol, impactés essentiellement par les conditions climatiques. Dans le sol, les échanges thermiques sont majoritairement des transferts de chaleur par conduction. Ainsi, les propriétés thermo-physiques du sol influencent la quantité d’énergie extraite par les échangeurs. Afin de quantifier les propriétés thermo-physiques d’un sol sous l’influence des conditions géologiques et climatiques, deux dispositifs expérimentaux sont élaborés, conçus, instrumentés et validés au sein de notre laboratoire. Les résultats expérimentaux enrichissent les connaissances scientifiques sur le comportement hydrique d’un sol soumis à des événements pluvieux et l’impact de la contrainte verticale sur les propriétés thermo-physiques d’un sol. De plus, une étude numérique, à partir d’une modélisation 2D par éléments finis d’un échangeur airsol, évalue les performances énergétiques de ce dernier en fonction de différentes humidifications du sol et différents scénarios de pluie. Les résultats numériques révèlent ainsi l’intérêt d’utiliser un sol d’enrobage très humide pour accroître significativement les performances énergétiques d’un échangeur air-sol. / Surface geothermal systems extract energy from the ground via a fluid circulating in an exchanger at a depth not exceeding 200 m. Two typologies of exchangers are generally used: systems with vertical exchangers, mainly affected by geological conditions; and horizontal exchangers, closer to the surface of ground, impacted mainly by weather conditions. Thermal exchanges in the soil are mainly conduction heat transfers. Thus, thermo-physical properties of soil influence, mostly, energy extracted by exchangers. In order to quantify influence of geological and meteorological conditions on thermo-physical properties of soil, two experimental devices are developed, designed, instrumented and validated. The experimental results provide more appropriate scientific knowledge on hydric behavior of a soil subjected to rain events and influence of compactness on thermal properties of soil. In addition, one numerical study, based on a finite element 2D modeling of an earth-air heat exchanger, evaluates their energy performance under different soil moisture conditions and rain scenarios thus revealing the utility of water to significantly improve its performance.

Master of Arts / This thesis is an exploratory study that aims to identify whether it is adequate to apply the current approach of considering policies related to geothermal energy under the general umbrella of renewable policies or we need to use a system-sector based approach specifically for geothermal energy systems. I have identified a total of twenty-three different policy types related to geothermal energy systems in U.S. states. To understand how geothermal related policies diffuse from one U.S. state to another, and, therefore, better design policies to promote the use of geothermal energy in U.S. states, we need to perform several diffusion studies. This process is time consuming and expensive. Thus, focusing on the most promising geothermal related policies, at least as a start, is crucial for future studies focusing on the diffusion of geothermal related policies between U.S. states. Therefore, this thesis focuses on the preliminary step of selecting a limited set of geothermal related policies for future policy diffusion studies. The main conclusions and answers provided in this thesis provide a strong support to the hypothesis that a system-sector based approach is needed when studying policies related to geothermal energy in U.S. states. I explicitly report that each of the three main geothermal systems is impacted by different set of policy categories and types. I also discuss that not all policies have the same impact on all sectors in which the geothermal energy is applied; in other words, the utilization of geothermal energy in the different sectors is promoted by different policies in distinguished ways. Moreover, the discussion in this thesis highlights the shortcomings of the common approach usually used in diffusion v studies of renewable energy policies. This approach considers all renewable energies as a general category, neglecting any potential impacts due to the unique characteristics of each renewable source. I show that, for example, the most popular policy types considered in policy diffusion studies for renewable energies are not the most significant ones for the different geothermal systems. I also highlight the fact that other policy types that are generally overlooked in policy diffusion studies of the generalized renewable energies are more significant for geothermal energy systems. These results indeed support my hypothesis regarding the importance of system-sector based approach when investigating geothermal energy policies.

Geothermal energy is the energy naturally present inside the earth crust. When a large volume of hot water and steam is trapped in subsurface porous and permeable rock structure and a convective circulating current is set up, it forms a geothermal reservoir. A geothermal system can be defined as - convective water in the upper crust of earth, which transfers heat from a heat source (in the reservoir) to a heat sink, usually the free surface. A geothermal system is made up of three main elements: a heat source, a reservoir and a fluid, which is the carrier that transfers the heat. As an alternative source of energy geothermal energy has been under attention of the researchers for quite some time. The reason behind this is the existence of several benefits like clean and renewable source of energy which has considerable environmental advantage, with no chemical pollutants or wastes are generated due to geothermal emissions, and the reliability of the power resource. Hence research has been directed in several directions like exploration of geothermal resources, modeling the characteristics of different types of geothermal reservoirs and technologies to extract energy from them. The target of these models has been the prediction of the production of the hot water and steam and thus the estimation of the electricity generating potential of a geothermal reservoir in future years. In a geothermal power plant reinjection of the heat depleted water extracted from the geothermal reservoir has been a common practice for quite some time. This started for safe wastewater disposal and later on the technology was employed to obtain higher efficiency of heat and energy extraction. In most of the cases a very small fraction of the thermal energy present in the reservoir can be recovered without the reinjection of geothermal fluid. Also maintaining the reservoir pressure is essential which gradually reduces due to continuous extraction of reservoir fluid without reinjection, especially for reservoirs with low permeabilities. Although reinjection of cold-water has several benefits, the possibility of premature breakthrough of the cold-water front, from injection well zone to production well zone, reduces the efficiency of the reservoir operation drastically. Hence for maintaining the reservoir efficiency and longer life of the reservoir, the injectionproduction well scheme is to be properly designed and injection and extraction rates are to be properly fixed. Modeling of flow and heat transport in a geothermal reservoir due to reinjection of coldwater has been attempted by several researchers analytically, numerically and experimentally. The analytical models which exist in this field deal mostly with a single injection well model injecting cold-water into a confined homogeneous porous-fractured geothermal reservoir. Often the thermal conductivity is neglected in the analytical study considering it to be negligible which is not always so, as proved in this study. Moreover heterogeneity in the reservoir is also a major factor which has not been considered in any such analytical study. In the field of numerical modeling there also exists a need of a general coupled three-dimensional thermo-hydrogeological model including all the modes of heat transport (advection and conduction), the heat loss to the confining rocks, the regional groundwater flow and the geothermal gradient. No study existing so far reported such a numerical model including those mentioned above. The present study is concerned about modeling the non-isothermal flow and heat transport in a geothermal reservoir due to reinjection of heat depleted water into a geothermal reservoir. Analytical and numerical models are developed here for the transient temperature distributions and advancement of the thermal front in a geothermal reservoir which is generated due to the cold-water injection. First homogeneous geothermal aquifers are considered and later heterogeneities of different kinds are brought into picture. Threedimensional numerical models are developed using a software code DuMux which solves flow and heat transport problems in porous media and can handle both single and multiphase flows. The results derived by the numerical models have been validated using the results from the analytical models derived in this study. Chapter 1 of the thesis gives a brief introduction about different types of geothermal reservoirs, followed by discussion on the governing differential equations, the conceptual model of a geothermal reservoir system, the efficiency of geothermal reservoirs, the modeling and simulation concepts (models construction, boundary conditions, model calibration etc.). Some problems related with geothermal reservoirs and geothermal power is also discussed. The scenario of India in the context having a huge geothermal power potential is described and different potential geothermal sites have been pointed out. In Chapter 2, the concept of reinjection of the heat depleted (cold) water into the geothermal reservoir is introduced. Starting with a brief history of the geothermal reinjection, the chapter describes the purpose and the need of reinjection of geothermal fluid giving examples of different geothermal fields over the world where reinjection has been in practice and benefitted by that. The chapter further discusses on the problems and obstacles faced by the geothermal projects resulting from the geothermal reinjection, most important of which is the thermal-breakthrough and cooling of production wells. Lastly the problem of this thesis is discussed which is to model the transient temperature distribution and the movement of the cold-water thermal front generated due to the reinjection. The need of this modeling is elaborated which represents the motivation of taking up the problem of the thesis. Chapter 3 describes an analytical model developed for the transient temperature in a porous geothermal reservoir due to injection of cold-water. The reservoir is composed of a confined aquifer, sandwiched between rocks of different thermo-geological properties. The heat transport processes considered are advection, longitudinal conduction in the geothermal aquifer, and the conductive heat transfer to the underlying and overlying rocks of different geological properties. The one-dimensional heat transfer equation has been solved using the Laplace transform with the assumption of constant density and thermal properties of both rock and fluid. Two simple solutions are derived afterwards, first neglecting the longitudinal conductive heat transport and then heat transport to confining rocks. The analytical solutions represent the transient temperature distribution in the geothermal aquifer and the confining rocks and model the movement of the cold-water thermal front in them. The results show that the heat transport to the confining rocks plays an influential role in the transient heat transport here. The influence of some parameters, e.g. the volumetric injection rate, the longitudinal thermal conductivity and the porosity of the porous media, on the transient heat transport phenomenon is judged by observing the variation of the transient temperature distribution with different values of the parameters. The effects of injection rate and thermal conductivity have been found to be high on the results. Chapter 4 represents another analytical model for transient temperature distribution in a heterogeneous geothermal reservoir underlain and overlain by impermeable rocks due to injection of cold-water. The heterogeneity of the porous medium is expressed by the spatial variation of the flow velocity and the longitudinal effective thermal conductivity of the medium. Simpler solutions are also derived afterwards first neglecting the longitudinal conduction, then the heat loss to the confining rocks depending on the situation where the contribution of them to the transient heat transport phenomenon in the porous media is negligible. Solution for a homogeneous aquifer with constant values of the rock and fluid parameters is also derived with an aim to compare the results with that of the heterogeneous one. The effect of heat loss to the confining rocks in this case is also determined and the influence of some of the parameters involved, on the transient heat transport phenomenon is assessed by observing the variation of the results with different magnitudes of those parameters. Results show that the heterogeneity plays a major role in controlling the cold-water thermal front movement. The transient temperature distribution in the geothermal reservoir depends on the type of heterogeneity. The heat loss to the confining rocks of the geothermal aquifer also has influence on the heat transport phenomenon. In Chapter 5 another analytical model is derived for a heterogeneous reservoir where the heterogeneous geothermal aquifer considered is a confined aquifer consisted of homogeneous layers of finite length and overlain and underlain by impermeable rock media. All the different layers in the aquifer and the overlying and underlying rocks are of different thermo-hydrogeological properties. Results show that the advancement of the cold-water thermal front is highly influenced by the layered heterogeneity of the aquifer. As the cold-water thermal front encounters layers of different thermo-hydrogeological properties the movement of it changes accordingly. The analytical solution derived here has been compared with a numerical model developed by the multiphysics software code COMSOL which shows excellent agreement with each other. Lastly it is shown that approximation of the properties of a geothermal aquifer by taking mean of the properties of all the layers present will lead to erroneous estimation of the temperature distribution. Chapter 6 represents a coupled three-dimensional thermo-hydrogeological numerical model for transient temperature distribution in a confined porous geothermal aquifer due to cold-water injection. This 3D numerical model is developed for solving more practical problems which eliminate the assumptions taken into account in analytical models. The numerical modeling is performed using a software code DuMux as mentioned before. Besides modeling the three-dimensional transient temperature distribution in the model domain, the chapter investigates the regional groundwater flow has been found to be a very important parameter to consider. The movement of the thermal front accelerates or decelerates depending on the direction of the flow. Influence of a few parameters involved in the study on the transient heat transport phenomenon in the geothermal reservoir domain, namely the injection rate, the permeability of the confining rocks and the thermal conductivity of the geothermal aquifer is also evaluated in this chapter. The models have been validated using analytical solutions derived in this thesis. The results are in very good agreement with each other. In Chapter 7 the main conclusions drawn from the study have been enlisted and the scope of further research is also pointed out.