This study presents a probabilistic analysis of 3D Navier‐Stokes (NS) fluid flow through 30 randomly generated sheared fractures with equal roughness properties (Hurst exponent = 0.8). The results of ...numerous 3D NS realizations are compared with the highly simplified local cubic law (LCL) solutions regarding flow orientations and regimes. The transition between linear and nonlinear flow conditions cannot be described with a generally valid critical Reynolds number (Recrit), but rather depends on the individual fracture's void geometry. Over 10% reduction in flow is observed for increased global Re (>100) due to the increasing impact of nonlinear conditions. Furthermore, the fracture geometry promotes flow anisotropy and the formation of channels. Flow perpendicular to the shearing leads to increased channeling and fluid flow (∼40% higher) compared to flow parallel to the shearing. In the latter case, dispersed flow and irregular flow paths cause a reduction of LCL validity.
Plain Language Summary
The movement of fluid and heat through fractured rocks is massively affected by the void space of the contributing fractures and the velocity of the fluid. The opposing offset of the two fracture surfaces creates complex and variable void geometries. A calculation of the expected flow rates is very difficult. In most studies, simplifications are adopted neglecting these complex features by using a plane surface and assuming that the flow is taking place smoothly. We therefore simulate the real flow using the three‐dimensional fracture void space and compare the results with simpler two‐dimensional models. It can be demonstrated that the difference between the two types of simulations depends not only on the local simplification but also on the flow direction within the fractures and that the difference increases with higher flow velocities. If the fractures are not considered as a whole continuum but on the local scale, preferential fluid pathways or channels are formed in which most of the flow takes place. Depending on the flow direction, channels are more or less pronounced. We can show that in well‐developed channels the differences between the calculation methods are much smaller than in the other parts of the domain.
Key Points
Shearing of fractures generates a directional flow anisotropy, with an increased flow rate perpendicular to the shearing
Preferential channels are formed which localize the largest part of the volumetric flow in small parts of the entire fracture domain
Spatial differences between the complex Navier‐Stokes equations and the local cubic law can be explained by channeling processes
Silica nanoparticles have become an important tool in material sciences, nanomedicine, biotechnology, and pharmaceutics, with recent suggested applications also in environmental sciences. In life and ...environmental sciences, the application field is usually aqueous media; however, the crucial issue of silica nanoparticle dissolution behavior and rate in the target medium is often neglected, overlooked, or taken for granted. Silica nanoparticles are not stable in aqueous solutions until equilibrium silica concentrations are reached. While for life science applications, the degradability of silica nanoparticles is prerequisite for biocompatibility, this characteristic impedes the successful application of silica nanoparticles as environmental tracer, where long-term stability is needed. In this study, the impact of external (temperature, pH values, salinity, availability of silica) and internal (degree of condensation, size, porosity) parameters on the stability of ~ 45-nm-sized silica nanoparticles is characterized. Results show that external factors such as elevated temperature and alkaline pH-values accelerate the dissolution, acidic pH, high salinities, and high initial silica concentrations exhibit a contrary effect. Consequently, in applications, where external parameters cannot be controlled (e.g., in vivo, subsurface reservoirs), dissolution control and stability improvement of silica nanoparticles can be achieved by various means, such as adding a protective layer or by condensation of the silanol bonds through calcination.
Graphical abstract
Zusammenfassung
Die hier vorgestellte Arbeit schätzt den Stand der Extraktionstechnologien zur Lithiumgewinnung aus geothermalen Wässern basierend auf aktuellen wissenschaftlichen Studien ab und ...identifiziert mögliche technische Herausforderungen. Bewertet werden häufig diskutierte Technologien wie Flüssig-Flüssig-Extraktion, selektive Extraktion durch anorganische Sorptionsmittel, elektrochemische Methoden und Membrantechnologien hinsichtlich ihrer Anwendbarkeit und Integrierbarkeit in die geothermische Energieproduktion. Aktuelle Forschungsprojekte haben verschiedene Extraktionsmethoden im Labor- und teilweise Prototypenmaßstab validiert. Eine Skalierung zu einem industriellen Prozess existiert bisher nicht. Dementsprechend fehlen Informationen bezüglich Dauerbetriebs sowie Einfluss standortspezifischer Hürden (Wasserchemie, Volumenstrom, Fließraten etc.) und zur tatsächlichen Wirtschaftlichkeit. Die Menge des rückgewinnbaren Lithiums ergibt sich in erster Linie aus der Konzentration des im Wasser gelösten Lithiums, der Extraktionseffizienz und -geschwindigkeit, sowie der Menge des verwendeten Extraktionsmittels. Das Zusammenspiel dieser Faktoren bestimmt die Verfahrenstechnik und die Größe der Extraktionsinfrastruktur. Je nach Verfahren werden die physikochemischen Eigenschaften des Wassers (pH, Eh, T, p etc.) während der Extraktion verändert, wodurch das Scaling- und Korrosionspotenzial gesteigert werden kann.
Der aktuelle Stand der Technik zeigt ein frühes bis mittleres Technologiereifestadium bei Lithium-Extraktionseffizienzen in Laborexperimenten von 50–90 %. Unter den ungleich höheren Herausforderungen im laufenden Betrieb eines Geothermiekraftwerks, werden Extraktionseffizienzen im unteren Bereich dieser Bandbreite als realistisch angesehen.
Accurate quantification of spatially resolved fluid flow within fractures is crucial for successful reservoir development, such as Enhanced Geothermal Systems. This study presents an innovative ...workflow designed to model and characterize preferential flow paths (channels) within rough‐walled shear fractures. A set of 30 rough‐walled self‐affine fractures, all possessing identical roughness characteristics, is stochastically generated. By solving the nonlinear Navier‐Stokes equations in 420 individual realizations, the transition from linear to nonlinear flow regimes and the two extreme flow directions perpendicular and parallel to the shearing are numerically captured. A distinguishing feature of this approach is its comprehensive statistical analysis, which encompasses both the geometric and transport properties of flow paths in the non‐simplified three‐dimensional fractured void space under typical geothermal flow conditions. In a perpendicular orientation of flow and shearing, fluid flow exhibits pronounced localization, with more than one‐third of the volumetric flow concentrated within 15% of the fracture volume. In contrast, parallel to the shearing, a complex pattern of individual tortuous channels emerges, with flow occurring in 22% of the void space. Nonlinear effects primarily manifest outside these channels, suggesting that complex flow phenomena may dominate irregular fracture structures, such as contact zones or asperities. In the parallel case, increased flow rates lead to an amplification of channeling processes resulting in less affected volume and diminished tortuosity of the main flow path, while in perpendicular orientation nonlinear effects are only of minor importance. The small‐scale flow regime of both extreme cases tends to converge with increasing flow rates.
Plain Language Summary
Understanding fluid flow in subsurface rock is important to successful geothermal operations. In Enhanced Geothermal Systems, the water flows in special paths within the rocks. These paths are like shortcuts that fluid takes because they're easier to move through. It is therefore important to quantify these emerging channels in terms of their geometric appearance as well as their transport properties. This study could show that the extent of channeling depends strongly on the flow direction along or perpendicular to the shearing. Tortuous and geometry‐dependent flow paths are formed parallel to the shearing, whereas perpendicular to the shearing, channels tend to be straight and highly localized. Furthermore, channeling depends on the volumetric flow in the fracture, as higher fluid flow and rising nonlinear effects favor increased channeling with less heat exchange area and less tortuous paths.
Key Points
Sheared rough fractures exhibit preferential flow paths that localize a large part of the volume flow in small fracture void space
The 3D analysis shows that the geometric properties (height differences, tortuosity, and aperture) strongly affect the local flow regime
Increasing flow rates cause an increase in channeling with a simultaneous decrease in flow path length
•Application of solute geothermometer often yield large uncertainties (up to 200 K).•Improvement approach of SiO2 and Na-K geothermometers using laboratory experiments and numerical ...modeling.•Correction of site-dependet effects (reservoir lithology, pH value, dilution).•Strongly converging SiO2 and Na-K temperatures (≤10 K).
Solute geothermometry often leads to a broad range and often inconsistent calculated reservoir temperatures, in particular when exploring geothermal systems, where only limited information (geology, borehole data etc.) is available. The application of different Na-K and SiO2 geothermometer, the most widely used methods, not uncommonly lead to deviations of results by up to 200 K for one sample.
In this study, the most effective interfering factors for these geothermometer applications are identified. A multi-step approach is proposed, combining experimental and numerical methods with specific fluid characterization to quantify these factors and to transfer these findings to the natural system enabling the correction of temperatures to realistic in-situ values.
Taking into account dilution with surface water, a chlorofluorocarbon concentration based mixing model was set up to correct analysed SiO2 concentrations to original in-situ concentrations. A numerical model was used to determine the in-situ pH, which is highly sensitive to silica solubility. Results from long-term laboratory equilibration experiments were evaluated to identify the reservoir type dependent equilibrated SiO2 polymorph.
In the case of the Na-K geothermometer, it is shown that the Na+/K+ concentration ratio in fluids is obviously not unequivocally controlled by temperature but is also dependent upon reservoir rock composition. Thus, different reservoir lithologies lead to different equilibration states in terms of Na+/K+. This is obviously one reason for the existence of the large number of different Na-K geothermometers. By modelling the stability of the Na+/K+ ratio governing feldspars, albite and orthoclase, we suggest a method that reveals the Na+/K+ equilibration state for each fluid supporting the allocation of the appropriate geothermometer equation.
The improvement procedure is demonstrated in a case study evaluating fluid data of geothermal springs from the Villarrica geothermal system, Southern Chile. It is shown that initially highly scattered results strongly converge after corrections, leading to a substantial improvement in in-situ temperature estimations with small deviations of ≤10 K between SiO2 and Na-K geothermometers. Also absolute temperature calculated for each spring in the study area, ranging from 84 to 184 °C agree well (within ΔT <20 K) with results of multicomponent geothermometry temperatures reported in a previous work.
The extraction of rare metals like lithium (Li) from geothermal fluids is a promising alternative to conventional mining. Membrane distillation (MD) could support energy-efficient fluid treatment ...enabling further freshwater production. For the operation of geothermal plants and MD uncontrolled precipitation of silica (Si) represents a major hurdle. Herein, we demonstrate the transfer of a Si treatment from lab to field demonstrator scale, tested under conditions of an operating geothermal power plant.
For the treatment, lime precipitation was chosen showing good Si reduction rates using artificial fluids. The high alkaline conditions of this process (pH > 10) in combination with the high salinities of the geothermal brines (TDS > 100 g/L) are transferred into real geothermal environment with a newly developed numerical design calculation. The resulting demonstrator consists of three major process steps - 1) Si-reduction, 2) liquid/solid separation, and 3) post-concentration using MD. The Si treatment efficiently reduced 98 % of Si in <5 min reaction time, without influencing the lithium concentration negatively. The MD resulted in Li concentrations of ∼500 mg/L while producing fresh water. Beyond the approval of the concept, neuralgic points for improvement were identified expanding fundamental knowledge about the material use of geothermal fluids.
Display omitted
•Geothermal brines can contain high concentrations of raw materials such as Lithium.•Mineral scaling impedes combined geothermal energy and brine treatment processes.•Chemical brine treatment enables use of membrane distillation in geothermal cycles.•Numerical design simulation facilitates transferring the process to power plants.•Demonstrator is developed and continuously operated in a geothermal power plant.
Abstract
Conventional methods to estimate the static formation temperature (SFT) require borehole temperature data measured during thermal recovery periods. This can be both economically and ...technically prohibitive under real operational conditions, especially for high-temperature boreholes. This study investigates the use of temperature logs obtained under injection conditions to determine SFT through inverse modelling. An adaptive sampling approach based on machine-learning techniques is applied to explore the model space efficiently by iteratively proposing samples based on the results of previous runs. Synthetic case studies are conducted with rigorous evaluation of factors affecting the quality of SFT estimates for deep hot wells. The results show that using temperature data measured at higher flow rates or after longer injection times could lead to less-reliable results. Furthermore, the estimation error exhibits an almost linear dependency on the standard error of the measured borehole temperatures. In addition, potential flow loss zones in the borehole would lead to increased uncertainties in the SFT estimates. Consequently, any prior knowledge about the amount of flow loss could improve the estimation accuracy considerably. For formations with thermal gradients varying with depth, prior information on the depth of the gradient change is necessary to avoid spurious results. The inversion scheme presented is demonstrated as an efficient tool for quantifying uncertainty in the interpretation of borehole data. Although only temperature data are considered in this work, other types of data such as flow and transport measurements can also be included in this method for geophysical and rock physics studies.
•Numeric model of complex counterflow related processes in geothermal single well.•Fully coupled THC model to simulate halite scaling formation in a geothermal well.•Application of Pitzer model to ...handle fluid salinities up to halite saturation.
Using abandoned wells from oil and gas industry or dry holes, geothermal single well applications enable the prevention of drilling costs and prospecting risks which are crucial inhibitions in geothermal development.
Recently, deep geothermal single wells were numerically investigated for their thermal and hydraulic performance. For the first time, we now include the chemistry of the produced brine in a fully coupled THC model for studies of scaling formation. As a case study, the 2011 circulation test of the GeneSys well Gt1 Groß-Buchholz (Hanover, Germany) was modeled successfully using the fully coupled THC code TOUGHREACT. High salinity requires application of the Pitzer ion interaction model which was verified for this particular chemical system using literature data.
Modeled wellhead temperature is in very good accordance with measurements. Also simulated depth of scaling formation and total scaling volume fit to onsite observations. Evaluation of initial reservoir brine composition reveals its large impact on the range of depth where scaling occurs. Furthermore, it is shown that the presence of CaCl2 reduces halite solubility considerably and favors scaling formation.
Results show that our modeling concept is capable of quantifying the complex coupled THC processes in single wells.
Mesoporous silica nanoparticles hosting guest molecules are a versatile tool with applications in various fields such as life and environmental sciences. Current commonly applied pore blocking ...strategies are not universally applicable and are often not robust enough to withstand harsh ambient conditions (
e.g.
geothermal). In this work, a titania layer is utilized as a robust pore blocker, with a test-case where it is used for the encapsulation of fluorescent dyes. The layer is formed by a hydrolysis process of a titania precursor in an adapted microemulsion system and demonstrates effective protection of both the dye payload and the silica core from disintegration under otherwise damaging external conditions. The produced dye-MSN@TiO
2
particles are characterized by means of electron microscopy, elemental mapping, ζ-potential, X-ray diffraction (XRD), nitrogen adsorption, Thermogravimetric analysis (TGA), fluorescence and absorbance spectroscopy and Fourier Transform Infrared Spectroscopy - Total Attenuated Reflectance (FT-IR ATR). Finally, the performance of the titania-encapsulated MSNs is demonstrated in long-term aqueous stability and in flow-through experiments, where owing to improved dispersion encapsulated dye results in improved flow properties compared to free dye properties. This behavior exemplifies the potential advantage of carrier-borne marker molecules over free dye molecules in applications where accessibility or targeting are a factor, thus this encapsulation method increases the variety of fields of application.
A robust and stable encapsulation method for mesoporous silica nanoparticles, protecting the payload, preventing leakage and stabilizing the nanoparticles.
Solute geothermometry often leads to a broad range and often inconsistent calculated reservoir temperatures, in particular when exploring geothermal systems, where only limited information (geology, ...borehole data etc.) is available. The application of different Na-K and SiO2 geothermometer, the most widely used methods, not uncommonly lead to deviations of results by up to 200 K for one sample. In this study, the most effective interfering factors for these geothermometer applications are identified. A multi-step approach is proposed, combining experimental and numerical methods with specific fluid characterization to quantify these factors and to transfer these findings to the natural system enabling the correction of temperatures to realistic in-situ values. Taking into account dilution with surface water, a chlorofluorocarbon concentration based mixing model was set up to correct analysed SiO2 concentrations to original in-situ concentrations. A numerical model was used to determine the in-situ pH, which is highly sensitive to silica solubility. Results from long-term laboratory equilibration experiments were evaluated to identify the reservoir type dependent equilibrated SiO2 polymorph. In the case of the Na-K geothermometer, it is shown that the Na+/K+ concentration ratio in fluids is obviously not unequivocally controlled by temperature but is also dependent upon reservoir rock composition. Thus, different reservoir lithologies lead to different equilibration states in terms of Na+/K+. This is obviously one reason for the existence of the large number of different Na-K geothermometers. By modelling the stability of the Na+/K+ ratio governing feldspars, albite and orthoclase, we suggest a method that reveals the Na+/K+ equilibration state for each fluid supporting the allocation of the appropriate geothermometer equation. The improvement procedure is demonstrated in a case study evaluating fluid data of geothermal springs from the Villarrica geothermal system, Southern Chile. It is shown that initially highly scattered results strongly converge after corrections, leading to a substantial improvement in in-situ temperature estimations with small deviations of ≤10 K between SiO2 and Na-K geothermometers. Also absolute temperature calculated for each spring in the study area, ranging from 84 to 184 °C agree well (within ΔT <20 K) with results of multicomponent geothermometry temperatures reported in a previous work.
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