Modeling water-rock interactions in underground environments requires the use of thermodynamic databases that account for the complexity of groundwater chemical compositions under relevant pressure and temperature conditions. A key issue concerning saline groundwaters is their high ionic strength, which are often not reliably calculated and calls for the use of specific thermodynamic approaches. Among such approaches, the Pitzer model appears to be one of the most promising. However, its applicability to underground fluids is limited by the fact that the values of its numerous parameters are only known for a subset of chemical systems and, more particularly, are often missing for trace elements and at temperatures other than 25 degrees C. This paper describes a new way to develop a relevant thermodynamic database to account for the trace elements in these types of deep environments. It combines two in-house databases: Thermoddem, which relies on the B-dot Extended Debye-H & uuml;ckel activity model, and PhreeSCALE, which uses the Pitzer formalism. The resulting Hybrid Pitzer-Thermoddem (HyPiT) thermodynamic database (TDB) integrates a simplified Pitzer model to account for major and trace elements. Based on a series of recent works (Simoes et al., 2016, 2017a, 2017b), the Pitzer binary interaction parameters, (i(0) and (i(1) and their first derivative with respect to temperature (d(i(0)/dT and d(i(1)/dT) are determined for simulating the first order interactions between trace elements and major species. The HyPiT database is successfully applied to different geochemical systems relevant to underground environments. They include (i) the solubility of gypsum in NaCl and sea-type brines of varying ionic strength at temperatures between 0 and 25 degrees C, (ii) the solubility of barite in NaCl brines at 25, 60 and 80 degrees C, (iii) the solubility of calcite in 0.1 and 4 M NaCl brines for temperatures and pressures up 250 degrees C and 1450 bars, respectively, (iv) the solubility of amorphous silica up to 150 degrees C in various single electrolyte solutions, namely HCl, NaCl, MgCl2, HNO3, NaNO3, Na2SO4, and MgSO4. Particular attention is also given to the modeling of iron-bearing systems such as the speciation of FeIII in LiCl solutions up to 20 mol/kg.

A new set of ternary interaction parameters is proposed to describe the excess properties of aqueous solutions in the CoSO4-Li2SO4-H2O chemical system between 283.15 K and 348.15 K at P = 101.3 kPa, using the Pitzer formalism. The parameters were determined using a combination of (i) experimental solubility values between 283.15 K and 323.15 K coming from the literature and a new set of experimental osmotic coefficients at 298.15 K and 308.15 K, (ii) reliable solid-liquid equilibrium models based on Pitzer equations for the CoSO4-H2O and Li2SO4-H2O binary systems, from literature. The new interaction parameters defined in this study allow to reproduce correctly the change in the hydration state of salts with temperature (Li2SO4.H2O and CoSO4.nH2O with n = 1, 6, 7) as well as their solubility products. They also allow to accurately represent the experimental phase diagram of the CoSO4-Li2SO4-H2O system according to temperature.

Solubility study for UO 2 (NO 3 ) 2 -NaNO 3 -H 2 O and UO 2 (NO 3 ) 2 -Mg(NO 3 ) 2 -H 2 O systems to evaluate existing data and improve the understanding on aqueous U( vi ) nitrate systems up to high ionic strength. Pitzer parameters based on available data are derived.

Geothermal fluids are highly variable in chemical composition and ion concentration. Parametrising density, as one of the most important fluid properties for geothermal reservoir development, is therefore challenging. The purpose of this paper is to test the hypothesis that saline geothermal fluids can well be characterised for density when only the dominating dissolved salts (i.e., NaCl, KCl, and CaCl2) are taken into account, thus neglecting any minor fluid constituents. For the example of four geothermal sites with known chemical fluid composition and significant differences in total salt content (Groß Schönebeck and Insheim, Germany, Balmatt, Belgium, and Heemskerk, The Netherlands) synthetic aqueous solutions of the main salts without and with three different other salts (i.e., LiCl, SrCl2, and MgCl2) representing the minor fluid constituents were parametrised for density at atmospheric pressure and temperatures between 293 K and 353 K. Moreover, density was derived numerically using the PHREESCALE chemical code and evaluated against the analytical data. The results demonstrate that: (1) an effect of the ion type is evident with density increasing in the order of added LiCl, MgCl2, and SrCl2 at a given concentration. (2) The uncertainty in density when neglecting any minor fluid components is at most 2

Understanding transport and mixing in stratified saline systems is critical for predicting the behavior of brines in natural aquifers, industrial reservoirs, and engineered disposal sites. These multi-ion solutions often exhibit complex instabilities driven by differential diffusion and compositional gradients. The onset and morphology of such convective mixing remain poorly predicted. We investigated double-diffusive (DD) and diffusion-layer convection (DLC) in superimposed aqueous solutions of the salts typical of saline aquifers, sodium chloride (NaCl) and sodium sulphate (Na2SO4). The study combines thermodynamic modeling, optical interferometry experiments, and nonlinear numerical simulations to explore convective instabilities in a ternary system. Our findings reveal a rich variety of convective scenarios depending on salt configuration and concentration ratios. When the faster-diffusing NaCl was placed above Na2SO4, diffusion-layer convection occurred with a delayed and asymmetric onset of instability, an experimentally demonstrated feature not reported previously. In contrast, when the positions were reversed, the system developed double-diffusive fingers that grew slowly due to the small density ratio. These fingers exhibited an unusual morphology, consisting of extremely fine, vertically textured structures that gradually merged away from the interface. This formed a large area of diffuse mixing and suppression of coherent convective structures. In all cases, classical stability criteria failed to fully predict the onset and nature of convection. Instead, we identified the initial position of the system on the stability map, as determined by the full diffusion matrix, as a critical factor.

The European project PUSH-IT’s ambition is to overcome the seasonal mismatch between heat demand and heat generation from sustainable sources using Underground Thermal Energy Storage (UTES) at high temperatures, i.e. up to 90°C (https://www.push-it-thermalstorage.eu). PUSH-IT showcases three UTES technologies at six pilot sites: Aquifer Thermal Energy Storage (ATES) in the Netherlands and Germany, Mine Thermal Energy Storage (MTES) in Germany and the UK, and Borehole Thermal Energy Storage (BTES) in Germany and the Czech Republic.Water quality issues represent technical challenges for viable long-term thermal energy storage, whatever the technology. These include:Biogeochemical perturbations in the storage reservoir: the temperature gradient influences the chemical composition of the fluids that may lead to (bio)clogging in near-wells and loss of injectivity/productivity, notably for ATES; Corrosion and scaling in the wells and facilities; Shallower aquifer perturbation by temperature increase and saline brines from deeper aquifers. The lessons learnt over decades of geothermal exploitation and low temperature thermal energy storage provide a solid basis for anticipating, monitoring, managing and remediating these issues in a variety of geological contexts. However, for increasing the temperatures of thermal energy storage in geothermal reservoirs several specificities have not been thoroughly investigated, such as the thermal storage at high temperature, the need to use reversible wells, reversible fluxes in the reservoir, temperature and pressure cycling. Furthermore, the role of microbiology has often been neglected and is anticipated to be more sensitive in storage applications.At the current stage of the project, the main concerns for each site have been mapped. The modelling and monitoring works are at different stages of progress depending on the sites. Significant methodological work is being carried out on microbiological phenomena and on the elaboration of monitoring protocols.The project will go beyond the simple feedback on the experience from pilot sites by providing recommendations to anticipate and deal with water quality issues for future sites. We will provide guidelines to assist decision-makers in identifying the main issues, in implementing the necessary modelling, in sizing and adjusting the monitoring plan, in assessing possible water treatment benefit vs. environmental impact. Anticipating and managing water-related issues as far as possible is the best way of making the right choices, guaranteeing the performance and longevity of operations, limiting environmental impact and encouraging society engagement and support. Acknowledgements: Funded by the European Union under grant agreement 1011096566 (PUSH-IT project). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor CINEA can be held responsible for them.

ThermoChimie is a thermodynamic database project intended for use across the radioactive waste management community to support repository performance assessment, research and development activities, and potentially inform decisions about waste conditioning, reprocessing, and disposability. Established in 1995 by Andra (France), and now managed by a consortium that also includes NWS (UK) and ONDRAF/NIRAS (Belgium), it is currently moving towards the end of Phase 3 (December 2025) of the project with the recent release of ThermoChimie version 12a and a further version update anticipated in the next 18 months. This paper provides an overview of the ThermoChimie database with emphasis on the activities undertaken during Phase 3, including major updates of the Pd, Fe, Se, Ni, U, Np, Pu, Am, Cm, and Tc systems, inclusion of data for Cu, Be, and Hg, the further development of the XCheck (c) tool that facilitates database management, increased database compatibility with a range of geochemical codes (PHREEQC, TOUGHREACT, CrunchFlow, CHESS, GWB, Spana, PFLOTRAN), and expert consultation on the subjects of cement and zeolite phases and redox reactions to inform future development of the database. This considerable body of work ensures that the ThermoChimie database continues to provide accurate, consistent and complete (as far as practicable) thermodynamic data to the radioactive waste community and beyond.

Solid-liquid equilibria of the ternary CoSO4-Li2SO4-H2O chemical system have been determined at T = 283.15, 298.15, 308.15, and 323.15 K using discontinuous isoperibolic thermal analysis, inductively coupled plasma-optical emission spectrometry, and X-ray powder diffraction. The phase diagram that has been experimentally constructed details the evolution of the solubility according to the chemical composition and temperature. The studied chemical system is characterized by solid-brine equilibria involving only simple salts. This makes it possible to define ranges of composition favoring the precipitation of one type of salt over another and thus to separate them by selective crystallization according to a starting composition. It also suggests that in a specific domain of composition, it is possible to crystallize either CoSO4.nH2O (n = 6 or 7 depending on temperature) or Li2SO4.H2O by increasing or decreasing the temperature. This is due to the opposite behavior of their solubility as a function of temperature. This work proposes a set of experimental data (solubilities, polysaturated points, and boundaries between triphasic and biphasic domains) that could be used to be integrated into a thermodynamic model.

Applying the Nernst-Planck formalism of 1D diffusive flux equations for ionic species of salts dissolved in water, Pitzer formalism for activity coefficients and PHREEQC software for species diffusion coefficients, with ionic strength-dependence coefficients optimized, we obtained expressions for apparent diffusion coefficients of sodium chloride, calcium chloride, and sodium sulphate, prevalent in deep aquifers. PHREEQC 1D transport simulations of free-diffusion experiments yielded temporal evolution of the concentration and gradient vertical profiles. The dynamics of concentrations non-equilibrium fluctuations during the free-diffusion experiments have been recorded by shadowgraphy. Thanks to the calculated gradient profiles the thickness of the sample has been integrated in the analysis equations of the structure functions. From the earliest stages of the diffusion process we measured the diffusion coefficients for the three salts, in very good agreement with reference measurements, validating the technique and method of analysis for studying free-diffusion in reservoirs targeted for CO2 and energy vectors storage.

Thermodynamic description of complex sulfate systems of relevance for nuclear waste disposal. Pitzer and SIT models including aqueous complexes derived for the Eu(iii)–Mg/Na–SO4–H2O system based on solubility experiments and TRLFS measurements.

AbstractThis study reports on newly acquired density data of synthetically prepared pure and mixed NaCl and CaCl2 aqueous solutions that span a wide range of geothermally encountered concentrations and mixing ratios. The analytical data are provided for the temperature range of 293–353 K at ambient pressure. For the reproduction of that data, PHREESCALE was used. The predictive potential of this numerical tool regarding the density of geothermal fluids of known composition was the major target herein. As a result, the measured data are in good agreement with previous analytical studies found in the literature. Possible sources of errors are discussed in this paper. Density data of the mixed solutions at temperatures other than ambient are unique and close existing data gaps. The numerical model reproduces the newly measured and already existing density data within an error band of approximately 1%. For further use in geothermal applications, this can be considered an excellent agreement. Moreover, the model yields a direct calculation of density without the need to establish complex empirical equations of state and mixing rules. Finally, sensitivity calculations performed with a thermal–hydraulic (TH) numerical reservoir model demonstrate the required accuracy of fluid density for reliably predicting the long-term performance of deep geothermal energy systems. In terms of the productivity index and the timing of thermal breakthrough it shows that the present analytical and numerical uncertainty in density is small enough to reliably state both reservoir parameters.

Abstract We investigate how micro‐geoelectrical monitoring is promising for studying microscale coupled processes since it facilitates the upscaling of pore‐scale observations and enhances the petrophysical interpretation of the geoelectrical measurements. Microscale geophysics using microfluidics emerges and combines direct visualization of pore scale dynamics and chemical reactivity with geoelectrical monitoring. Calcite dissolution is a usual geochemical reaction considered as an analog of water–mineral interactions involved in the critical zone. We develop a numerical workflow combining image processing and geochemical simulation as inputs for the petrophysical modeling applied to a published data set of microscale induced polarization monitoring of calcite dissolution under partially saturated conditions. The successful interpretation provides the cation exchange capacity and specific surface area evolution; essential parameters in field‐scale surveys.

A thermodynamic description of complex sulfate systems of relevance for nuclear waste disposal is provided. A full dissociation Pitzer model is derived for the system Eu(iii)–Mg/Na–SO4–H2O based on solubility experiments and comprehensive solid phase characterization.