Dmitrii A. Kulik

GEM-Selektor geochemical modeling package: revised algorithm and GEMS3K numerical kernel for coupled simulation codes

Reactive mass transport (RMT) simulation is a powerful numerical tool to advance our understanding of complex geochemical processes and their feedbacks in relevant subsurface systems. Thermodynamic equilibrium defines the baseline for solubility, chemical kinetics, and RMT in general. Efficient RMT simulations can be based on the operator-splitting approach, where the solver of chemical equilibria is called by the mass transport part for each control volume whose composition, temperature, or pressure has changed. Modeling of complex natural systems requires consideration of multiphase–multicomponent geochemical models that include nonideal solutions (aqueous electrolytes, fluids, gases, solid solutions, and melts). Direct Gibbs energy minimization (GEM) methods have numerous advantages for the realistic geochemical modeling of such fluid–rock systems. Substantial improvements and extensions to the revised GEM interior point method algorithm based on Karpov’s convex programming approach are described, as implemented in the GEMS3K C/C+ + code, which is also the numerical kernel of GEM-Selektor v.3 package (http://gems.web.psi.ch). GEMS3K is presented in the context of the essential criteria of chemical plausibility, robustness of results, mass balance accuracy, numerical stability, speed, and portability to high-performance computing systems. The stand-alone GEMS3K code can treat very complex chemical systems with many nonideal solution phases accurately. It is fast, delivering chemically plausible and accurate results with the same or better mass balance precision as that of conventional speciation codes. GEMS3K is already used in several coupled RMT codes (e.g., OpenGeoSys-GEMS) capable of high-performance computing.

Sorption modelling by Gibbs energy minimisation: Towards a uniform thermodynamic database for surface complexes of radionuclides

Summary Radionuclide sorption on mineral-water interfaces can be thermodynamically modelled, similar to solid-solution aqueous-solution systems (only in chemical elemental stoichiometry), if definitions of the standard and reference states, surface activity terms (SAT), and elemental stoichiometries of surface-bound species are unequivocally established. A pre-requisite is that a unique common value of the reference (site) density (Γo) must be part of the definitions of standard and reference states, while the sample- and surface-specific maximum density parameters (Γmax) be included into the SAT corrections to reproduce saturation of physically available adsorption sites instead of introducing the additional mass balance constraints. Subsequently, the standard partial molal properties of surface complexes at multiple surface types on different sorbents can be found, comparable with the standard molar properties of solids, gases and aqueous species. Using surface complexation models (SCM) of U(VI) adsorption on quartz and amorphous silica (SiO2) phases in GEM (Gibbs energy minimization) implementation, a feasible way is shown how to construct a uniform, internally consistent thermodynamic dataset for surface species of radionuclides; to use standard partial molal Gibbs energies Go298 of surface species in sorption modelling; how to convert Go298 values from/to log K or intrinsic adsorption constants log Kint to use in the law-of-mass action (LMA) speciation codes, and finally, into “smart Kd” values for the applications related to waste repository performance assessment.

SOLUBILITY AND THERMODYNAMIC PROPERTIES OF CARBONATE-BEARING HYDROTALCITE–PYROAURITE SOLID SOLUTIONS WITH A 3:1 Mg/(Al+Fe) MOLE RATIO

The naturally occurring layered double hydroxides (LDH, or anionic clays) are of particular interest in environmental geochemistry because of their ability to retain hazardous cations and especially anions. However, incorporation of these minerals into predictive models of water-rock interaction in contaminant environments, including radioactive-waste repositories, is hampered by a lack of thermodynamic and stability data. To fill part of this gap the present authors have derived properties of one of the complex multicomponent solid solutions within the LDH family: the hydrotalcite-pyroaurite series, Mg3(Al1−xFex)(OH)8(CO3)0.5·2.5H2O.Members of the hydrotalcite-pyroaurite series with fixed MgII/(AlIII+FeIII) = 3 and various FeIII/(FeIII+AlIII) ratios were synthesized by co-precipitation and dissolved in long-term experiments at 23±2°C and pH = 11.40±0.03. The chemical compositions of co-existing solid and aqueous phases were determined by inductively coupled plasma-optical emission spectroscopy, thermogravimetric analysis, and liquid scintillation counting of 55Fe tracers; X-ray diffraction and Raman were used to characterize the solids. Based on good evidence for reversible equilibrium in the experiments, the thermodynamic properties of the solid solution were examined using total-scale Lippmann solubility products, ΣΠT. No significant difference was observed between values of SPT from co-precipitation and from dissolution experiments throughout the whole range of Fe/Al ratios. A simple ideal solid-solution model with similar end-member ΣΠT values (a regular model with 0 < WG < 2 kJ mol −1 sufficient to describe the full range of intermediate mineral compositions. In turn, this yielded the first estimate of the standard Gibbs free energy of the pyroaurite end member, G298,Pyro = −3882.60±2.00 kJ/mol, consistent with G298,Htlco = −4339.85 kJ/mol of the hydrotalcite end member, and with the whole range of solubilities of the mixed phases. The molar volumes of the solid-solution at standard conditions were derived from X-ray data. Finally, Helgeson’s method was used to extend the estimates of standard molar entropy and heat capacity of the end members over the pressure-temperature range 0−70°C and 1–100 bar.

Differential alteration of basaltic lava flows and hyaloclastites in Icelandic hydrothermal systems

Geological field observations evidence that active and fossil Icelandic hydrothermal systems are typically embedded into an intercalation of almost completely altered and nearly unaltered volcanic rock layers. We investigated the reasons for this finding with help of geochemical reaction path calculations, by studying the mineralogical evolution of contrasting lithofacies–basalt flows and hyaloclastites at various temperatures and pressures, different recharge water composition, and gas content. From this study, we conclude that the initial porosity of protoliths and volume changes due to their transformation into secondary minerals are sufficient to explain the different extents of alteration as observed in field studies. In addition, we present a generalized kinetic model to estimate the alteration time of glassy fragments in hyaloclastite as a function of grain size, surface roughness, and temperature. This time was found to be rather short, ranging from a few hours to a few years.

An internally consistent thermodynamic dataset for aqueous species in the system Ca-Mg-Na-K-Al-Si-O-H-C-Cl to 800 °C and 5 kbar

This study presents an internally consistent thermodynamic dataset for aqueous species in the system Ca-Mg-Na-K-Al-Si-O-H-C-Cl, obtained by adding species of calcium, magnesium and carbon to the core system Na-K-Al-Si-O-H-Cl (Miron and others, 2016). Critically evaluated experimental data on mineral solubility (Ca and Mg hydroxides, Ca and Mg silicates, anorthite, Ca and Mg carbonates) in water and aqueous electrolyte solutions over wide ranges in temperature and pressure were added to the database of experimental data. The complete experimental dataset was then used to simultaneously refine the standard state Gibbs energies of all aqueous ions and complexes in the framework of the revised Helgeson-Kirkham-Flowers (HKF) equation of state. The thermodynamic properties of the solubility-controlling minerals were accepted from the internally consistent dataset of Holland and Powell (1998; updated Thermocalc dataset ds55). The association equilibria of important hydroxide, chloride, carbonate and silicate complexes were critically reviewed, and their standard state properties and HKF parameters were independently derived from conductance, potentiometric and, in a few cases, solubility measurements. In a global optimization of standard Gibbs energies of aqueous species, performed with the GEMSFITS code (Miron and others, 2015), the association equilibria for aqueous complexes were always maintained. The new thermodynamic dataset reproduces all available fluid-mineral phase equilibria and mineral solubility data in the system Ca-Mg-Na-K-Al-Si-O-H-C-Cl with good accuracy over wide ranges in temperature (25–800 °C), pressure (1 bar – 5 kbar) and composition (salt concentrations up to 5 molal). This makes it possible to perform geochemical and reactive transport modeling of processes in natural and engineered georeservoirs over wide ranges of conditions with an unprecedented level of accuracy and reliability and to address processes of fluid flow and fluid-rock interaction in the Earths crust from a new perspective. Using the same strategy as applied in the present study, the internally consistent thermodynamic dataset can be further extended with additional major and trace elements, and the data refinement process can be repeated when new experimental data or next-generation equation of state or activity models for aqueous solutions become available.

Numerical Simulation of Reactive Fluid Flow on Unstructured Meshes

Reactive transport simulation on unstructured meshes can provide fundamental insight into the effect that geometric complexity of geologic structures has on fluid flow and development of reaction fronts. When applied to conditions ranging from ambient to hydrothermal and combined with compressible flow, accounting for geometric complexity provides an advantage for applications such as enhanced geothermal systems, carbon dioxide sequestration, hydrothermal ore formation, and radioactive waste disposal. We introduce CSMP–GEMS, a thermo–hydro and chemical multicomponent reactive transport code based on coupling of the Complex System Modeling Platform (CSMP) transport modeling framework with the GEMS3K chemical speciation solver. GEMS3K features a comprehensive suite of non-ideal activity and equation-of-state models of solution phases (aqueous electrolyte, gas and fluid mixtures, solid solutions). Current features include transient, compressible, single-phase advective and/or dispersive fluid flow, mass transport, heat transport in saturated porous media, and geochemical reactions in subsurface hydrothermal systems. We present two one-dimensional numerical experiments to compare CSMP–GEMS with the reactive transport codes OpenGeoSys–GEM and TOUGHREACT. Each experiment simulates calcite dissolution and dolomite precipitation during advection and hydrodynamic dispersion. One experiment corresponds to an existing isothermal

Reactive transport modelling of dolomitisation using the new CSMP++GEM coupled code: governing equations, solution method and benchmarking results

(25\,^{\circ }\mathrm{C})

Characterization and Solubility Determination of the Solid-Solution Between AFm-I 2 and AFm-SO 4

(25∘C) benchmark; the second explores the applicability of the codes to non-isothermal problems. We also present a two-dimensional example that illustrates the application of CSMP–GEMS on unstructured meshes that can represent complex geologic relations. The results suggest that all three codes are well suited to predicting fluid circulation, heat transport, and mineral stability within hydrothermal systems relevant to enhanced geothermal systems and carbon dioxide sequestration in deep aquifers. Self-consistent accounting for kinetic processes is a major advantage of TOUGHREACT, but published applications are restricted to orthogonal meshes, potentially limiting the applicability of TOUGHREACT to geometrically less complex natural systems. OpenGeoSys–GEM can operate on unstructured meshes that may include multiple element types, facilitating the examination of non-orthogonal domains. However, due to its reliance on the groundwater equations, OpenGeoSys–GEM may be best suited for application to systems in which flow includes dispersion/diffusion and is not compressible. CSMP–GEMS does not currently calculate reaction kinetics, but may be useful for application to geometrically complex systems.