PHREEQC is a geochemical computer code developed by the United States Geological Survey and used to model mineral solubilities, aqueous solutions, and water-rock interactions over a large range of ...conditions. This easy-to-use and open-source program is widely used in the fields of geochemistry, environmental science and engineering, petroleum industry, mining and chemical engineering and biogeochemistry. However, the quality of the modeling depends on the experience of the modeler, and the quality of thermodynamic data available for the code, which is addressed in this review. The authors of the PHREEQC program clearly stated that the choice of thermodynamic data files (TDFs) is the responsibility of the user. Thirteen, mostly “third party”, TDFs are distributed along with the software release without detailed documentation or critical evaluation. The quality of the TDFs varies with a wide spectrum of the internal consistency of the thermodynamic data, comprehensiveness of the chemical components and species, the temperature-pressure coverage of log K values, activity coefficient models suitable for ionic strength, and traceability of the sources of data.
In this review, we provide some basic documentation, evaluate and compare these TDFs, and attempt to outline the limits of their applicability. We also used the TDFs to model examples of river water, seawater, groundwater, oil-field brine, and selected mineral solubility data for some oxides, oxy-hydroxides, hydroxides, aluminosilicates, carbonates, and sulfates. These comparisons clearly demonstrate that, with the same PHREEQC computer code but a different TDF, significantly different modeling results can be produced. It is imperative that users specify which TDF they used when referring to their modeling results.
The research on Carbon Capture and Storage (CCS) has become fruitful as energy-intensive industries are working towards transitioning to low carbon energy industry. Shale gas reservoirs have been ...recently considered as suitable geological targets for carbon dioxide (CO2) storage. However, due to the high reactivity of shales to CO2, the mineralogical changes after CO2/brine/shale interactions play a decisive role in defining the sealing properties of shales at geological time scales. Up to date, this issue is rarely investigated; therefore, in this study, a simplified 1-D reactive transport model was constructed based on the properties obtained from Eagle Ford and Mancos shales. PHREEQC software was utilized to simulate equilibrium and kinetic behavior and evaluate the alterations in minerals at 177 atm and 70 °C. The equilibrium model indicated that calcite and clay minerals dissolved in CO2-saturated brine, while quartz grains precipitated, due to the formation of carbonic acid. This behavior confirmed the high reactivity of shales to the injection of CO2-saturated brines. The kinetic model indicated that the geological time scale for CO2/brine/shale interaction can be divided into three phases. Primary minerals alterations occurred during the first 10 years, however, the main alteration in mineralogy occurred between 10 and 100 years, whereas the reactants continued to dissolve in low portions until the equilibrium state was reached beyond 100 years. The model showed that carbonate and clay minerals dissolved during the CO2/brine/shale interaction, which could provide the potential for mineral trapping as an effective sealing mechanism in the middle phases of the storage lifetime in shales, confirming the high potential of shales for CO2 containment. The main observations and conclusions obtained from this work can be easily extrapolated to other shale formations with similar mineral compositions.
Transport of multicomponent electrolyte solutions in saturated porous media is affected by the electrostatic interactions between charged species. Such Coulombic interactions couple the displacement ...of the different ions in the pore water and remarkably impact mass transfer not only under diffusion, but also under advection‐dominated flow regimes. To accurately describe charge effects in flow‐through systems, we propose a multidimensional modeling approach based on the Nernst‐Planck formulation of diffusive/dispersive fluxes. The approach is implemented with a COMSOL‐PhreeqcRM coupling allowing us to solve multicomponent ionic conservative and reactive transport problems, in domains with different dimensionality (1‐D, 2‐D, and 3‐D), and in homogeneous and heterogeneous media. The Nernst‐Planck‐based coupling has been benchmarked with analytical solutions, numerical simulations with another code, and high‐resolution experimental data sets. The latter include flow‐through experiments that have been carried out in this study to explore the effects of electrostatic interactions in fully three‐dimensional setups. The results of the simulations show excellent agreement for all the benchmarks problems, which were selected to illustrate the capabilities and the distinct features of the Nernst‐Planck‐based reactive transport code. The outcomes of this study illustrate the importance of Coulombic interactions during conservative and reactive transport of charged species in porous media and allow the quantification and visualization of the specific contributions to the diffusive/dispersive Nernst‐Planck fluxes, including the Fickian component, the term arising from the activity coefficient gradients, and the contribution due to electromigration.
Key Points
Nernst‐Planck‐based approach to model multidimensional transport, charge interactions, and chemical reactions in flow‐through porous media
Computation and visualization of diffusive/dispersive, electromigration, and activity coefficients' gradient fluxes
Code validated with high‐resolution 2‐D experimental data and first fully 3‐D data set on multicomponent ionic transport
•Multiphase flow and multicomponent reactive transport in coupled compartments•Impact of atmospheric forcing on gas component transport and geochemical reactions•COMSOL-PhreeqcRM coupling to simulate ...porous medium/free-flow systems•Key features of the coupled model illustrated with benchmark and application examples
The exchange of gas components across the subsurface/atmosphere interface influences multiphase flow and reactive transport in the subsurface and is crucial for many biogeochemical processes, for the emission of greenhouse gases, and for the fate of volatile contaminants. In this study, we present a modeling approach to simulate non-isothermal multiphase flow and multicomponent reactive transport in coupled subsurface/atmosphere compartments. The model is based on a coupled porous medium/free flow domain in which the Navier-Stokes equation is used to describe single-phase (gaseous) flow in the free-flow subdomain and Darcy's law is applied for two-phase flow in the porous medium subdomain (i.e., two-domain approach). The implementation is performed by coupling COMSOL Multiphysics and PhreeqcRM, which enables the investigation of the interplay between multi-physical processes (i.e., flow, mass and heat transport) in the coupled compartments and geochemical reactions in the porous medium. We first present a set of benchmark examples in which key features of the proposed model are tested against other numerical simulators and an analytical solution. Successively, we take advantage of the unique capabilities of the proposed approach to explore conservative and reactive transport of gas components in a coupled porous medium/free flow domain. The results show that the exchange processes between the compartments control the location of reactive zones and the extent of geochemical reactions (i.e., mineral dissolution) by changing the spatiotemporal distribution of fluid phases and enhancing the interphase mass transfer of key gas components such as oxygen and carbon dioxide.
Fibrous carbonate cement is a common phase in marine phreatic environments — but its origin and diagenetic history are underexplored. This paper compares two fibrous fabrics that are different in ...terms of their peculiarities but share important similarities regarding their essentials: upper Ediacaran fibrous dolomite cement and Upper Ordovician fibrous calcite cement, both from the Tarim Basin, China. The focus is on the significance of these fabrics as archives of their marine paleoenvironment. Upper Ediacaran fibrous dolomite cement is present as (i) a fascicular-optic and (ii) a radial-fibrous phase. Judging from the optical length-slow character and luminescent zones, radial-fibrous dolomite cement precipitated directly from early diagenetic marine porewater. Fascicular-optic dolomite cement has a length fast optical character and hence likely formed as a high-Mg calcite precursor penecontemporaneously in a reefal, shallow-marine diagenetic environment. Similarly, two types of Upper Ordovician fibrous cement are present: (i) radiaxial-fibrous and (ii) fascicular-optic calcite cement. Fascicular-optic calcite precipitated in a marine phreatic platform environment, whereas radiaxial-fibrous calcite cement formed in an early marine diagenetic porewater environment. Both fabrics arguably replace a magnesium calcite precursor. For the case of the Tarim Basin, cathodoluminescence and redox-sensitive elements indicate predominantly anoxic, upper Ediacaran marine waters (Ce anomaly = 0.8 ± 0.2), and suboxic, Upper Ordovician shallow-marine seawater (0.7 ± 0.2). Based on PHREEQC modeling, the shallow burial, upper Ediacaran porewater (5.1) yields a higher dolomite saturation index than upper Ediacaran (4.8) and Upper Ordovician seawater (3.7). The direct precipitation of radial-fibrous dolomite from Ediacaran porewaters was likely induced due to organic matter decomposition and related seawater oxygen depletion, elevated Mg2+/Ca2+ ratios, and high alkalinity values. Modeling and proxy data show significant hydrogeochemical differences between seawater and porewater in the upper meters of the sediment column. The upper Ediacaran seawater apparently favored fibrous cement formation with magnesium calcite precursors. In contrast, fibrous dolomite cement was directly precipitated in an early-diagenetic marine porewater regime. The comparability of proxy data reflecting seawater and such recording early diagenetic marine porewater is critically discussed.
The Ediacaran early diagenetic marine porewater yields higher dolomite saturation index than the Ediacaran seawater and Ordovician seawater. Display omitted
•Radial-fibrous dolomite with length-slow character and well-preserved zone is primary.•Fascicular-optic calcite and radiaxial-fibrous calcite have precursor of Mg-calcite.•Aqueous chemistry shows variation from shallow seawater to early diagenetic porewater.•Anoxic, elevated Mg2+/Ca2+ ratios and alkalinity favored the precipitation of dolomite.
This study focuses on assessing the hydrogeochemical processes influencing the mobility of dissolved metal and metalloid species during mine effluent mixing. Field samples were collected to ...characterize effluents at an active gold mine located in the Abitibi Greenstone belt in western Québec, Canada. Controlled laboratory mixing experiments were further performed with real effluents. In situ physicochemical parameters, concentrations of major dissolved ions and trace elements were analyzed. Mineralogical analyses were also performed on precipitates from the laboratory mixtures. The data were used for statistical analyses and for modeling the geochemical evolution of effluents using PHREEQC with the wateq4f.dat database (with modifications). The results suggest that the formation of secondary minerals such as schwertmannite, Fe(OH)3, and jarosite could significantly affect the concentrations of trace elements in effluents. The precipitation of secondary minerals immobilized trace elements through coprecipitation and sorption processes. The main limitations of the modeling approach used here include the evaluation of the ion balance for low pH samples with high Fe and Al concentrations and the omission of biological processes. The approach provides insights into the geochemical evolution of mine effluents and could be adapted to several mining sites as a tool for improving water management.
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•Mine water management may involve mixing effluents of different compositions•Contaminant concentrations may deviate from conservative mixtures•Chemical and mineralogical analyses were performed on field and lab mixtures•Evidence is provided on the impact of schwertmannite precipitation on water quality•Recommendations are proposed for characterizing and managing mine effluents
The production of geothermal fluids can be adversely affected by the formation of free gas bubbles (degassing). Decreasing pressures can cause dissolved gas to exsolve, which can reduce water ...production. This study aims to improve the understanding of the conditions under which free gas nucleates in geothermal reservoirs. The focus is on CO2 degassing from brines with varying salinity. We report a series of depressurisation experiments at high pressure and temperature using a cell that allows for visual monitoring of the degassing process using a high-speed camera along with pressure and temperature logging. A geochemical model was created for simulating the degassing behaviour at the same conditions as those used in the experiments, thus allowing for direct comparison.
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•Robust method based on shadowgraphy to measure free-diffusion coefficients.•Consistent application of the Nernst-Planck formalism of 1D diffusive transport.•Context of CO2 storage in ...underground brine reservoirs.
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.
The complexation of Fe(II) with organic matter (OM) and especially with humic acids (HAs) remains poorly characterized in the literature. In this study, batch experiments were conducted on a pH range ...varying from 1.95 to 9.90 to study HA-mediated Fe(II) binding. The results showed that high amounts of Fe(II) are complexed with HA depending on the pH. Experimental data were used to determine a new set of binding parameters by coupling PHREEPLOT and PHREEQC-Model VI. The new binding parameters (log KMA=2.19±0.16, log KMB=4.46±0.47 and ΔLK2=3.90±1.30) were validated using the LFER (linear free energy relationship) method and published adsorption data between Fe(II) and Suwannee River fulvic acid (SRFA) (Rose and Waite, 2003). They were then put in PHREEQC-Model VI to determine the distribution of Fe(II) onto HA functional groups. It was shown that Fe(II) forms mainly bidentate complexes, some tridentate complexes and only a few monodentate complexes with HA. Moreover, Fe(II) is mainly adsorbed onto carboxylic groups at acidic and neutral pH, whereas carboxy-phenolic and phenolic groups play a major role at basic pH. The major species adsorbed onto HA functional groups is Fe2+; Fe(OH)+ appears at basic pH (from pH 8.13 to 9.9). The occurrence of OM and the resulting HA-mediated binding of Fe(II) can therefore influence Fe(II) speciation and bioavailability in peatlands and wetlands, where seasonal anaerobic conditions prevail. Furthermore, the formation of a cationic bridge and/or the dissolution of Fe(III)-(oxy)hydroxides by the formation of Fe(II)-OM complexes can influence the speciation of other trace metals and contaminants such as As.
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•We measured Fe(II)-humic acid adsorption isotherm and pH sorption edge.•Results were modeled using a coupling of PHREEPLOT–PHREEQC-Model VI.•We determined the binding parameters for Fe(II)-humic acid complex.•We validated the binding parameters using LFERs and published datasets.•We determined the speciation of Fe(II)-humic acid complexes using PHREEQC-Model VI.
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