Mineral Reaction Rates Research Articles

Unveiling the Essential Parameters Driving Mineral Reactions during CO2 Storage in Carbonate Aquifers through Proxy Models

Numerical simulation is a commonly employed technique for studying carbon dioxide (CO2) storage processes in porous media, particularly saline aquifers. It enables the representation of diverse trapping mechanisms and the assessment of CO2 retention capacity within the subsurface. The intricate physicochemical phenomena involved necessitate the incorporation of multiphase flow, accurate depiction of fluid and rock properties, and their interactions. Among these factors, geochemical reaction rates and mechanisms are pivotal for successful CO2 trapping in carbonate reactive rocks. However, research on kinetic parameters and the influence of lithology on CO2 storage remains limited. This limitation is partly due to the challenges faced in laboratory experiments, where the time scale of the reactions and the lack of in situ conditions hinder accurate measurement of mineral reaction rates. This study employs proxy models constructed using response surfaces calibrated with simulation results to address uncertainties associated with geochemical reactions. Monte Carlo simulation is utilized to explore a broader range of parameters and identify influential factors affecting CO2 mineralization. The findings indicate that an open database containing kinetic parameters can support uncertainty assessment. Additionally, the proxy models effectively represent objective functions related to CO2 injectivity and mineralization, with calcite dissolution playing a predominant role. pH, calcite concentration, and CO2 injection rate significantly impact dolomite precipitation, while quartz content remains unaffected.

Evolution of Mineral-Accessible Surface Area Induced by Geochemical Reactions

Mineral dissolution and precipitation reactions occur in a wide range of porous media systems, driven by deviations from an equilibrium between solid and fluid phases. Reactions occur at mineral surfaces in contact with reactive fluids or accessible mineral surface areas. These mineral surface areas can be determined using a multiscale imaging approach and have been shown to improve the simulation of mineral reaction rates in porous media compared to other estimates of reactive surface area. As reactions progress, mineral surface area evolves, and reactive transport simulations often use a simplified model assuming spherical grains to estimate mineral surface area evolution. This, however, does not depict the evolution of reactive surfaces in porous media systems with varying mineral accessibility. This work aims to quantitatively assess the evolution of accessible mineral surface area in porous media for a multimineralic system undergoing mineral dissolution reactions induced by acid exposure. Before and after the reaction, accessible mineral surface areas are determined from 2D scanning electron microscopy, and 3D X-ray computed tomography imaging of a sandstone sample. The quantified evolution of accessible surface area is compared to the calculated mineral surface area evolution using current approaches. Results show an overall increase in total surface area due to the reaction; however, individual mineral surface areas may increase or decrease. Variation is observed in mineral surface area values measured from imaging and equations used in models. The evolution of mineral surface area is largely impacted by the total surface area as well as the pore connectivity rather than porosity and volume fraction evolution.

Impact of Pore Connectivity on Quantification of Mineral Accessibility in Sandstone Samples

Characterization of microscale features and mineral distributions in rock samples can be facilitated non-destructively with imaging analysis. Scanning electron microscopy combined with backscattered electron and energy-dispersive spectroscopy is particularly valuable and can be utilized to identify minerals. Mineral segmentation coupled with quantitative image processing can yield mineral volume fractions and accessibility from these images. Prior estimates of mineral accessibility from images have improved the simulations of mineral reaction rates, but it is unclear how pore connectivity should be accounted for. This is further complex in samples with clay minerals where nanopores in clays need to be considered. Here, the impacts of different approaches to assess pore connectivity on quantification of mineral accessibility are considered for seven sandstone samples with varying composition. Mineral accessibilities are calculated by counting the interfacial pixels between the associated minerals and the adjacent pores from the 2D mineral segmented maps. Three types of accessibilities are considered: the first approach accounts for all the macropore space, the second approach considers only the 2D connected macropores, and the third approach includes the 2D connected porosity considering nanopores in clays. The observed variations in accessibility for most mineral phases are within 1 order of magnitude when nanopore connectivity is considered and thus not anticipated to largely impact the simulated reactivity of samples. However, greater variations were observed for clay minerals, which may impact long-term simulations (years). Larger variations in accessibility were also noted for carbonate minerals, but only some samples contained carbonate phases, and additional data is needed to assess the trends.

A reactive transport modeling perspective on the dynamics of interface-coupled dissolution-precipitation

In interface coupled dissolution-precipitation systems, the dynamics of the mineral-fluid interface depends on two intertwining processes: the dissolution of the primary mineral that is needed for subsequent precipitation and the passivation of the dissolution reaction as a result of secondary mineral precipitation. The resulting thickness and texture of the precipitating coating layer will affect the progression of geochemical reactions, flow and transport processes at the macroscopic scale. Understanding the interplay between macroscopic flow regimes and microscopic reaction mechanisms (e.g., nucleation and crystal growth pathways) in controlling the dynamics of the mineral-fluid interface has important implications for predicting natural weathering processes, scaling in the subsurface energy production systems, etc. In this study, we use a micro-continuum pore-scale reactive transport model to investigate the feedback loop between reaction rate and solute transport with explicit consideration of the surface passivation and the diffusion process through the coating layer, as well as the impacts of saturation-dependent nucleation rate on the textures of precipitates that will largely dictate the diffusion properties of the coating layer. Our model results highlight that the drastically different coating behaviors at the macroscopic scale and their dependence on solution supersaturation observed in previous column experiments are primarily controlled by the interplay between mineral reaction rates, advective flow, and diffusion through the dynamically forming coating layer. The diffusion properties of the coating layer also play a secondary but non-negligible role in shaping the evolution of the co-dissolution and precipitation system. The probabilistic nucleation model building on the framework of classical nucleation theory highlights the complex dependence of precipitates’ texture on solution chemistry and substrate properties, which can affect the diffusion process within the precipitates. The modeling observations also underscore the necessity of further investigations to better characterize the properties of the coating layer and to improve modeling descriptions of the nucleation processes.

The Effect of Pore-Scale Two-Phase Flow on Mineral Reaction Rates

In various natural and engineered systems, mineral–fluid interactions take place in the presence of multiple fluid phases. While there is evidence that the interplay between multiphase flow processes and reactions controls the evolution of these systems, investigation of the dynamics that shape this interplay at the pore scale has received little attention. Specifically, continuum scale models rarely consider the effect of multiphase flow parameters on mineral reaction rates or apply simple corrections as a function of the reactive surface area or saturation of the aqueous phase, without developing a mechanistic understanding of the pore-scale dynamics. In this study, we developed a framework that couples the two-phase flow simulator of OpenFOAM (open field operation and manipulation) with the geochemical reaction capability of CrunchTope to examine pore-scale dynamics of two phase flow and their impacts on mineral reaction rates. For our investigations, flat 2D channels and single sine wave channels were used to represent smooth and rough geometries. Calcite dissolution in these channels was quantified with single phase flow and two phase flow at a range of velocities. We observed that the bulk calcite dissolution rates were not only affected by the loss of reactive surface area as it becomes occupied by the non-reactive non-aqueous phase, but also largely influenced by the changes in local velocity profiles, e.g., recirculation zones, due to the presence of the non-aqueous phase. The extent of the changes in reaction rates in the two-phase systems compared to the corresponding single phase system is dependent on the flow rate (i.e., capillary number) and channel geometry, and follows a non-monotonic relationship with respect to aqueous saturation. The pore-scale simulation results highlight the importance of interfacial dynamics in controlling mineral reactions and can be used to better constrain reaction rate descriptions in multiphase continuum scale models. These results also emphasize the need for experimental studies that underpin the development of mechanistic models for multiphase flow in reactive systems.

The impact of mineral reactive surface area variation on simulated mineral reactions and reaction rates

Reactive transport modeling is an essential tool to simulate complex geochemical reactions in porous media that can impact formation properties including porosity and permeability. However, simulating these reactions is challenging due to uncertainties in model parameters, particularly mineral surface areas. Imaging has emerged as a powerful means of estimating model parameters including porosity and mineral abundance, accessibility and accessible surface area. However, these parameters, particularly mineral accessible surface area, vary with image resolution. This work aims to enhance understanding of the impact of image resolution and other means of estimating mineral reactive surface area on simulated mineral reactions and reaction rates. Mineral surface areas calculated from images with resolutions of 0.34 μm and 5.71 μm were used to simulate mineral reactions in the context of geologic CO2 sequestration in the Paluxy formation at the continuum scale. Additional simulations were carried out using BET surface areas collected from the literature and geometric surface areas. Simulations were run for 7300 days and mineral volume fractions and effluent ion concentrations tracked and compared. Variations in mineral surface areas measured from images are within 1 order of magnitude and yield similar simulation results, indicating the impact of image resolution on simulated reactions and reaction rates is minimum for the resolutions and sample considered. In comparison, surface areas obtained from BET and geometric approaches are 1–4 orders of magnitude higher than image-obtained surface areas and result in greater simulated reaction rates and extents. Minerals with high reaction rates (calcite and siderite) are most impacted by surface area values at short times where simulated mineral volume fractions at longer times agree relatively well, even for simulations with several orders of magnitude variation in surface area. Phases with lower reaction rates, such as K-feldspar and muscovite, are predominantly impacted over longer times where variations in surface areas impact reaction extents and porosity evolution. Overall, variations in surface areas due to image resolution are small and result in little variation in simulated reactions and reaction rates while there are significant variations in simulation results when other surface area estimates are used. These variations, however, depend on the reactivity of the mineral phase where surface areas of fast-reacting phases largely impact simulations at short (hours to days) timescales and surface areas of slower-reactive phases impact longer term simulations (years).

Modelling of multi-minerals kinetic evolution in hyper-alkaline leachate for a 15-year experiment.

Cement has been widely used for low- to intermediate-level radioactive waste management; however, the long-term modelling of multiple mineral transfer between the cement leachate and the host rock of a geological disposal facility remains a challenge due to the strong physical-chemical interactions within the chemically disturbed zone. This paper presents a modelling study for a 15-year experiment simulating the reaction of crystalline basement rock with evolved near-field groundwater (pH = 10.8). A mixed kinetic equilibrium (MKE) modelling approach was employed to study the dolomite-rich fracture-filling assemblage reacting with intermediate cement leachate. The study found that the mineralogical and geochemical transformation of the system was driven by the kinetically controlled dissolution of the primary minerals (dolomite, calcite, quartz, k-feldspar and muscovite). The initial high concentration of calcium ions appeared to be the main driving force initiating the dedolomitization process, which played a significant role in the precipitation of secondary talc, brucite and Mg-aluminosilicate minerals. The modelling study also showed that most of the initially precipitated calcium silicon hydrate phases redissolved and formed more stable calcium silicon aluminium hydrate phases. The findings highlight the importance of a deep and insightful understanding of the geochemical transformations based on the type and characteristics of the host rock, where the system is under out of equilibrium conditions, and the rates of mineral reactions.

Mineral dissolution and reprecipitation mediated by an amorphous phase

Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth’s internal dynamics. Element exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al–Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.

Field Reaction Experiments of Carbonate Minerals in Spring Waters: Natural Analogue of Geologic CO2 Storage

To diminish the uncertainty of the mineral trapping rate during geologic CO2 storage, the growth rate of carbonate minerals was measured in CO2-containing spring waters, which can be regarded as a natural analogue of geologic CO2 storage. The authors’ approach, using nanoscale analysis of seed crystal surfaces after immersion into spring waters, enables rapid and accurate measurement of mineral reaction rates. The results show that calcite growth rates in spring waters were lower by 1–3 orders than the values given in a database of laboratory experiment results. We verified the traditional paradigm that Mg2+ controls carbonate reaction kinetics. An increase of the Mg/Ca ratio to around 5 by adding Mg2+ to spring waters markedly reduced the calcite growth rate. However, even if effects of Mg2+ and flow rate are considered, we were unable to explain satisfactorily the difference of the calcite growth rates between those of spring waters and laboratory experiments. Therefore, other factors might also be related to the slow growth rate in nature. The present results, including the fact such that neither dolomite nor magnesite was formed even at the high Mg/Ca ratio, are expected to provide an important constraint to overestimation of the mineral trapping rate.

Geochemical Evolution of Groundwater Flowing Through Arsenic Source Sediments in an Aquifer System of West Bengal, India

AbstractThe source of geogenic arsenic (As) contaminating a shallow aquifer in West Bengal was traced to fine‐grained sediments deposited in an abandoned river channel. Along with As‐bearing phases, these sediments contain 0.46% codeposited organic carbon. The release of As and the geochemistry of groundwater within the channel‐fill deposits is investigated using a detailed mass balance model supported by aqueous, solid‐phase, and mineralogical data. The model describes the evolution of groundwater chemistry along a flow path extending from its recharge in an abandoned channel pond, through the channel fill, to the underlying aquifer. Variations in groundwater composition are explained in terms of mineral weathering of host sediments driven by organic carbon decay. Arsenic is released through the reductive dissolution of goethite and the weathering of chlorite. Concomitantly, some As is sequestered in precipitating vivianite. These competing processes reach equilibrium deeper in the channel‐fill sequence as groundwater As concentrations stabilize. The model yields estimates of mineral reaction (or precipitation) rates including rates of organic carbon oxidation (1.15 mmol C L−1 a−1) and net As release ( mmol L−1 a−1). Fine‐grained, slightly permeable, deposits such as channel fill containing reactive organic carbon and As‐bearing goethite and phyllosilicates are centers of intense chemical weathering conducive to As mobilization.

Effects of atmospheric composition on apparent activation energy of silicate weathering: I. Model formulation

We have developed a weathering model to comprehensively understand the determining factors of the apparent activation energy of silicate weathering in order to better estimate the silicate-weathering flux in the Precambrian. The model formulates the reaction rate of a mineral as a basis, then the elemental loss by summing the reaction rates of whole minerals, and finally the weathering flux from a given weathering profile by integrating the elemental losses along the depth of the profile. The rate expressions are formulated with physicochemical parameters relevant to weathering, including solution and atmospheric compositions. The apparent activation energies of silicate weathering are then represented by the temperature dependences of the physicochemical parameters based on the rate expressions. It was found that the interactions between individual mineral-reactions and the compositions of solution and atmosphere are necessarily accompanied by those of temperature-dependence counterparts. Indeed, the model calculates the apparent activation energy of silicate weathering as a function of the temperature dependence of atmospheric CO2(ΔHCO2′). The dependence of the apparent activation energy of silicate weathering on ΔHCO2′ may explain the empirical dependence of silicate weathering on the atmospheric composition. We further introduce a compensation law between the apparent activation energy and the pre-exponential factor to obtain the relationship between the silicate-weathering flux (FCO2), temperature and the apparent activation energy. The model calculation and the compensation law enable us to predict FCO2 as a function of temperature, once ΔHCO2′ is given. The validity of the model is supported by agreements between the model prediction and observations of the apparent activation energy and FCO2 in the modern weathering systems. The present weathering model will be useful for the estimation of FCO2 in the Precambrian, for which ΔHCO2′ can be deduced from the greenhouse effect of atmospheric CO2.

Evaluation of accessible mineral surface areas for improved prediction of mineral reaction rates in porous media

The rates of mineral dissolution reactions in porous media are difficult to predict, in part because of a lack of understanding of mineral reactive surface area in natural porous media. Common estimates of mineral reactive surface area used in reactive transport models for porous media are typically ad hoc and often based on average grain size, increased to account for surface roughness or decreased by several orders of magnitude to account for reduced surface reactivity of field as opposed to laboratory samples. In this study, accessible mineral surface areas are determined for a sample from the reservoir formation at the Nagaoka pilot CO2 injection site (Japan) using a multi-scale image analysis based on synchrotron X-ray microCT, SEM QEMSCAN, XRD, SANS, and FIB-SEM. This analysis not only accounts for accessibility of mineral surfaces to macro-pores, but also accessibility through connected micro-pores in smectite, the most abundant clay mineral in this sample. While the imaging analysis reveals that most of the micro- and macro-pores are well connected, some pore regions are unconnected and thus inaccessible to fluid flow and diffusion. To evaluate whether mineral accessible surface area accurately reflects reactive surface area a flow-through core experiment is performed and modeled at the continuum scale. The core experiment is performed under conditions replicating the pilot site and the evolution of effluent solutes in the aqueous phase is tracked. Various reactive surface area models are evaluated for their ability to capture the observed effluent chemistry, beginning with parameter values determined as a best fit to a disaggregated sediment experiment (Beckingham et al., 2016) described previously. Simulations that assume that all mineral surfaces are accessible (as in the disaggregated sediment experiment) over-predict the observed mineral reaction rates, suggesting that a reduction of RSA by a factor of 10–20 is required to match the core flood experimental data. While the fit of the effluent chemistry (and inferred mineral dissolution rates) greatly improve when the pore-accessible mineral surface areas are used, it was also necessary to include highly reactive glass phases to match the experimental observations, in agreement with conclusions from the disaggregated sediment experiment. It is hypothesized here that the 10–20 reduction in reactive surface areas based on the limited pore accessibility of reactive phases in core flood experiment may be reasonable for poorly sorted and cemented sediments like those at the Nagaoka site, although this reflects pore rather than larger scale heterogeneity.

Influence of grain size, water, and deformation on dolomite reaction rim formation

Solid-solid mineral reaction rates are influenced by the microfabrics of reactant phases and concurrent deformation. To investigate this interplay in carbonate systems, we performed annealing and deformation experiments on polycrystalline and single-crystal calcite and magnesite, forming dolomite (Dol) and magnesio-calcite (Mg-Cal). At a fixed temperature of T = 750 °C and confining pressure of P = 400 MPa, samples were either annealed for 29 h, or deformed in triaxial compression or torsion for 18 h using a Paterson-type gas deformation apparatus. At the contact interface of the starting reactants, Dol reaction rims and polycrystalline Mg-Cal layers were formed. The widths of the layers were in the ranges 4–117 and 30–147 μm, respectively, depending on the microstructure of starting materials and experimental conditions. Annealing experiments with polycrystalline reactants in contact with each other resulted in a ~22-fold increase in Dol rim thickness compared to a contact between two single crystals and a larger Mg-Cal layer width by a factor of 5 (cf. Helpa et al. 2014). This suggests that the microstructure of magnesite controls migration of the reaction front. For polycrystalline starting materials, axial stress accelerated Mg-Cal growth rates but not Dol growth rates. Highly strained torsion samples showed Dol formation along grain boundaries in Mg-Cal as well as in the polycrystalline calcite reactant. A reduction of Dol rim thickness between polycrystalline reactants deformed in torsion is possibly caused by concurrent grain coarsening of polycrystalline magnesite. Dol and Mg-Cal growth kinetics between single crystals were unaffected by the addition of ~0.3 wt% water. The experiments demonstrate that Dol reaction kinetics strongly correlate with magnesite reactant grain sizes, while Mg-Cal growth depends on the calcite reactant grain sizes. The dolomite-forming mineral reaction kinetics are not significantly affected by concurrent deformation. In contrast, deformation enhances Mg-Cal formation, especially at small calcite grain sizes that promote efficient grain boundary diffusion. Therefore, the fastest reactions forming Dol and Mg-Cal in nature are expected to occur in very fine-grained reactants. Concurrent deformation may drastically enhance reaction kinetics if grain size reduction of the reactants occurs by, for example, cataclasis or dynamic recrystallization.

On the use of Li isotopes as a proxy for water–rock interaction in fractured crystalline rocks: A case study from the Gotthard rail base tunnel

We present Li isotope measurements of groundwater samples collected during drilling of the 57km long Gotthard rail base tunnel in Switzerland, to explore the use of Li isotope measurements for tracking water–rock interactions in fractured crystalline rocks at temperatures of up to 43°C. The 17 groundwater samples originate from water-conducting fractures within two specific crystalline rock units, which are characterized by a similar rock mineralogy, but significantly different fluid composition. In particular, the aqueous Li concentrations observed in samples from the two units vary from 1–4mg/L to 0.01–0.02mg/L. Whereas δ7Li values from the unit with high Li concentrations are basically constant (δ7Li=8.5–9.1‰), prominent variations are recorded for the samples from the unit with low Li concentrations (δ7Li=10–41‰). This observation demonstrates that Li isotope fractionation can be highly sensitive to aqueous Li concentrations. Moreover, δ7Li values from the unit with low Li concentrations correlate well with reaction progress parameters such as pH and [Li]/[Na] ratios, suggesting that δ7Li values are mainly controlled by the residence time of the fracture groundwater. Consequently, 1D reactive transport modeling was performed to simulate mineral reactions and associated Li isotope fractionation along a water-conducting fracture system using the code TOUGHREACT. Modeling results confirm the residence time hypothesis and demonstrate that the absence of δ7Li variation at high Li concentrations can be well explained by limitation of the amount of Li that is incorporated into secondary minerals. Modeling results also suggest that Li uptake by kaolinite forms the key process to cause Li isotope fractionation in the investigated alkaline system (pH >9), and that under slow flow conditions (<10m/year), this process is associated with a very large Li isotope fractionation factor (ε≈−50‰). Moreover, our simulations demonstrate that for simple and well-defined systems with known residence times and low Li concentrations, δ7Li values may help to quantify mineral reaction rates if more thermodynamic data about the temperature-dependent incorporation of Li in secondary minerals as well as corresponding fractionation factors become available in the future. In conclusion, δ7Li values may be a powerful tool to track water–rock interaction in fractured crystalline rocks at temperature higher than those at the Earth’s surface, although their use is restricted to low Li concentrations and well defined flow systems.

CO2 sequestration in feldspar-rich sandstone: Coupled evolution of fluid chemistry, mineral reaction rates, and hydrogeochemical properties

To investigate CO2 Capture, Utilization, and Storage (CCUS) in sandstones, we performed three 150°C flow-through experiments on K-feldspar-rich cores from the Eau Claire formation. By characterizing fluid and solid samples from these experiments using a suite of analytical techniques, we explored the coupled evolution of fluid chemistry, mineral reaction rates, and hydrogeochemical properties during CO2 sequestration in feldspar-rich sandstone. Overall, our results confirm predictions that the heightened acidity resulting from supercritical CO2 injection into feldspar-rich sandstone will dissolve primary feldspars and precipitate secondary aluminum minerals. A core through which CO2-rich deionized water was recycled for 52days decreased in bulk permeability, exhibited generally low porosity associated with high surface area in post-experiment core sub-samples, and produced an Al hydroxide secondary mineral, such as boehmite. However, two samples subjected to ∼3day single-pass experiments run with CO2-rich, 0.94mol/kg NaCl brines decreased in bulk permeability, showed generally elevated porosity associated with elevated surface area in post-experiment core sub-samples, and produced a phase with kaolinite-like stoichiometry. CO2-induced metal mobilization during the experiments was relatively minor and likely related to Ca mineral dissolution. Based on the relatively rapid approach to equilibrium, the relatively slow near-equilibrium reaction rates, and the minor magnitudes of permeability changes in these experiments, we conclude that CCUS systems with projected lifetimes of several decades are geochemically feasible in the feldspar-rich sandstone end-member examined here. Additionally, the observation that K-feldspar dissolution rates calculated from our whole-rock experiments are in good agreement with literature parameterizations suggests that the latter can be utilized to model CCUS in K-feldspar-rich sandstone. Finally, by performing a number of reactive transport modeling experiments to explore processes occurring during the flow-through experiments, we have found that the overall progress of feldspar hydrolysis is negligibly affected by quartz dissolution, but significantly impacted by the rates of secondary mineral precipitation and their effect on feldspar saturation state. The observations produced here are critical to the development of models of CCUS operations, yet more work, particularly in the quantification of coupled dissolution and precipitation processes, will be required in order to produce models that can accurately predict the behavior of these systems.

The effects of physical and geochemical heterogeneities on hydro-geochemical transport and effective reaction rates

The role of coupled physical and geochemical heterogeneities in hydro-geochemical transport is investigated by simulating three-dimensional transport in a heterogeneous system with kinetic mineral reactions. Ensembles of 100 physically heterogeneous realizations were simulated for three geochemical conditions: 1) spatially homogeneous reactive mineral surface area, 2) reactive surface area positively correlated to hydraulic heterogeneity, and 3) reactive surface area negatively correlated to hydraulic heterogeneity. Groundwater chemistry and the corresponding effective reaction rates were calculated at three transverse planes to quantify differences in plume evolution due to heterogeneity in mineral reaction rates and solute residence time (τ). The model is based on a hypothetical CO2 intrusion into groundwater from a carbon capture utilization and storage (CCUS) operation where CO2 dissolution and formation of carbonic acid created geochemical dis-equilibrium between fluids and the mineral galena that resulted in increased aqueous lead (Pb2+) concentrations. Calcite dissolution buffered the pH change and created conditions of galena oversaturation, which then reduced lead concentrations along the flow path. Near the leak kinetic geochemical reactions control the release of solutes into the fluid, but further along the flow path mineral solubility controls solute concentrations. Simulation results demonstrate the impact of heterogeneous distribution of geochemical reactive surface area in coordination with physical heterogeneity on the effective reaction rate (Krxn,eff) and Pb2+ concentrations within the plume. Dissimilarities between ensemble Pb2+ concentration and Krxn,eff are attributed to how geochemical heterogeneity affects the time (τeq) and therefore advection distance (Leq) required for the system to re-establish geochemical equilibrium. Only after geochemical equilibrium is re-established, Krxn,eff and Pb2+ concentrations are the same for all three geochemical conditions. Correlation between reactive surface area and hydraulic conductivity, either positive or negative, results in variation in τeq and Leq.

Silicon Isotopes as a New Method of Measuring Silicate Mineral Reaction Rates at Ambient Temperature

Experiments of albite dissolution under ambient temperature at pH 3 and 5 demonstrated the feasibility of using 29Si doping method to precisely measure silicate mineral dissolution rates. Initial solutions were doped with 29Si to result in an isotopic composition of 0.04% 28Si, 99.9% 29Si, and 0.06% 30Si, which compares with the Amelia and Evja albite with Si isotopic composition of 92.23% 28Si, 4.67% 29Si, and 3.10% 30Si. The isotopic contrast allows the detection of dissolution of a tiny amount of albite into the aqueous solution. Experimental results show rapid increase of 28Si abundance at the initial stage of fresh albite dissolution, reflecting reaction with high energy sites on the surface. Steady state dissolution rates were reached after 20-30 days at 22 oC. The evolution of 28Si and 29Si abundances (%) with time tightly constrains the albite dissolution rates, which are consistent with literature data. In experiments seeded with kaolinite, both albite dissolution rates and kaolinite precipitation rates were determined with a combination of Si isotopes and isotope dilution method of determining Si concentrations.The new experiments have also verified the assumptions that are necessary for the isotope doping method to be applicable. Experimental data showed no Si isotope fractionation during albite dissolution. The ion-exchange method in sample pre-treatments does not alter Si isotopic composition or silica concentrations. The errors from neglecting isotope fractionation during secondary mineral precipitation are small on the determined albite dissolution rates when the isotope doping is overwhelming. In conclusion, we now have substantial experimental data show that the 29Si doping is a new method applicable to a wide range of silicate mineral dissolution rate determinations. Particularly, our albite experimental results show promise to use isotope doping method to measure silicate dissolution rates at various saturation states and determine the rate – ΔGr relationships.

Upscaling Calcite Growth Rates from the Mesoscale to the Macroscale

Quantitative prediction of mineral reaction rates in the subsurface remains a daunting task partly because a key parameter for macroscopic models, the reactive site density, is poorly constrained. Here we report atomic force microscopy (AFM) measurements on the {1014} calcite surface of monomolecular step densities, treated as equivalent to the reactive site density, as a function of aqueous calcium-to-carbonate ratio and saturation index. Data for the obtuse step orientation are combined with existing step velocity measurements to generate a model that predicts overall macroscopic calcite growth rates. The model is quantitatively consistent with several published macroscopic rates under a range of alkaline solution conditions, particularly for two of the most comprehensive data sets, without the need for additional fit parameters. The model reproduces peak growth rates, and its functional form is simple enough to be incorporated into reactive transport or other macroscopic models designed for predictions in porous media. However, it currently cannot model equilibrium or pH effects and it may overestimate rates at high aqueous calcium-to-carbonate ratios. The discrepancies in rates at high calcium-to-carbonate ratios may be due to differences in pretreatment, such as exposing the seed material to SI ≥ 1.0 to generate/develop growth hillocks, or other factors.

Human Health Risk Assessment of CO2 Leakage into Overlying Aquifers Using a Stochastic, Geochemical Reactive Transport Approach

Increased human health risk associated with groundwater contamination from potential carbon dioxide (CO2) leakage into a potable aquifer is predicted by conducting a joint uncertainty and variability (JUV) risk assessment. The approach presented here explicitly incorporates heterogeneous flow and geochemical reactive transport in an efficient manner and is used to evaluate how differences in representation of subsurface physical heterogeneity and geochemical reactions change the calculated risk for the same hypothetical aquifer scenario where a CO2 leak induces increased lead (Pb(2+)) concentrations through dissolution of galena (PbS). A nested Monte Carlo approach was used to take Pb(2+) concentrations at a well from an ensemble of numerical reactive transport simulations (uncertainty) and sample within a population of potentially exposed individuals (variability) to calculate risk as a function of both uncertainty and variability. Pb(2+) concentrations at the well were determined with numerical reactive transport simulation ensembles using a streamline technique in a heterogeneous 3D aquifer. Three ensembles with variances of log hydraulic conductivity (σ(2)lnK) of 1, 3.61, and 16 were simulated. Under the conditions simulated, calculated risk is shown to be a function of the strength of subsurface heterogeneity, σ(2)lnK and the choice between calculating Pb(2+) concentrations in groundwater using equilibrium with galena and kinetic mineral reaction rates. Calculated risk increased with an increase in σ(2)lnK of 1 to 3.61, but decreased when σ(2)lnK was increased from 3.61 to 16 for all but the highest percentiles of uncertainty. Using a Pb(2+) concentration in equilibrium with galena under CO2 leakage conditions (PCO2 = 30 bar) resulted in lower estimated risk than the simulations where Pb(2+) concentrations were calculated using kinetic mass transfer reaction rates for galena dissolution and precipitation. This study highlights the importance of understanding both hydrologic and geochemical conditions when numerical simulations are used to perform quantitative risk calculations.

Kinetic modelling of CO2–water–rock interactions

A system perturbated by CO2 injection reacts by dissolving primary minerals and form new secondary phases. The importance of such mineral reactions for safe long-term storage is highly system dependent, with some reservoirs rich in reactive phases that contain divalent metal cations, which promote mineral carbonte growth, and other reservoirs being almost pure quartz (SiO2) sands. Because of the complexity in the reaction paths of natural systems, numerical simulations have proved to be valuable to predict the reactivity on time scales from laboratory experiments to tens or hundreds of thousands of years. Such numerical simulations require quality thermodynamic and kinetic data, as well as proper mathematical expressions.We here provide a guide to model kinetically constrained CO2–water–rock interactions aimed for both experienced geochemists and researchers with more limited background in modelling geochemical reactions. Because rates of mineral reactions are coupled through common aqueous species, and rates vary by more than ten orders of magnitude, the system of ordinary differential equations (ODE's) is commonly stiff. This leads to challenges in solving such equations and limits the possible system size and spatial and geometrical complexity that can be solved. We have here focused on how different simplifications can be made to reduce the CPU time required to solve mineral reactions and to allow reaction path modelling even for the largest scales. For a 10,000 years batch reaction simulation, we showed how the CPU run time decreased from hours when all minerals were defined by ODE's (fully kinetic), to a few seconds if secondary phases were allowed to grow according to the local equilibrium assumption (semi-kinetic). We then showed how the system can be further simplified by allowing far-from-equilibrium dissolving minerals to be expressed by first-order decay analytical expressions. We finally suggest a step-wise procedure where significant mineral reactions were first identified by running batch simulations, and the system were then simplified according to the system size, geometric complexity, and the time of interest.