Calcite Dissolution Rates Research Articles

Dissolution of minerals with rough surfaces

We study dissolution of minerals with initial rough surfaces using kinetic Monte Carlo simulations and a scaling approach. We consider a simple cubic lattice structure, a thermally activated rate of detachment of a molecule (site), and rough surface configurations produced by fractional Brownian motion algorithm. First we revisit the problem of dissolution of initial flat surfaces, in which the dissolution rate rF reaches an approximately constant value at short times and is controlled by detachment of step edge sites. For initial rough surfaces, the dissolution rate r at short times is much larger than rF; after dissolution of some hundreds of molecular layers, r decreases by some orders of magnitude across several time decades. Meanwhile, the surface evolves through configurations of decreasing energy, beginning with dissolution of isolated sites, then formation of terraces with disordered boundaries, their growth, and final smoothing. A crossover time to a smooth configuration is defined when r=1.5rF; the surface retreat at the crossover is approximately 3 times the initial roughness and is temperature-independent, while the crossover time is proportional to the initial roughness and is controlled by step-edge site detachment. The initial dissolution process is described by the so-called rough rates, which are measured for fixed ratios between the surface retreat and the initial roughness. The temperature dependence of the rough rates indicates control by kink site detachment; in general, it suggests that rough rates are controlled by the weakest microscopic bonds during the nucleation and formation of the lowest energy configurations of the crystalline surface. Our results are related to recent laboratory studies which show enhanced dissolution in polished calcite surfaces. In the application to calcite dissolution in alkaline environment, the minimal values of recently measured dissolution rate spectra give rF∼10-9 mol/(m2 s), and the calculated rate laws of our model give rough rates in the range 10-6–10-5 mol/(m2 s). This estimate is consistent with the range of calcite dissolution rates obtained in a recent work after treatment of literature data, which suggests the universal control of kink site dissolution in short term laboratory works. The weak effects of lattice size on our results also suggest that smoothing of mineral grain surfaces across geological times may be a microscopic explanation for the difference of chemical weathering rate of silicate minerals in laboratory and in the environment.

Carbonate dissolution rates in high salinity brines: Implications for post-Noachian chemical weathering on Mars

A diverse suite of carbonate minerals including calcite (CaCO3) and magnesite (MgCO3) have been observed on the martian surface and in meteorites. Terrestrial carbonates usually form via aqueous processes and often record information about the environment in which they formed, including chemical and textural biosignatures. In addition, terrestrial carbonates are often found in association with evaporite deposits on Earth. Similar high salinity environments and processes were likely active on Mars and some areas may contain active high salinity brines today. In this study, we directly compare calcite and magnesite dissolution in ultrapure water, dilute sulfate and chloride solutions, as well as near-saturated sulfate and chloride brines with known activity of water (ɑH2O) to determine how dissolution rates vary with mineralogy and ɑH2O, as well as aqueous cation and anion chemistry to better understand how high salinity fluids may have altered carbonate deposits on Mars. We measured both calcite and magnesite initial dissolution rates at 298 K and near neutral pH (6–8) in unbuffered solutions containing ultrapure water (18 MΩ cm−1 UPW; ɑH2O = 1), dilute (0.1 mol kg−1; ɑH2O = 1) and near-saturated Na2SO4 (2.5 mol kg−1, ɑH2O = 0.92), dilute (0.1 mol kg−1, ɑH2O = 1) and near-saturated NaCl (5.7 mol kg−1, ɑH2O = 0.75). Calcite dissolution rates were also measured in dilute and near-saturated MgSO4 (0.1 mol kg−1, ɑH2O = 1 and 2.7 mol kg−1, ɑH2O = 0.92, respectively) and MgCl2 (0.1 mol kg−1, ɑH2O = 1 and 3 mol kg−1, ɑH2O = 0.73, respectively), while magnesite dissolution rates were measured in dilute and near-saturated CaCl2 (0.1 mol kg−1, ɑH2O = 1 and 9 mol kg−1, ɑH2O = 0.35).Initial calcite dissolution rates were fastest in near-saturated MgCl2 brine, while magnesite dissolution rates were fastest in dilute (0.1 mol kg−1) NaCl and CaCl2 solutions. Calcite dissolution rates in near-saturated Na2SO4 were similar to those observed in the dilute solutions (−8.00 ± 0.12 log mol m−2 s−1), while dissolution slowed in both NaCl solutions (0.1 mol kg−1; −8.23 ± 0.10 log mol m−2 s−1 and (5.7 mol kg−1; −8.44 ± 0.11 log mol m−2 s−1), as well as near-saturated MgSO4 brine (2.7 mol kg−1; −8.35 ± 0.05 log mol m−2 s−1). The slowest calcite dissolution rates observed in the near-saturated NaCl brine. Magnesite dissolution rates were ∼5 times faster in the dilute salt solutions relative to UPW, but similar to UPW (−8.47 ± 0.06 log mol m−2 s−1) in near-saturated Na2SO4 brines (−8.41 ± 0.18 log mol m−2 s−1). Magnesite dissolution slowed significantly in near-saturated CaCl2 brine (−9.78 ± 0.10 log mol m−2 s−1), likely due to the significantly lower water activity in these experiments. Overall, magnesite dissolution rates are slower than calcite dissolution rates and follow the trend: All dilute salt solutions >2.5 mol kg−1 Na2SO4 ≈ UPW > 5.7 mol kg−1 NaCl >> 9 mol kg−1 CaCl2. Calcite rates follow the trend 3 mol kg−1 MgCl2 > 2.5 mol kg−1 Na2SO4 ≈ UPW ≈ all dilute salt solutions >2.7 mol kg−1 MgSO4 ≈ 5.7 mol kg−1 NaCl. Magnesite dissolution rates in salt solutions generally decrease with decreasing ɑH2O in both chloride and sulfate brines, which indicates water molecules act as ligands and participate in the rate-limiting magnesite dissolution step. However, there is no general trend associated with water activity observed in the calcite dissolution rates. Calcite dissolution accelerates in near-saturated MgCl2, but slows in near-saturated NaCl brine despite both brines having similar water activities (ɑH2O = 0.73 and 0.75, respectively). High Mg calcite was observed as a reaction product in the near-saturated MgCl2, indicating Mg2+ from solution likely substituted for Ca2+ in the initial calcite, releasing additional Ca2+ into solution and increasing the observed calcite dissolution rate. Calcite dissolution rates also increase slightly as Na2SO4 concentration increases, while calcite dissolution rates slow slightly with increasing concentration of MgSO4 and NaCl. However, all of the carbonate rates vary by less than 0.5 log units and are within or near the standard deviation observed for each set of replicate experiments.Carbonate mineral lifetimes in high salinity brines indicate magnesite may be preferentially preserved compared to calcite on Mars. Therefore, Mg-carbonates that have experienced post-depositional aqueous alteration are more likely to preserve paleoenvironmental indicators and potential biosignatures. Rapid weathering of carbonates in circum-neutral pH sulfate brines may provide a potential source of cations for abundant sulfate minerals observed on Mars, Ceres, and other planetary bodies.

Wormhole Generations in Indiana Limestone with CO2 Intrusion: Numerical Simulations Based on Core Flooding Experiments

In this paper, we examined possible impact factors for calcite dissolution and wormhole generation in limestone formations with CO2 saturated brine intrusion by continuum-scale reactive transport simulations. Previous core-flood experiments that mimic near-wellbore region conditions of Geological CO2 Storage (GCS) fields were used as verification cases. Simulation results reasonably reproduce the experimental results. The results suggest that the initial permeability distribution of the core and pre-existing large permeability zones along the core were key factors for wormhole generation. Porosity distribution also affects wormhole generation, but not significantly. According to our simulation results, CO2 saturated brine injection rate, injection pattern, calcite dissolution rate, and permeability anisotropy do not significantly impact calcite dissolution and wormhole generation. The Damkohler (Da) numbers were 0.45–0.9 for the experiments at the core scale in this study, which are in accordance with the...

Calcite dissolution rate spectra measured by in situ digital holographic microscopy

Digital holographic microscopy in reflection mode is used to track in situ, real-time nanoscale topography evolution of cleaved (104) calcite surfaces exposed to flowing or static deionized water. The method captures full-field holograms of the surface at frame rates of up to 12.5s−1. Numerical reconstruction provides 3D surface topography with vertical resolution of a few nanometers and enables measurement of time-dependent local dissolution fluxes. A statistical distribution, or spectrum, of dissolution rates is generated by sampling multiple area domains on multiple crystals. The data show, as has been demonstrated by Fischer et al. (2012), that dissolution is most fully described by a rate spectrum, although the modal dissolution rate agrees well with published mean dissolution rates (e.g., 0.1μmolm−2s−1 to 0.3μmolm−2s−1). Rhombohedral etch pits and other morphological features resulting from rapid local dissolution appear at different times and are heterogeneously distributed across the surface and through the depth. This makes the distribution in rates measured on a single crystal dependent both on the sample observation field size and on time, even at nominally constant undersaturation. Statistical analysis of the inherent noise in the DHM measurements indicates that the technique is robust and that it likely can be applied to quantify and interpret rate spectra for the dissolution or growth of other minerals.

Calcite dissolution kinetics at the sediment-water interface in natural seawater

Abstract The absorption, or uptake, of anthropogenic CO 2 by the oceans results in a decrease in pH and carbonate ion concentration, [CO 3 2 − ]; as a consequence, the saturation state of seawater with respect to CaCO 3 minerals (calcite, aragonite) falls, leading to a shallowing of their saturation depths and triggering an increase in their dissolution at the seafloor. Nearly one-third of the seabed is composed of CaCO 3 -rich sediments, and their dissolution is the ultimate marine sink of anthropogenic CO 2 . Despite numerous past studies, much confusion and uncertainty still surround our understanding of the rates and kinetics of CaCO 3 dissolution at the deep seafloor. Results from in situ studies disagree with laboratory studies, most of which have been carried out under conditions, e.g., mineral suspensions, that are not representative of processes at the seafloor. Herein, we report measurements of the dissolution rate of calcite, formed into synthetic sediment disks by mixing various amounts of this mineral with montmorillonite. These disks were placed in a stirred-flow reactor and exposed to a range of saturation states and shear stress conditions to simulate conditions at the sediment-water interface. The dissolution rates, normalized to the interfacial area of the sediment disks, were linearly dependent on the undersaturation state of the experimental seawater solution and displayed a square-root dependence on the calcite content, under both quiescent and stirred conditions. The rate of release of reaction products from the sediment increased with stirring rate, i.e., shear stress, until it became invariant at higher stirring rates. This latter result argues that calcite dissolution is transport (water-side) controlled for shear stress levels known to exist at the seafloor, which advises a simpler kinetic description of benthic calcite dissolution.

Effect of $$\hbox {CO}_2$$ CO 2 solubility on dissolution rate of calcite in saline aquifers for temperature range of 50–100 $$^{\circ } \hbox {C}$$ ∘ C and pressures up to 600 bar: alterations of fractures geometry in carbonate rocks by $$\hbox {CO}_2$$ CO 2 -acidified brines

An empirical model is developed to predict the dissolution rate of calcite in saline solutions that are saturated with respect to dissolved \(\hbox {CO}_2\) over a broad range of both subcritical and supercritical conditions. The focus is on determining the rate of calcite dissolution within a temperature range of 50–100 \(^\circ \hbox {C}\) and pressures up to 600 bar, relevant for \(\hbox {CO}_2\) sequestration in saline aquifers. A general reaction kinetic model is used that is based on the extension of the standard Arrhenius equation with an added, solubility-dependent, pH term to account for the saturated concentration of dissolved \(\hbox {CO}_2\). On the basis of this kinetic model, a new rate equation is obtained using multi-parameter, nonlinear regression of experimental data to determine the dissolution of calcite as a function of temperature, pressure and salinity. Different models for the activity coefficient of \(\hbox {CO}_2\) dissolved in saline solutions are accounted for. The new rate equation helps us obtain good agreement with experimental data, and it is applied to study the geochemically induced alterations of fracture geometry due to calcite dissolution.

Effect of dissolved H2SO4 on the interaction between CO2-rich brine solutions and limestone, sandstone and marl

The effect of pure and impure carbon dioxide (CO2) on geological storage is uncertain. Oxidation of impurities such as sulfur dioxide (SO2) and hydrogen sulfide (H2S), which may be catalyzed by co-injected oxygen or nitrogen oxides, leads to the formation of sulfuric acid (H2SO4) and a decrease in pH of the formation water. We investigated the effect of 0.005molL−1 H2SO4 (corresponding to a worst case of 0.4% SO2 in the flue gas, anticipating total conversion of SO2 into H2SO4) on the reactivity of the reservoir (limestone and sandstone) and cap (marl) rocks of Hontomin (Spain) at P=pCO2=10bar and 60°C during 24days using flow-through column experiments. Aqueous element concentrations were measured from fluid extracts obtained periodically throughout the experiments to infer fluid-rock reactions over time. Results were modeled with the CrunchFlow reactive transport code.The added H2SO4 lowered the pH of the injected brine by ~1.5pH units with respect to the pH of ~3.6 of the H2SO4-free brine. In both H2SO4-free and H2SO4-rich brine, calcite dissolution fostered gypsum precipitation. A comparison between the reactivity of the rocks reacted in H2SO4-free and H2SO4-rich brine showed that calcite dissolution and gypsum precipitation rates were increased by 27–48% and 25–75%, respectively, in H2SO4-rich brine. Overall rock porosity increments in H2SO4-rich brine were 2.9–3.6%, 3.7–4.8%, and 2.1–2.7% for sandstone, limestone and marl, respectively. Porosities in H2SO4-rich brine were 6%, 23% and 250%, higher, respectively, than under pure CO2. Modeled porosity increments in the acid inlet zone in H2SO4-rich brine for sandstone, limestone and marl were 28–29%, 44–45% and 24–28%, respectively, corresponding to an increase of 1%, 250% and 25%, respectively, relative to H2SO4-free brine. Gypsum precipitation was consistently higher in marl than in limestone and sandstone, indicating kinetically favorable conditions for gypsum precipitation within the cap rock. Our results provide relevant data for long-term storage simulations of impure CO2 injection.

Preparation of porous β-tricalcium phosphate using starfish-derived calcium carbonate as a precursor

Porous β-tricalcium phosphate (β-TCP) was successfully prepared from starfish-derived calcium carbonate (sf-bone) under several hydrothermal conditions. The sf-bone, obtained from Patiria Pectinifera by bleaching to remove organic substances, was Mg-containing calcite granules with an interconnected microporous structure of approximately 10−50µm of pore, and was hydrothermally treated with ammonium phosphate aqueous solutions at various pHs and temperatures. The sf-bone was converted to Mg-containing β-TCP with maintaining its microporous structure by the hydrothermal treatment for 1day or longer in (NH4)2HPO4 aqueous solution at 200°C. This conversion was based on dissolution-reprecipitation process of Mg-containing calcite in the phosphate salt aqueous solution. Thus, conditions during the conversion, pH and temperature, affected the morphologies and crystal phases of sf-bone after the treatment depended upon both calcite dissolution and calcium phosphate-formation rates.

The Alkaline Dissolution Rate of Calcite.

Due to the widespread presence of calcium carbonate on Earth, several geochemical systems, among which is the global CO2 cycle, are controlled to a large extent by the dissolution and precipitation of this mineral. For this reason, the dissolution of calcite has been thoroughly investigated for decades. Despite this intense activity, a consensual value of the dissolution rate of calcite has not been found yet. We show here that the inconsistency between the reported values stems mainly from the variability of the chemical and hydrodynamic conditions of measurement. The spreading of the values, when compared in identical conditions, is much less than expected and is interpreted in terms of sample surface topography. This analysis leads us to propose benchmark values of the alkaline dissolution rate of calcite compatible with all the published values, and a method to use them in various chemical and hydrodynamic contexts.

First principles model of carbonate compaction creep

AbstractRocks under compressional stress conditions are subject to long‐term creep deformation. From first principles we develop a simple micromechanical model of creep in rocks under compressional stress that combines microscopic fracturing and pressure solution. This model was then upscaled by a statistical mechanical approach to predict strain rate at core and reservoir scale. The model uses no fitting parameter and has few input parameters: effective stress, temperature, water saturation porosity, and material parameters. Material parameters are porosity, pore size distribution, Young's modulus, interfacial energy of wet calcite, the dissolution, and precipitation rates of calcite, and the diffusion rate of calcium carbonate, all of which are independently measurable without performing any type of deformation or creep test. Existing long‐term creep experiments were used to test the model which successfully predicts the magnitude of the resulting strain rate under very different effective stress, temperature, and water saturation conditions. The model was used to predict the observed compaction of a producing chalk reservoir.

Determination of mineral dissolution regimes using flow-through time-resolved analysis (FT-TRA) and numerical simulation

Flow-through time resolved analysis (FT-TRA) involves subjecting small mineral samples (<10mg) inserted in a miniature flow-through cell (50μL) to controlled flows of eluent analyzed on-line by ICP-MS. In this study, FT-TRA is used to empirically determine the dissolution regimes for the two well-studied minerals forsterite and calcite, representing minerals with relatively slow and fast dissolution kinetics. A proportional increase in steady-state effluent [Mg, Si] concentrations with increasing flow-through cell eluent residence times confirms a dominantly surface-controlled dissolution regime for a powdered forsterite sample at pH2.3, implying that transport limitations are negligible. In contrast, the relationship between flow rates and dissolution rates for single grain calcite samples at pH2.3–4 reveals that transport limitations affect the rate of calcite dissolution. To provide a quantitative and process-based assessment of the effect of diffusive transport limitations, simulations of the calcite experiments were performed with a high resolution, pore-scale model that considers the geometry of the calcite grain and the FT-TRA flow-through reactor. The pore-scale model reproduces the observed effluent [Ca] concentrations for all experimental conditions using a single set of surface kinetic parameters, by accounting for the formation of a diffusive boundary layer (DBL) that varies in thickness as a function of flow rates. These results demonstrate that combining FT-TRA with pore-scale modeling makes it possible to obtain unprecedented insights not achievable by either method separately, including quantification of DBL thicknesses and the determination of transport controls as a function of pH, flow velocity and residence times.

Pore‐scale simulation of carbonate dissolution in micro‐CT images

AbstractWe present a particle‐based method to simulate carbonate dissolution at the pore scale directly on the voxels of three‐dimensional micro‐CT images. The flow field is computed on the images by solving the incompressible Navier‐Stokes equations. Rock‐fluid interaction is modeled using a three‐step approach: solute advection, diffusion, and reaction. Advection is simulated with a semianalytical pore‐scale streamline tracing algorithm, diffusion by random walk is superimposed, while the reaction rate is defined by the flux of particles through the pore‐solid interface. We derive a relationship between the local particle flux and the independently measured batch calcite dissolution rate. We validate our method against a dynamic imaging experiment where a Ketton oolite is imaged during CO2‐saturated brine injection at reservoir conditions. The image‐calculated increases in porosity and permeability are predicted accurately, and the spatial distribution of the dissolution front is correctly replicated. The experiments and simulations are performed at a high flow rate, in the uniform dissolution regime – Pe ≫ 1 and PeDa ≪ 1—thus extending the reaction throughout the sample. Transport is advection dominated, and dissolution is limited to regions with significant inflow of solute. We show that the sample‐averaged reaction rate is 1 order of magnitude lower than that measured in batch reactors. This decrease is the result of restrictions imposed on the flux of solute to the solid surface by the heterogeneous flow field, at the millimeter scale.

Influence of the flow rate on dissolution and precipitation features during percolation of CO2-rich sulfate solutions through fractured limestone samples

Calcite dissolution and gypsum precipitation are expected to occur when injecting CO2 in a limestone reservoir with sulfate-rich resident brine. If the reservoir is fractured, these reactions will take place mainly in the fractures, which serve as preferential paths for fluid flow. As a consequence, the geometry of the fractures will vary leading to changes in their hydraulic and transport properties. In this study, a set of percolation experiments which consisted of injecting CO2-rich solutions through fractured limestone cores was performed under P=150bar and T=60°C. Flow rates ranging from 0.2 to 60mL/h and sulfate-rich and sulfate-free solutions were used.Variation in fracture volume induced by calcite dissolution and gypsum precipitation was measured by X-ray computed microtomography (XCMT) and aqueous chemistry. An increase in flow rate led to an increase in volume of dissolved limestone per unit of time, which indicated that the calcite dissolution rate in the fracture was transport controlled. Moreover, the dissolution pattern varied from face dissolution to wormhole formation and uniform dissolution by increasing the flow rate (i.e., Pe from 1 to 346). Fracture permeability always increased and depended on the type of dissolution pattern.

Explicit Planktic Calcifiers in the University of Victoria Earth System Climate Model, Version 2.9

ABSTRACTMarine calcifiers as a plankton functional type (PFT) are a crucial part of the global carbon cycle, being responsible for much of the carbon export to the deep ocean entering via biological pathways. Deep ocean carbon export through calcifiers is controlled by physiological, ecological, and biogeochemical factors. This paper describes the implementation of a calcifying phytoplankton PFT in the University of Victoria Earth System Climate Model, version 2.9 (UVic ESCM), and mechanistic improvements to the representation of model carbon export (a full calcite tracer, carbonate chemistry dependent calcite dissolution rates, and a ballasting scheme). An iterative method for stabilizing and tuning the biogeochemistry is furthermore described. The UVic ESCM now fills a niche in Earth system modelling that was previously unoccupied in that it is relatively inexpensive to run, yet resolves the complete Earth system carbon cycle including prognostic calcium carbonate and a separate phytoplankton calcifier PFT. The model is now well suited to testing feedbacks between the carbonate and carbon cycles and the climate system as transient simulations. The modifications described improve the UVic ESCM's mechanistic realism without compromising performance with respect to observed carbon and nutrient fluxes. Primary production, export production, particulate organic carbon, and calcite fluxes all fall within independently observed estimates.

Natural variations in calcite dissolution rates in streams: Controls, implications, and open questions

AbstractModels of bedrock channel evolution typically assume that chemical erosion is negligible in comparison to mechanical erosion. While this assumption is reasonable for channels in silicate rocks, it is questionable within highly soluble strata such as carbonates. The magnitude and variability of calcite dissolution rates in streams has remained as a critical unknown for models of bedrock incision and karst conduit formation. Here we use U.S. Geological Survey data to estimate calcite dissolution rates from 77 different streams located in a wide range of settings. The calculated rates are commonly on the order of ∼1 mmyr−1, which is 1 to 2 orders of magnitude larger than previous estimates. We also find that is the strongest control on at‐a‐site variability, though some sites also display dilution‐controlled variability. Typically, dissolution rates vary within a relatively narrow range, which has important implications for the relative importance of chemical and mechanical erosion.

Kinetics of calcite dissolution in CO2-saturated water at temperatures between (323 and 373) K and pressures up to 13.8 MPa

We report measurements of the calcite dissolution rate in CO2-saturated water at pressures ranging from (6.0 to 13.8)MPa and temperatures from (323 to 373)K. The rate of calcite dissolution in HCl(aq) at temperatures from (298 to 353)K was also measured at ambient pressure with pH between 2.0 and 3.3. A specially-designed batch reactor system, implementing a rotating disc technique, was used to obtain the dissolution rate at the solid/liquid interface of a single crystal, free of mass transfer effects. We used vertical scanning interferometry to examine the texture of the calcite surface produced by the experiment and the results suggested that at far-from-equilibrium conditions, the measured calcite dissolution rate was independent of the initial defect density due to the development of a dynamic dissolution pattern which became steady-state shortly after the onset of dissolution. The results of this study indicate that the calcite dissolution rate under surface-reaction-controlled conditions increases with the increase of temperature from (323 to 373)K and CO2 partial pressure from (6.0 to 13.8)MPa. Fitting the conventional first order transition state kinetic model to the observed rate suggested that, although sufficient to describe calcite dissolution in CO2-free HCl(aq), this model clearly underestimate the calcite dissolution rate in the (CO2+H2O) system over the range of conditions studied. A kinetic model incorporating both pH and the activity of CO2(aq) has been developed to represent the dissolution rates found in this study. We report correlations for the corresponding reaction rate coefficients based on the Arrhenius equation and compare the apparent activation energies with values from the literature. The results of this study should facilitate more rigorous modelling of mineral dissolution in deep saline aquifers used for CO2 storage.

Suitability of Existing Numerical Model Codes and Thermodynamic Databases for the Prognosis of Calcite Dissolution Processes in Near-Surface Sediments Due to a CO2 Leakage Investigated by Column Experiments

Among the risks of CO2 storage is the potential of CO2 leakage into overlaying formations and near-surface potable aquifers. Through a leakage, the CO2 can intrude into protected groundwater resources, which can lead to groundwater acidification followed by potential mobilisation of heavy metals and other trace metals through mineral dissolution or ion exchange processes. The prediction of pH buffer reactions in the formations overlaying a CO2 storage site is essential for assessing the impact of CO2 leakages in terms of trace metal mobilisation. For buffering the pH-value, calcite dissolution is one of the most important mechanisms. Although calcite dissolution has been studied for decades, experiments conducted under elevated CO2 partial pressures are rare. Here, the first study for column experiments is presented applying CO2 partial pressures from 6 to 43 bars and realising a near-natural flow regime. Geochemical calculations of calcite dissolution kinetics were conducted using PHREEQC together with different thermodynamic databases. Applying calcite surface areas, which were previously acquired by N2-BET or calculated based on grain diameters, respectively, to the rate laws according to Plummer et al. (Am J Sci 278:179–216, doi: 10.2475/ajs.278.2.179 , 1978) or Palandri and Kharaka (US Geol Surv Open file Rep 2004–1068:71, 2004) in the numerical simulations led to an overestimation of the calcite dissolution rate by up to three orders of magnitude compared to the results of the column experiments. Only reduction of the calcite surface area in the simulations as a fitting procedure allowed reproducing the experimental results. A reason may be that the diffusion boundary layer (DBL), which depends on the groundwater flow velocity and develops at the calcite grain surface separating it from the bulk of the solution, has to be regarded: The DBL leads to a decrease in the calcite dissolution rate under natural laminar flow conditions compared to turbulent mixing in traditional batch experiments. However, varying the rate constants by three orders of magnitudes in a field scale PHREEQC model simulating a CO2 leakage produced minor variations in the pH buffering through calcite dissolution. This justifies the use of equilibrium models when calculating the calcite dissolution in CO2 leakage scenarios for porous aquifers and slow or moderate groundwater flow velocities. However, the selection of the thermodynamic database has an impact on the dissolved calcium concentration, leading to an uncertainty in the simulation results. The resulting uncertainty, which applies also to the calculated propagation of an aquifer zone depleted in calcite through dissolution, seems negligible for shallow aquifers of approximately 60 m depth, but amounts to 35 % of the calcium concentration for aquifers at a depth of approximately 400 m.

Direct nanoscale observations of the coupled dissolution of calcite and dolomite and the precipitation of gypsum.

In-situ atomic force microscopy (AFM) experiments were performed to study the overall process of dissolution of common carbonate minerals (calcite and dolomite) and precipitation of gypsum in Na2SO4 and CaSO4 solutions with pH values ranging from 2 to 6 at room temperature (23 ± 1 °C). The dissolution of the carbonate minerals took place at the (104) cleavage surfaces in sulfate-rich solutions undersaturated with respect to gypsum, by the formation of characteristic rhombohedral-shaped etch pits. Rounding of the etch pit corners was observed as solutions approached close-to-equilibrium conditions with respect to calcite. The calculated dissolution rates of calcite at pH 4.8 and 5.6 agreed with the values reported in the literature. When using solutions previously equilibrated with respect to gypsum, gypsum precipitation coupled with calcite dissolution showed short gypsum nucleation induction times. The gypsum precipitate quickly coated the calcite surface, forming arrow-like forms parallel to the crystallographic orientations of the calcite etch pits. Gypsum precipitation coupled with dolomite dissolution was slower than that of calcite, indicating the dissolution rate to be the rate-controlling step. The resulting gypsum coating partially covered the surface during the experimental duration of a few hours.

Calcite dissolution rates in texturally diverse calcareous rocks

Abstract The injection of reactive fluids into carbonate reservoirs during enhanced recovery operations can induce important changes in rock permeability. However, reliably predicting these changes requires accurate knowledge of calcite reaction rates. While numerous studies have examined calcium carbonate dissolution rates, most have focused on pure calcite crystals rather than actual rocks. In this study, two types of flow-through experiments were carried out to determine the calcite dissolution rates in texturally diverse calcareous rocks: the first type of experiment had a duration of 3 days, while the second type ran for 3 months. Our experiments show that rocks with differing textures and roughness (samples included coarse-grained oolitic limestone, fine-grained Solnhofen limestone, marble and calcite spar) do in fact dissolve by different mechanisms. However, despite these differences, bulk reaction rates were found to be remarkably similar (with a relative standard deviation of &lt;20%) and consistent with previously reported reaction rates for calcite. Thus, our results suggest that textural differences between rock types are unlikely to have an important impact on overall reaction rates in fractured carbonate reservoirs.

Pore-scale controls on calcite dissolution rates from flow-through laboratory and numerical experiments.

A combination of experimental, imaging, and modeling techniques were applied to investigate the pore-scale transport and surface reaction controls on calcite dissolution under elevated pCO2 conditions. The laboratory experiment consisted of the injection of a solution at 4 bar pCO2 into a capillary tube packed with crushed calcite. A high resolution pore-scale numerical model was used to simulate the experiment based on a computational domain consisting of reactive calcite, pore space, and the capillary wall constructed from volumetric X-ray microtomography images. Simulated pore-scale effluent concentrations were higher than those measured by a factor of 1.8, with the largest component of the discrepancy related to uncertainties in the reaction rate model and its parameters. However, part of the discrepancy was apparently due to mass transport limitations to reactive surfaces, which were most pronounced near the inlet where larger diffusive boundary layers formed around grains and in slow-flowing pore spaces that exchanged mass by diffusion with fast flow paths. Although minor, the difference between pore- and continuum-scale results due to transport controls was discernible with the highly accurate methods employed and is expected to be more significant where heterogeneity is greater, as in natural subsurface materials.