Reactive Transport Simulations Research Articles

Effect of X‐Ray Computed Tomography Imaging Parameters on Quantification of Petrophysical Properties

AbstractThree‐dimensional (3D) X‐ray computed tomography (X‐ray CT) imaging has emerged as a nondestructive means of microstructural characterization. However, obtaining and processing high‐quality and high‐resolution images is time‐consuming and often requires high‐performance computing, particularly with a high number of projections. This work evaluates the effect of 3D X‐ray CT imaging parameters on pore connectivity and surface area quantification in sandstone samples of varied composition. Samples from Bentheimer and Torrey Buff formations are imaged via 3D X‐ray CT imaging at resolutions ranging from 1.25 to 15 μm, bin sizes 1, 2, and 4, and number of projections from 400 to 4,500. Collected images are processed and analyzed using ImageJ and MATLAB to discern petrophysical properties and the results are compared with each other and Mercury Intrusion Capillary Pressure (MICP) results. Overall, little variation in bulk porosity with changing scanning parameters is observed. However, for low resolution and projection numbers, connected porosity is lower compared to bulk porosity due to a failure to capture microfeatures. Overall, mineral surface area is observed to decrease with increasing bin size, voxel size, and projection numbers, except an observed increase with projection numbers for Torrey Buff. The Torrey Buff samples contains comparatively more clays and even the highest resolution (1.25 μm) fails to separate the micrograins, which is reflected in the pore size distribution. Identifying these variations are helpful as discrepancies in imaged pore connectivity and surface area can largely impact assessments of fluid flow and transport in reactive transport simulations informed by this data.

A coupled flow and transport model for simulation of multi-species reactive transport in unconfined aquifer using meshless local Petrov Galerkin (MLPG) method.

An understanding of natural degradation of multiple reactive contaminants in the aquifers is essential before designing the monitoring or remediation programs for polluted aquifers. Since such reactive contaminants are ubiquitous, a number of research works has been performed in the past three decades for the modelling of multi-species reactive transport (MSRT) phenomenon. The widely used finite difference method (FDM) and finite element method (FEM)-based models suffer a drawback of relying on a grid/mesh, which makes the solution unstable. Addressing such difficulties, the latest research on the MSRT models is directed towards the meshless methods. In this study, the meshless local Petrov Galerkin (MLPG) method-based multi-species reactive transport model (MLPG-MSRT) is presented, with an objective to create a robust simulation tool for the prediction of fate of multiple contaminants of the first-order reaction network. The developed model is validated for reversible as well as irreversible reaction networks with the available analytical solutions. Also, the MLPG model for unconfined aquifer flow (UF) is developed, validated, and coupled with the MLPG-MSRT model. The MLPG-UF-MSRT model results are further compared with the established FDM-based MODFLOW-RT3D model solutions for a rectangular and a real field type study. The results showed that the proposed model can simulate MSRT as accurately as the FDM-based models with an additional advantage of simplicity and stability, and thus, is more efficient for complex field problems.

Machine learning model predicting hydrothermal dolomitisation for future coupling of basin modelling and geochemical simulations

In the energy transition context, basin modelling, initially dedicated to the understanding of oil and gas systems, can address several challenges that the sustainable energy sector faces, such as identifying large storage volumes for carbon sequestration, determining the thermal recharge sources for geothermal exploration, or characterising the generation, migration, sink, and accumulation of critical element resources. In this regard, there is a need for spatiotemporal information on the mineral reactions that control the petrophysical properties of the rocks in order to map the best locations in the basin for capturing geothermal energy or storing carbon in basin-wide reservoirs for CO2 mitigation. Hydrothermal dolomitisation is one of these well-known diagenetic reactions that can enhance or destroy reservoir porosity. It has been extensively characterised by static reactive transport modelling, but not kinematic ones, due to the high computational cost of such reactive transport simulations. To address this issue, we built a machine-learning-based (ML) surrogate geochemical model for hydrothermal dolomitisation valid over a broad range of physical and chemical conditions encountered in such a context with the end goal of developing a generic exploration tool to be deployed in the basin calculator ArcTem for emerging energy needs. We train and test five regression algorithms (Random Forest, Gradient Boosting, AdaBoost, Support-Vector Machine, and Multilayer Perceptron) upon a synthetic dataset to predict the dissolved/precipitated amount of calcite and dolomite (referred to as δCaCO3 and δCaMg(CO3)2, respectively). The dataset contains 370 × 103 samples and six features (T, pH, NaCl, Ca, Mg, C) generated by a Latin Hypercube Sampling (LHS) between two end-members: (1) the modern seawater composition and a temperature of 80 °C for the lower bound and (2) a highly saline mantle-derived fluid and a temperature of 250 °C for the upper bound. The target values, i.e. dissolved/precipitated mole number of calcite/dolomite, are obtained by performing thermodynamic equilibrium calculations with the LHS samples as the initial unequilibrated solutions. Our results show that the artificial neural network (ANN) model best performs among the algorithms with rmse of 0.007 mol on δCaMg(CO3)2 prediction and 0.0084 mol on δCaCO3 prediction, R2 values above 0.99, and APE smaller than 0.3%, indicating its good generalisability to provide accurate predictions on unseen data in the studied range. Further, the ANN model is ∼1000 times faster than the geochemical calculator when solving the same system at once, illustrating the great potential of ML methods in emulating computationally intensive components of the numerical simulations.

Relevance of diffuse-layer, Stern-layer and interlayers for diffusion in clays: A new model and its application to Na, Sr, and Cs data in bentonite

The diffusive flux of cations is enhanced and that of anions is decreased compared to that of water tracers in bentonite or other clays, as a result of electrostatic interactions between ions and the charged clay surfaces. Clays are often used or foreseen as barriers in waste disposal in order to protect the environment from hazardous materials. A consistent description of diffusion of various ions and uncharged species is important in this context, especially if long-term interactions between clays and other materials shall be predicted by reactive transport simulations. Here, diffusion of a suite of tracers (HTO, Cl, Na, Sr and Cs) in bentonite was investigated. Experimental through-diffusion data at different bentonite dry densities were described with models of increasing complexity. First, ‘standard’ empirical single ion transport models (uncoupled, simple Fickian diffusion) were applied for each density. These models served as reference cases for comparisons. For sorbing tracers (Na, Sr, Cs), surface diffusion models were used in a next step, where average surface mobilities for the cations were determined based on comparisons with transport parameters from HTO diffusion. Finally, a more complex model was developed in order to describe anion and cation diffusion in a coupled way. This model accounts for locally parallel diffusion in different environments, namely in ‘free’ water unaffected by surface charges, in diffuse (Donnan) layer water, within the Stern layer, and within interlayer water (D-S-I model). This coupled model requires additional parameters related to the bentonite microstructure as well as to cation mobilities in the Stern layer and in the interlayer. The latter were taken from literature. Microstructural parameters were constrained in a manner that overall anion exclusion matches anion accessible porosities found by the simple Fickian diffusion model. This was possible with a reasonable choice of microstructure parameters that are consistent with literature values. A good agreement between the experimental data and the simulated cation diffusion coefficients of the D-S-I model was found, using constraints by the HTO and Cl diffusion data. The interlayer pathway was found to be most important for diffusion of Na, Sr and Cs through bentonite. Stern layer diffusion was significant and more important than diffusion through the diffuse layer for Cs and Sr, while for Na the two pathways were of equal importance.

Hydrological and Geological Controls for the Depth Distribution of Dissolved Oxygen and Iron in Silicate Catchments

AbstractDissolved Oxygen (DO) plays a key role in reactive processes and microbial dynamics in the critical zone. Recent observations showed that fractures can provide rapid pathways for oxygen penetration in aquifers, triggering unexpected biogeochemical processes. In the shallow subsurface, DO reacts with electron donors, such as Fe2+ coming from mineral dissolution. Yet, little is known about the factors controlling the spatial heterogeneity and distribution of oxygen with depth. Here we present a reduced analytical model describing the coupled evolution of DO and Fe2+ as a function of fluid travel time in silicate catchments. Our model, validated from fully resolved reactive transport simulations, predicts a linear decay of DO with time, followed by a rapid non‐linear increase of Fe2+ concentrations up to a far‐from‐equilibrium steady‐state. The relative effects of geological and hydrological forcings are quantified through a Damköhler number (Da) and a lithological number (Λ). We use this framework to investigate the depth distribution of DO and Fe2+ in two catchments with similar environmental contexts but contrasted hydrochemical properties. We show that hydrochemical differences are explained by small variations in Da but orders of magnitude variations in Λ. Therefore, we demonstrate that the hydrological and geological drivers controlling hydrochemistry in silicate catchments can be discriminated by analyzing jointly the O2 and Fe2+ evolution with depth. These findings provide a new conceptual framework to understand and predict the evolution of DO in modern groundwater, which plays an important role in critical zone processes.

CO2 Leakage Scenarios in Shale Overburden

Potential CO2 leakage from deep geologic reservoirs requires evaluation on a site-specific basis to assess risk and arrange mitigation strategies. In this study, a heterogeneous and realistic numerical model was developed to investigate CO2 migration pathways and uprising time in a shaly overburden, located in the Malaysian off-shore. Fluid flow and reactive transport simulations were performed by TOUGHREACT to evaluate the: (1) seepage through the caprock; (2) CO2-rich brine leakage through a fault connecting the reservoir with seabed. The effect of several factors, which may contribute to CO2 migration, including different rock types and permeability, Fickian and Knudsen diffusion and CO2 adsorption in the shales were investigated. Obtained results show that permeability mainly ruled CO2 uprising velocity and pathways. CO2 migrates upward by buoyancy without any important lateral leakages due to poor-connection of permeable layers and comparable values of vertical and horizontal permeability. Diffusive flux and the Knudsen flow are negligible with respect to the Darcy regime, despite the presence of shales. Main geochemical reactions deal with carbonate and pyrite weathering which easily reach saturation due to low permeability and allowing for re-precipitation as secondary phases. CO2 adsorption on shales together with dissolved CO2 constituted the main trapping mechanisms, although the former represents likely an overestimation due to estimated thermodynamic parameters. Developed models for both scenarios are validated by the good agreement with the pressure profiles recorded in the exploration wells and the seismic data along a fault (the F05 fault), suggesting that they can accurately reproduce the main processes occurring in the system.

Impact of aquifer properties on the extent and timeline of CO2 trapping

AbstractGeologic CO2 sequestration in porous saline aquifers is a promising approach to reducing atmospheric concentrations of CO2. Reactive transport simulations provide the opportunity to analyze which factors influence geochemical reactivity in the reservoir, understand those most important for promoting CO2 trapping, and assess individual sites. Field‐scale aquifer characterization is time and resource intensive such that here, reactive transport simulations are leveraged to enhance understanding of selected aquifer properties including porosity, permeability, depth of storage, and carbonate mineralogy on the overall CO2 trapping potential to better select sites promoting geochemical reactivity for CO2 trapping. There are different mechanisms for sequestrating CO2. Once injected, CO2 will dissolve into the brine to create an acidic environment, resulting in the dissolution of pre‐injection formation minerals. Released ions can reprecipitate as secondary minerals. The dissolved CO2 and mineralized CO2 are considered as a more secure form of CO2 trapping in this study compared to the free supercritical CO2. Here, a framework leveraging a controlled set of field scale simulations is developed to facilitate rapid, optimized site selection. Simulations vary aquifer properties to understand the impact of each unique property on CO2 trapping, tracking, and comparing the amount of supercritical, aqueous, and mineralized CO2. The rate at which the CO2 injected into the aquifer is converted to aqueous or mineralized CO2 is newly defined here as the sequestration efficiency and used to compare simulation results. The reservoir depth and fraction of carbonate minerals in the formation are shown to be more important factors than reservoir porosity and permeability in affecting CO2 trapping. However, the impact of aquifer properties on the evolution of injected CO2 depends on the stage of the sequestration project. © 2023 Society of Chemical Industry and John Wiley & Sons, Ltd.

Evaluation of rock-fluid interaction in a carbonate reservoir: Backflow test and reactive transport simulations

Water injection is one of the most used secondary oil recovery methods. However, when injected into a carbonate reservoir, it disturbs the equilibrium between the porous rock and the formation water, altering the geochemical system. The aim of this work is to evaluate the rock-fluid chemical interaction associated with seawater injection into a carbonate reservoir, based on field flow tests and flow simulations using COORES™ and ARXIM. Though previous studies were carried out under laboratory conditions, in this work, for the first time, rock-fluid interaction was observed and quantified during a field test in a high-temperature and high-pressure reservoir. Chemical analysis of produced water during the flow test showed that calcium and alkalis concentrations were higher than those calculated by the simple mixture of injected and formation waters, confirming the existence of chemical reactions under reservoir conditions. Reactive transport simulations were used to determine kinetic parameters related to calcite dissolution and dolomite precipitation in the reservoir. The results of this study enhance the understating of chemical interactions between injected fluids and carbonate reservoirs and provide data for more reliable simulations. These could lead to more accurate predictions regarding produced water composition and pH, anticipating the likelihood of reservoir collapse and scaling issues.

Numerical evaluation of photosensitive tracers as a strategy for separating surface and subsurface transient storage in streams

We numerically evaluated photosensitive tracers as a potential strategy for separating the effects of surface and hyporheic storage zones (SSZs and HSZs, respectively) on stream corridor transport. Correctly separating HSZ and SSZ effects is critical to estimating the hydro-biogeochemical function of a stream because HSZs and SSZs expose solutes to significantly different biogeochemical conditions, like sunlight exposure, microbial processes, and oxygen availability. Our numerical experiments used a multiscale river-corridor transport model implemented in the ATS code, which accommodates multiple storage zones with distinct travel time distributions and biogeochemical reactions. For parameter inferences, we used Bayesian inverse modeling. We found that breakthrough curves for photo-decaying tracers from day and night injection can delineate surface and hyporheic transient storage contributions, but only when interpreted jointly through a two-storage zone model. Numerical experiments that used only daytime injection or interpreted breakthrough curves with a single storage zone model yielded good fit to breakthrough curves, but parameter estimates were biased and controlling processes misattributed, examples of good model fits for the wrong reasons. Using those biased parameter estimates in reactive transport simulations resulted in significantly different projections of denitrification, which underscores the potential for stream function to be mischaracterized if tracer tests are interpreted through an inappropriately simplified model for transient storage. More generally, this study highlights the role of modeling in evaluating the experimental design and identifying the potential of system mischaracterization and its implications.

Simulation of Reactive Transport in Fractured Porous Media

Numerical simulations of reactive transport in fractured porous media require the solution of coupled physical and chemical processes that depend on the fractures. Such coupled processes are described by a system of nonlinear partial differential-algebraic equations, while strong heterogeneities characterise fractures. This paper presents an approach to simulate single-phase flow and non-isothermal reactive transport with mineral dissolution and precipitation in fractured porous media. Our numerical solution strategy is based on two ingredients. First, the model equations consist of coupled partial differential equations for the fluid flow, heat transfer and solute transport and nonlinear algebraic equations representing the chemical reactions. Second, fractures are explicitly represented and treated as lower-dimensional objects. The partial differential equations are discretised using finite-volume methods, and at each time step, we solve a nonlinear system of equations using Newton’s method. With numerical simulations, we illustrate our model’s ability to accurately describe the two-way interaction between coupled multi-physical processes and two- and three-dimensional porous media with intersecting fractures.

A Compositional Global Implicit Approach for Modeling Coupled Multicomponent Reactive Transport

AbstractReactive transport modeling has become widely used to help improve understanding of hydrogeochemical processes from the pore scale to the watershed scale. In recent years, the scope of reactive transport applications has increased toward a higher level of complexity and process coupling. For example, the production and consumption of water as well as porosity evolution associated with the dissolution and precipitation of hydrated minerals can impact system evolution. Waste rock weathering, carbon sequestration, or the degradation of engineered barriers in radioactive waste repositories all constitute applications in which geochemistry and hydrodynamics can strongly influence each other. For these purposes, the traditional formulation of reactive transport simulators, which decouples groundwater flow and reactive transport processes, is limited. We present a global implicit compositional approach, which integrates the flow processes directly into the reactive transport and geochemical framework. This approach solves the flow field implicitly with the reactive transport equations, simultaneously accounting for water consumption and production due to geochemical reactions. Applications show that the model allows tackling complex reactive transport problems while accounting for intra‐aqueous reactions, redox reactions, and reactions involving mass transfer with the gas and solid phases. The presented simulations also demonstrate that the compositional and traditional approaches yield similar results for complex geochemical systems with relatively low reactivity.

Reactive transport modelling under supercritical conditions

The utilisation of supercritical geothermal fluids depends on the assessment of the long-term effects of fluid-rock interaction on flow and transport behaviour. Reactive transport simulations are used for these assessments quantitatively based on high-quality thermodynamic and chemical data, understanding of the thermochemical constraints, and capability for accurate, stable, and rapid simulations. This work explores the precipitation/dissolution behaviour of basalt in water under supercritical conditions (400 °C and 502 bar) based on a comprehensive laboratory study conducted by Passarella (2021).The mineral reactions in the experiment were found to be largely kinetically controlled based on the model results. The final model dissolution rate constants were generally within an order of magnitude of the values calculated using the Arrhenius Law at 400 °C, based on published k25 and Ea values, paired with the reactive surface area estimates following the work of White and Peterson (1990). This indicates that the reaction rates at the experimental supercritical conditions are consistent with those observed at subcritical conditions for each mineral. It is theorised that this is because the properties of supercritical water at high pressures are comparable to subcritical water, specifically in terms of viscosity and the dielectric constant. A similar review of the final model precipitation rate constants showed better agreement with neutral dissolution rates extended to precipitation than the estimates following Horiuti (1957) for the experimental conditions.Mineral alterations during the experiment depended primarily on glass dissolution based on the model. Fluid saturation relative to the other minerals increased from the release of glass dissolution products, affecting their dissolution rates. Glass dissolution was also the primary source of components for secondary mineral formation, including celadonite, which also depended on olivine dissolution to provide magnesium. Upon glass depletion, the dissolution rates of the remaining primary minerals increased, and celadonite dissolved. Chlorite was deposited later in the modelled experiment than the other secondary minerals.

Improving the spatial control of soil biocementation using indigenous microorganisms: Column experiments and reactive transport modeling

Microbially-Induced Calcite Precipitation (MICP), or biocementation, is a biomediated soil improvement process that uses microbial ureolytic activity to enable the precipitation of CaCO3 on soil particle surfaces and contacts. As the technology advances towards practical adoption, approaches that can modulate the spatial uniformity of biocementation and maximize treatment extent will be critical towards reducing implementation costs and impacts. In this study, treatment strategies capable of enriching indigenous ureolytic microorganisms at controlled ureolytic activities and the effects of these techniques on resulting spatial distributions of MICP were explored using centimeter- and meter-scale soil column experiments and reactive transport simulations. In column experiments, differences in treatment solution supplied yeast extract concentrations were shown to reliably control the enrichment of indigenous ureolytic microorganisms and achieve a wide range of ureolytic activities. Reactive transport simulations further demonstrated that reductions in stimulated ureolytic rates could enable more uniform improvement and increases in the extent of improvement with reduced sensitivity to changes in solution injection velocities. A reaction-to-injection duration ratio (RTIDR) parameter was proposed and captured the collective impacts of changes in injection rates and ureolytic reaction rates on reactive transport conditions thereby unifying outcomes from both simulations and experiments. A nonuniformity area (NA) parameter was also introduced to quantitatively characterize differences in biocementation uniformity independent of length scale and CaCO3 magnitudes. Results from this study collectively demonstrate the utility of changes in stimulated ureolytic activities towards controlling the spatial uniformity and extent of biocementation over meter-scale distances.

Improvement on Coupling Technique Between COMSOL and PHREEQC for the Reactive Transport Simulation

Improvement on Coupling Technique Between COMSOL and PHREEQC for the Reactive Transport Simulation

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.

Single-Well Push–Pull Tracer Test Analyses to Determine Aquifer Reactive Transport Parameters at a Former Uranium Mill Site (Grand Junction, Colorado)

At a former uranium mill site where tailings have been removed, prior work has determined several potential ongoing secondary uranium sources. These include locations with uranium sorbed to organic carbon, uranium in the unsaturated zone, and uranium associated with the presence of gypsum. To better understand uranium mobility controls at the site, four single-well push–pull tests (with a drift phase) were completed with the goal of deriving aquifer flow and contaminant transport parameters for inclusion in a future sitewide reactive transport model. This goes beyond the traditional use of a constant sorption distribution coefficient (Kd) and allows for the evaluation of alternative remedial injection fluids, which can produce variable Kd values. Dispersion was first removed from the resulting data to determine possible reactions before conducting reactive transport simulations. These initial analyses indicated the potential need to include cation exchange, uranium sorption, and gypsum dissolution. A reactive transport model using multiple layers to account for partially penetrating wells was completed using the PHT-USG reactive transport modeling code and calibrated using PEST. The model results quantify the hydraulic conductivity and dispersion parameters using the injected tracer concentrations. Uranium sorption, cation exchange, and gypsum dissolution parameters were quantified by comparing the simulated versus observed geochemistry. All simulations required some cation exchange and calcite equilibrium, and one simulation required gypsum dissolution to improve the model fit for calcium and sulfate. Uranium sorption parameters were not strongly influenced by the other parameter values but were highly influenced by uranium concentrations during the drift phase, with possible kinetic rate limitations. Thus, a future recommendation for such push–pull tests is to collect more geochemical data during the drift phase. The final uranium sorption parameters were within the range of values determined from prior column testing. The flow and transport parameters derived from these single-well push–pull tests will provide initial parameters for any future sitewide reactive transport model.

Chemical impacts of subsurface CO2 and brine on shallow groundwater quality

Leakage from geologic CO2 sequestration (GCS) sites to overlying shallow drinking water aquifers is a tangible risk. A primary purpose of this study is to assess the potential impacts of CO2 leakage into a fresh-water aquifer with associated CO2-water-sediment interactions. The study site is the Ogallala aquifer overlying an active demonstration-scale GCS site in north Texas, USA. Using the results of combined batch experiments and reactive transport simulations, we discuss the effects of salinity on potential trace metal release and the potential for groundwater quality recovery after leakage ceases. Resultssuggest that trace metals are released from sediment due to impure carbonate mineral dissolution and cation exchange with exposure to aqueous CO2. Concentrations of Mn, Zn and Sr might exceed the U.S. Environmental Protection Agency's (EPA) limits. After CO2 leakage stops, most cation concentrations decrease to levels observed before leakage quickly, suggesting that water quality may not be a long-term concern. However, saline water that co-leaks with CO2 may increase salinity of a shallow aquifer and induce more trace metals release from the sediment. In most cases, pH is sensitive to even small increases of CO2, suggesting that pH may be a sufficiently sensitive parameter for detecting CO2 leakage.

Interaction between CO2-rich water and hydrated Portland cement and ultramafic rocks

CO2 geological storage in deep rock formations is a possible strategy to mitigate the atmospheric CO2 concentration. In storage sites, CO2 is injected through wells. The space between the metal casing of the wellbore and the host rock is filled with Portland cement in which micro-fractures on flow paths between cement and rock may originate. It has been amply shown that the interaction between CO2-rich water, hydrated Portland cement and sedimentary rocks leads to alteration of cement (dissolution of cementitious phases) and rock (dissolution of silicates) and precipitation of secondary minerals. These processes may enhance or seal the existing microfractures through which CO2 may leak. However, the integrity of the cement and (ultra)mafic rocks under acidic conditions has not been investigated.In this study, the geochemical processes associated with the interaction between CO2-rich water and mortar (composed by hydrated Portland cement and quartz aggregates) and two mafic rocks (peridotite and obsidian) were studied to evaluate the stability of wellbores in mafic rocks, i.e., the role of carbonation and dissolution mechanisms during long-term exposures to CO2-rich brines. Three column experiments were performed at 10 bar pressure, room temperature (20 ± 2°C) and two ionic strengths (4.0·10-4 M and 2.8·10-2 M).The chemical composition of the effluents was examined over time and reproduced by 2D reactive transport simulations. Experimental and model results show that dissolution of the cement phases contributed to cement alteration and buffered the solution pH, rendering low rock alteration. Water residence time affected the precipitation of secondary CO2-bearing minerals.

Fracture Intensity Impacts on Reaction Front Propagation and Mineral Weathering in Three‐Dimensional Fractured Media

AbstractStudying reaction front propagation in heterogeneous natural settings is challenging, but numerical simulations can provide insight into the varying spatial and temporal scales of reaction front propagation. Here, the impact of increasing fracture intensity on mineral dissolution rates, and the extent of reaction front propagation is investigated using reactive transport simulations in upscaled discrete fracture network domains with varied fracture intensity. Domain‐averaged dissolution rates vary less than 0.5 log units regardless of the fracture intensity, but the spatial distribution of reactions is controlled by the location and number of dead‐end fractures and the number of connected flowpaths through the domain. Higher fracture intensities lead to more weathering in the domain because of more available mineral for water‐rock interactions. We find that reaction fronts propagate through the primary flowpaths in the first 10,000 years of the simulation for a 10‐m length domain, then propagate into secondary flowpaths and dead‐end fractures between 10,000 and 100,000 years, and finally into the matrix over timescales of hundreds of thousands of years. The domain‐averaged reaction rates decrease through time corresponding to a transition from dissolution in advection‐dominated, fast‐flowing pathways, to dissolution in transport‐limited zones of disconnected fractures and matrix. Matrix dissolution, or dissolution under transport‐limited conditions, is the dominant process at late times in these simulations. The results of these simulations recreate the observed paradox found in nature where highly fractured hillslopes tend to be more weathered but have slower weathering rates, while hillslopes with fewer fractures, are less weathered but have higher dissolution rates.

Reconciling predicted and observed carbon mineralization in siliciclastic formations

The study of natural CO2 storage reservoirs has demonstrated that carbon mineral trapping is the safest mechanism for subsurface CO2 storage. The slow rate of fluid-rock reactions makes it imperative to use reactive transport simulations as a tool for estimating reservoir scale carbon mineral trapping capacities. However, there is often a disparity in the estimations of carbon mineral trapping capacities determined from reactive transport simulations and recorded in natural CO2 storage reservoirs. This might be due to several factors such as the uncertainty around simulations parameters such as mineral reactive surface areas as well as the neglect of sub-core scale lithological heterogeneity in reservoir scale simulations. This study points to the importance of the latter to reconcile the mismatch. Multiphase reactive transport simulations were run on a cm-scale resolution geological model (CRM) of the Paaratte Formation, Otway Basin (Australia), where mm- to cm-scale lithological heterogeneity is incorporated via upscaled rock parameters. Simulations were also run on a m-scale resolution model (MRM) of the same transect to quantify the difference in the predicted secondary mineral composition due to the incorporation of sub-core scale heterogeneity in reservoir scale simulations. The simulation outcomes for both the models were compared to the data from the Pretty Hill Formation, a natural CO2 storage analogue within the Otway Basin with similar primary mineral composition. The results from the CRM compare well with the secondary carbonate mineralogy of the Pretty Hill Formation. In contrast, the MRM strongly underestimates carbonate mineral formation. The results highlight that the incorporation of mm- to cm-scale lithological interfaces in reservoir scale reactive transport simulations can increase carbon mineralization estimates in siliciclastic formations by 3–6 orders of magnitude as these interfaces are sites for mineral precipitation.