Reactive Transport Simulations Research Articles

Speeding Up Reactive Transport Simulations in Cement Systems by Surrogate Geochemical Modeling: Deep Neural Networks and k-Nearest Neighbors

We accelerate reactive transport (RT) simulation by replacing the geochemical solver in the RT code by a surrogate model or emulator, considering either a trained deep neural network (DNN) or a k-nearest neighbor (kNN) regressor. We focus on 2D leaching of hardened cement paste under diffusive or advective-dispersive transport conditions, a solid solution representation of the calcium silicate hydrates and either 4 or 7 chemical components, and use the HPx (coupled Hydrus-PHREEQC model) reactive transport code as baseline. We find that after training, both our DNN-based and kNN-based codes, \(\hbox {HPx}_{\rm{py}}\)-DNN and \(\hbox {HPx}_{\rm{py}}\)-kNN, can make satisfactorily to very accurate predictions while providing either a 3 to 9 speedup factor compared to HPx with parallelized geochemical calculations over 4 cores. Benchmarking against single-threaded HPx, these speedup factors become 8 to 33. Overall, \(\hbox {HPx}_{\rm{py}}\)-DNN and \(\hbox {HPx}_{\rm{py}}\)-kNN are found to achieve a close to optimal speedup when DNN regression and kNN search are performed on a GPU. Importantly, for the more complex 7-components cement system, no emulator that is globally accurate over the full space of possible geochemical conditions could be devised. Instead we therefore build “local” emulators that are only valid over a relevant fraction of the input parameter space. This space is identified by running a coarse and thus computationally cheap full RT simulation, and subsequently explored by kernel density sampling. Future work will focus on improving accuracy for this type of cement systems.

Insights in the chemical fundamentals of ASR and the role of calcium in the early stage based on a 3D reactive transport model

Prediction of alkali silica reaction is still difficult due to the lack of a comprehensive understanding of its chemical fundamentals. In-site experimentally revealing the fundamentals is not realistic as ASR shows over several years or even decades and is affected by many factors. In this paper, by utilizing a 3D reactive-transport simulation model at microscale, we have numerically explored the fundamentals of ASR in the early stage under the influence of reactive silica fraction, alkali concentration, silica disorder degree and aggregate porosity. Based on the simulation results, the chemical sequences of ASR, the initial location of ASR products, the mechanism behind and the role of calcium under the influence of the above factors are elaborated. Furthermore, a comprehensive mechanism to explain the pessimum reactive aggregate content is derived. The results of this paper give some insights about ASR in the early stage such as the initial expansion locations.

Impact of carbonation on leaching of constituents from a cementitious waste form for treatment of low activity waste at the DOE Hanford site

Carbonation can be a major aging process during disposal of alkaline cementitious waste forms and can impact constituent leaching by changes in material alkalinity, pore structure, and controlling mineral phases. The effect of carbonation on the leaching of major and trace constituents from Cast Stone, a cementitious waste form developed to treat high salt content low activity waste, was studied through a combination of leaching experiments and reactive transport simulations. Diffusive transport of constituents in the waste form was evaluated using reactive transport modeling of diffusion-controlled leaching test results and a geochemical speciation model derived from pH-dependent leaching. Comparisons between Cast Stone materials aged under nitrogen, air, and 2% carbon dioxide in nitrogen showed that carbonation impacts solubility, physical retention and observed diffusivity of major and trace constituents. Carbonation under 2% CO2 decreased the diffusion-controlled leaching of chromium by two orders of magnitude. Modeling results suggest that carbonation may also decrease solubility of technetium while changes to microstructure by carbonation increases effective diffusivity of constituents in Cast Stone.

Three-dimensional reactive transport simulation of Uranium in situ recovery: Large-scale well field applications in Shu Saryssu Bassin, Tortkuduk deposit (Kazakhstan)

Uranium in situ recovery (ISR) is the most widely used uranium mining technique worldwide. It consists of the dissolution of the ore by a mining solution, directly within the deposit. By predicting fluid flow and geochemical reactions in reservoirs, reactive transport (RT) modelling is a powerful tool to better understand and pilot ISR production. However, very few industrial uses have been reported thus far. This paper fills the gaps by illustrating a large-scale well-field application of RT modelling at one of the largest ISR mines worldwide, which is operated by Katco. This study highlights the robustness of a complex workflow based on the coupled reactive transport software HYTEC and its added value for the operator in the context of uranium ISR. The robustness demonstration is performed on 2394 wells covering 39 different production areas (blocks). The model reproduces the observed uranium concentrations and pH of pumped solutions over time scales up to 12 years. Only three parameters are manually adjusted to calibrate the model: global initial grades in clays (beidellite), calcite, and iron hydroxide (goethite). The discrepancy between simulated and observed uranium production and acid consumption decreases as the observation scale widens, showing that local errors compensate for each other. These deviations are mainly explained by the uncertainties of the 3D geological models and not by the RT simulations. Furthermore, the robustness of the model is a key asset for decision-making as it enables accurate predictions. This accuracy is illustrated through a case study of four of the simulated blocks. In 2019, after 10 years of production, the well field was redesigned to target the remaining uranium using the RT-based workflow. Several scenarios were simulated and sequentially optimised using a geometallurgical approach. The final adopted design predicted a 28% increase in uranium production and 35% in economic gains over the first two years of simulation alone (2019–2021). These theoretical gains were validated in practice as the comparison between the 2019 predictions and observations over 16 months showed a deviation less than 10% in the total uranium production, which decreased to 1.9% using the observed operational conditions, which reinforces the predictability of the workflow and validates the forecasted gains.

Oxidative Dissolution of Arsenic-Bearing Sulfide Minerals in Groundwater: Impact of Hydrochemical and Hydrodynamic Conditions on Arsenic Release and Surface Evolution.

The dissolution of sulfide minerals can lead to hazardous arsenic levels in groundwater. This study investigates the oxidative dissolution of natural As-bearing sulfide minerals and the related release of arsenic under flow-through conditions. Column experiments were performed using reactive As-bearing sulfide minerals (arsenopyrite and löllingite) embedded in a sandy matrix and injecting oxic solutions into the initially anoxic porous media to trigger the mineral dissolution. Noninvasive oxygen measurements, analyses of ionic species at the outlet, and scanning electron microscopy allowed tracking the propagation of the oxidative dissolution fronts, the mineral dissolution progress, and the change in mineral surface composition. Process-based reactive transport simulations were performed to quantitatively interpret the geochemical processes. The experimental and modeling outcomes show that pore-water acidity exerts a key control on the dissolution of sulfide minerals and arsenic release since it determines the precipitation of secondary mineral phases causing the sequestration of arsenic and the passivation of the reactive mineral surfaces. The impact of surface passivation strongly depends on the flow velocity and on the spatial distribution of the reactive minerals. These results highlight the fundamental interplay of reactive mineral distribution and hydrochemical and hydrodynamic conditions on the mobilization of arsenic from sulfide minerals in flow-through systems.

Machine learning to predict effective reaction rates in 3D porous media from pore structural features

Large discrepancies between well-mixed reaction rates and effective reactions rates estimated under fluid flow conditions have been a major issue for predicting reactive transport in porous media systems. In this study, we introduce a framework that accurately predicts effective reaction rates directly from pore structural features by combining 3D pore-scale numerical simulations with machine learning (ML). We first perform pore-scale reactive transport simulations with fluid–solid reactions in hundreds of porous media and calculate effective reaction rates from pore-scale concentration fields. We then train a Random Forests model with 11 pore structural features and effective reaction rates to quantify the importance of structural features in determining effective reaction rates. Based on the importance information, we train artificial neural networks with varying number of features and demonstrate that effective reaction rates can be accurately predicted with only three pore structural features, which are specific surface, pore sphericity, and coordination number. Finally, global sensitivity analyses using the ML model elucidates how the three structural features affect effective reaction rates. The proposed framework enables accurate predictions of effective reaction rates directly from a few measurable pore structural features, and the framework is readily applicable to a wide range of applications involving porous media flows.

Effects of Hydrogeological Heterogeneity on CO2 Migration and Mineral Trapping: 3D Reactive Transport Modeling of Geological CO2 Storage in the Mt. Simon Sandstone, Indiana, USA

We used three-dimensional (3D), high-resolution simulations facilitated by parallel computation to assess the effect of hydrogeological heterogeneity in the Mt. Simon Sandstone on CO2 plume evolution and storage and geochemical reactions in a portion of the Illinois Basin, Indiana. Two scenarios were selected to investigate the effects of the hydrogeological heterogeneity in 3D reactive transport simulations: a heterogeneous case with variable porosity and permeability, and a homogenous case with constant porosity and permeability. The initial pressure, temperature, and mineralogical distributions are consistently applied in both the heterogeneous case and the homogeneous case. Results indicate that including hydrogeological heterogeneity in 3D reservoir simulations for geological CO2 storage significantly impacts modeling results for plume migration patterns, CO2-water-mineral interaction, reservoir quality, and CO2 plume containment. In particular, results indicate that (1) the CO2 plume reached the top of the Mt. Simon Sandstone in the homogeneous case, but was restrained to the lower third of the formation when hydrogeologic heterogeneity was considered; (2) the dominant trapping mechanism in the heterogeneous case was mineral trapping (43%), while it was solubility trapping (47%) in the homogeneous case (at 10,000 years); (3) incorporating reservoir heterogeneity in the model leads to a higher likelihood of long-term containment.

Petrophysical initialization of core-scale reactive transport simulations on Indiana limestones: Pore-scale characterization, spatial autocorrelations, and representative elementary volume analysis

Initializations of the petrophysical parameters significantly affect the results of reactive transport simulations on unstable dissolution fronts. As the phenomenon arises from the inherent heterogeneity of the rocks, heterogenous initialization of the petrophysical fields is needed to model such a phenomenon. While modeling of unstable dissolution fronts has been extensively studied in the past, the geostatistical information required to initialize the heterogeneous petrophysical fields is usually assumed and still missing. The objective of this paper is to acquire such information for Indiana limestones on a core-scale. This is done by combining high resolution tomography with pore scale calculations to determine: i) the rock minimum representative volume (REV), ii) the frequency distributions of the petrophysical parameters, namely; porosity, permeability, and reactive surface area, iii) the relationships between the petrophysical parameters, and iv) the rock spatial correlation model and lengths. The results obtained should help reduce the uncertainties associated with petrophysical initialization of core-scale reactive transport simulations of carbonate rocks in general, and Indiana limestones in particular.

Removal of toxic metals from petroleum produced water by dolomite filtration

We conducted experiments and reactive transport simulations to assess the removal of high concentrations of toxic metals (100 mg/L) commonly found in produced water from high saline waters (45,000–115,000 mg/L) using dolomite filters made of compressed powdered dolomite. The focus was on Ba, Sr, Cd, Pb, and As, as well as on guar gum as a metal complexing additive. We found that salinity has a significant impact on the removal of Sr and Ba but a mild impact on the removal of Cd. An increase of pH due the dissolution of dolomite increases the removal of Cd more significantly than the removal of Ba and Sr. The formation of guar-gum complexes of metals in the aqueous phase attenuates the removal of Sr, Ba, and Cd by dolomite filtration, this attenuation process is more significant for Cd than for Sr and Ba. The affinity sequence for the sorption of toxic metals at salinity levels of PW is Pb > Cd > As>Ba>Sr. Higher removal levels predicted for Sr than for Ba and over predictions of Cd removal using previously proposed surface complexation models for dolomite highlights the necessity of further experimental research to incorporate molecular scale surface crystal structure parameters into surface complexation models.

The PFLOTRAN Reaction Sandbox

Abstract. As modern reactive transport simulators evolve to accommodate the demands of a user community, researchers need a platform for prototyping new biogeochemical processes, many of which are niche and specific to laboratory or field experiments. The PFLOTRAN Reaction Sandbox leverages modern, object-oriented Fortran in an attempt to provide such an environment within an existing reactive transport simulator. This work describes the PFLOTRAN Reaction Sandbox concept and implementation through several illustrative examples. Reaction Sandbox Biodegradation Hill customizes the existing microbially mediated biodegradation reaction formulation within PFLOTRAN to better match empirical data. Reaction Sandbox Simple provides an isolated environment for testing numerous preconfigured kinetic rate expressions and developing user intuition. Reaction Sandbox Example serves as a template for creating new sandboxes within PFLOTRAN.

Accelerated reactive transport simulations in heterogeneous porous media using Reaktoro and Firedrake

This work investigates the performance of the on-demand machine learning (ODML) algorithm introduced in Leal et al. (Transp. Porous Media133(2), 161–204, 2020) when applied to different reactive transport problems in heterogeneous porous media. This approach was devised to accelerate the computationally expensive geochemical reaction calculations in reactive transport simulations. We demonstrate that even with a strong heterogeneity present, the ODML algorithm speeds up these calculations by one to three orders of magnitude. Such acceleration, in turn, significantly advances the entire reactive transport simulation. The performed numerical experiments are enabled by the novel coupling of two open-source software packages: Reaktoro (Leal 2015) and Firedrake (Rathgeber et al. ACM Trans. Math. Softw.43(3), 2016). The first library provides the most recent version of the ODML approach for the chemical equilibrium calculations, whereas, the second framework includes the newly implemented conservative Discontinuous Galerkin finite element scheme for the Darcy problem, i.e., the Stabilized Dual Hybrid Mixed(SDHM) method Núñez et al. (Int. J. Model. Simul. Petroleum Industry, 6, 2012).

Dimensionality effects on multicomponent ionic transport and surface complexation in porous media

Coulombic interactions between charged species in pore water and at surface/solution interfaces are of pivotal importance for multicomponent reactive transport in porous media. In this study, we investigate the impact of domain dimensionality on electrostatically coupled dispersion and surface-solution reactions during transport of acidic plumes and major ions in porous media.Column and quasi two-dimensional flow-through experiments were performed, with identical silica porous media and under the same advection-dominated conditions. Equal mass fluxes of different electrolyte solutions (i.e., HCl - pH ∼ 2.8, NaBr - 100 mM, HCl - pH ∼ 2.8 plus NaBr - 100 mM) were continuously injected in the 1-D and 2-D experiments and breakthrough curves of pH and major ions were measured at the outlet of the domains. The presence of pronounced ionic strength gradients in the transverse direction in the 2-D setup caused distinct retardation and transport behaviors of protons and major ions which were not observed in the one-dimensional column experiments. Furthermore, in the cases of salt electrolytes injection, considerably enhanced release of H+ (>61%) from the quartz surface was observed in the multidimensional system compared to the one-dimensional setup.Reactive transport modeling was performed to reproduce the experimental outcomes and to analyse the coupling between transport processes, based on the Nernst-Planck formulation of diffusive/dispersive fluxes and on surface complexation reactions at the solid-solution interface. Electrostatic interactions between Na+, Br−, and H+, and deprotonation of the quartz surface upon the formation of sodium outer-sphere complexes, are the primary controllers of the spatial and temporal features displayed by the pH and major ions measurements. The reactive transport simulations allowed us to interpret the experimental observations, to visualize the distribution and spatio-temporal evolution of dissolved and solid species, to identify a spatially heterogeneous zonation of Coulombic interactions with distinct behavior at the fringe and core of the injected plumes in the multidimensional setup, and to quantify the different components of the Nerst-Planck fluxes of the charged solutes. This study demonstrates that the domain dimensionality directly affects electrostatic interactions between charged aqueous species in the pore water and surface complexation reactions at the solid-solution interface. The non-trivial effects of dimensionality on multicomponent ionic transport result in a significantly different behavior in 1-D and 2-D systems.

POET (v0.1): speedup of many-core parallel reactive transport simulations with fast DHT lookups

Abstract. Coupled reactive transport simulations are extremely demanding in terms of required computational power, which hampers their application and leads to coarsened and oversimplified domains. The chemical sub-process represents the major bottleneck: its acceleration is an urgent challenge which gathers increasing interdisciplinary interest along with pressing requirements for subsurface utilization such as spent nuclear fuel storage, geothermal energy and CO2 storage. In this context we developed POET (POtsdam rEactive Transport), a research parallel reactive transport simulator integrating algorithmic improvements which decisively speed up coupled simulations. In particular, POET is designed with a master/worker architecture, which ensures computational efficiency in both multicore and cluster compute environments. POET does not rely on contiguous grid partitions for the parallelization of chemistry but forms work packages composed of grid cells distant from each other. Such scattering prevents particularly expensive geochemical simulations, usually concentrated in the vicinity of a reactive front, from generating load imbalance between the available CPUs (central processing units), as is often the case with classical partitions. Furthermore, POET leverages an original implementation of the distributed hash table (DHT) mechanism to cache the results of geochemical simulations for further reuse in subsequent time steps during the coupled simulation. The caching is hence particularly advantageous for initially chemically homogeneous simulations and for smooth reaction fronts. We tune the rounding employed in the DHT on a 2D benchmark to validate the caching approach, and we evaluate the performance gain of POET's master/worker architecture and the DHT speedup on a 3D benchmark comprising around 650 000 grid elements. The runtime for 200 coupling iterations, corresponding to 960 simulation days, reduced from about 24 h on 11 workers to 29 min on 719 workers. Activating the DHT reduces the runtime further to 2 h and 8 min respectively. Only with these kinds of reduced hardware requirements and computational costs is it possible to realistically perform the long-term complex reactive transport simulations, as well as perform the uncertainty analyses required by pressing societal challenges connected with subsurface utilization.

An interdisciplinary view of the long-term evolution of repository systems across scales: the iCROSS project

Abstract. The interdisciplinary project “Integrity of nuclear waste repository systems – Cross-scale system understanding and analysis (iCROSS)” combines research competencies of Helmholtz scientists related to the topics of nuclear, geosciences, biosciences and environmental simulations in collaborations overarching the research fields energy and earth and environment. The focus is to understand and analyze close-to-real long-term evolutionary pathways of radioactive waste repositories across nanoscales to repository scales. The project is subdivided into work packages dealing with laboratory studies, field experiments in underground research laboratories (URLs), advanced modelling studies and the integration and alignment of data and information using virtual reality methods. In this sense, the project structure aims at a holistic view on relevant processes across scales in order to comprehensively simulate potential repository evolutions. Within the multi-barrier system of a repository for heat-generating radioactive waste, a number of complex reactions proceed, including dissolution, redox processes, biochemical reactions, gas evolution and solid/liquid interface and (co)precipitation reactions. At the same time, thermal and external mechanical stress has an impact on the conditions in a deep geological repository. All those processes are highly coupled, with multiple interdependencies on various scales and have a strong impact on radionuclide mobility and retention. In recent years, substantial progress was achieved in describing coupled thermal-hydro-mechanical-chemical-biological (THM/CB) processes in numerical simulations. A realistic and concise description of these coupled processes on different time and spatial scales is, at present, a largely unresolved scientific and computational challenge. The close interaction of experimental and simulation teams aims at a more accurate quantification and assessment of processes and thus, the reduction of uncertainties and of conservative assumptions and eventually to a close-to-real perception of the repository evolution. One focus of iCROSS is directed to relevant processes in a clay rock repository. In this context, the iCROSS team became a full member of the international Mont Terri consortium and worked in close collaboration with international and German institutions in URL projects. Respective experiments specifically deal with coupled processes at the reactive interfaces in a repository near field (e.g. the steel/bentonite and bentonite/concrete interfaces). Within iCROSS, the impact of secondary phase formation on radionuclide transport is investigated. At Mont Terri, experiments are in preparation to study radionuclide transport phenomena in clay rock formations within temperature gradients and in facies exhibiting significant heterogeneities on different scales (nm to cm). Beside those studies, high resolution exploration methods for rock characterization are developed and tested and the effect of temperature and other boundary conditions on the strength, creep properties and healing of faults within Opalinus clay are quantified. Multiphysics models coupled to reactive transport simulation have been further developed and applied to laboratory and field experiments. Results are digitally analyzed and illustrated in a visualization center, in order to enhance the comprehension of coupled processes in repository systems across scales. The present contribution provides an overview on the project and reports selected results. The impact of considering complex coupled processes in repository subsystems for the assessment of the integrity of a given (generic) repository arrangement is discussed.

Upscaling Mixing-Controlled Reactions in Unsaturated Porous Media

We study mixing-controlled chemical reactions in unsaturated porous media from a pore-scale perspective. The spatial heterogeneity induced by the presence of two immiscible phases, here water and air, in the pore space generates complex flow patterns that dominate reactive mixing across scales. To assess the impact of different macroscopic saturation states (the fraction of pore volume occupied by water) on mixing-controlled chemical reactions, we consider a fast irreversible reaction between two initially segregated dissolved species that mix as one solution displaces the other in the heterogeneous flow field of the water phase. We use the pore-scale geometry and water distributions from the laboratory experiments reported by Jiménez-Martínez et al. (Geophys. Res. Lett. 42: 5316–5324, 2015). We analyze reactive mixing in three complementary ways. Firstly, we post-process experimentally observed spatially distributed concentration data; secondly, we perform numerical simulations of flow and reactive transport in the heterogeneous water phase, and thirdly, we use an upscaled mixing model. The first approach relies on an exact algebraic map between conservative and reactive species for an instantaneous irreversible bimolecular reaction that allows to estimate reactive mixing based on experimental conservative transport data. The second approach is based on reactive random walk particle tracking simulations in the numerically determined flow field in the water phase. The third approach uses a dispersive lamella approach that accounts for the impact of flow heterogeneity on mixing in terms of effective dispersion coefficients, which are estimated from both experimental data and numerical random walk particle tracking simulations. We observe a significant increase in reactive mixing for decreasing saturation, which is caused by the stronger heterogeneity of the water phase and thus of the flow field. This is consistently observed in the experimental data and the direct numerical simulations. The dispersive lamella model, parameterized by the effective interface width, provides robust estimates of the evolution of the product mass obtained from the experimental and numerical data.

Aerobic respiration in riparian exchange zones of regulated river corridors

AbstractRiver stage fluctuations drive surface water‐groundwater exchanges within river corridors. This study evaluates how repeated daily stage fluctuations, representative of hydropeaking conditions, influence aerobic respiration of river‐sourced dissolved organic carbon (DOC) in the riparian exchange zone using reactive flow and transport simulations. Over 50 hypothetical scenarios were modelled to evaluate how the duration of the daily flood signal, river DOC concentration, aquifer hydraulic conductivity and ambient groundwater flow condition affect the fate and transport of DOC and DO in the riparian aquifer. Time series subsurface snapshots highlight how the various factors influence the subsurface distribution of DOC and DO. The total mass of DOC respired per meter of river had a wide range depending on the parameters, spanning from 1.4 to 71 g over 24‐h, with high hydraulic conductivity and losing ambient groundwater flow conditions favouring the largest amount of DOC respired. The ratio of DOC mass entering the riparian zone with the mass returning to the river showed that as little as 5% to as much as 76% of the DOC that enters the bank during stage fluctuations returns to the river. This return ratio is dependent on river DOC concentration, hydraulic conductivity and ambient groundwater flow. The results illustrate that stage variations due to river regulation can be a significant control on aerobic respiration in riparian exchange zones.

Numerical simulation of reactive particle transport in a randomly-orientated rough fracture with reversible and irreversible surface attachments

A novel and robust methodology has been developed to simulate reactive, dense, and polydisperse particle transport behavior in a randomly-orientated rough fracture by applying particle tracking algorithms and the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. More specifically, particle attachment onto an aperture surface is described by both reversible and irreversible adsorptions, which are respectively controlled by the energy barrier and the dominant forces at the secondary minimum on the DLVO interaction profiles. After the simulation model is validated, a number of simulation scenarios are conducted to examine the effect of dominant factors on the particle transport behavior subject to aperture surface attachment. It is found from the simulated results that a large spreading of particle plume is induced with the presence of particle gravity settling and surface attachment. Particle attachment is positively affected by particle size, fracture heterogeneity (i.e., σln(2b)2) and particle density (i.e., ρp) but negatively affected by hydraulic gradient (i.e., ∂h/∂x), fracture inclination (i.e., θ), and the ratio (i.e., λx/λy) of the x-directional to the y-directional correlation length scales of an aperture field. The sensitivity of particle attachment follows a strong-to-weak order for the non-DLVO parameters as ∂h/∂x > θ ≈ ρp > σln(2b)2 > λx/λy. Inconsistencies are also found between mass and number attachments for the associated quantities as well as sensitivities to the dominant factors. Among the non-DLVO factors, reversible attachment is more sensitively affected by ∂h/∂x, σln(2b)2, and λx/λy as they are capable of influencing the velocity field. An increased Hamaker constant (i.e., A123), particle surface potential (i.e., Ψp), and ionic strength (i.e., Is) respectively enhances, reduces, and enhances the overall particle attachment, while the irreversible attachment can be mildly mitigated by the increased A123 and Is when a deep secondary minimum appears on the DLVO profile. This study technically improves the previous work by jointly incorporating fracture orientation, particle gravity settling, and the reversible/irreversible surface attachment into the original particle transport models through a scientific manner. Also, this work further reduces the deviation between the existing theoretical modeling and the real particle transport/filtration behavior in fractured media, while the associated findings in this study provide better understanding and significance on this research topic.

Water‐Mineral Reactions in a Translated Single Realistic Fracture: Consequences for Contaminant Uptake by Matrix Diffusion

AbstractWe use a realistic fracture along with pore‐scale reactive transport simulations to investigate the impact of fracture filling minerals and water‐mineral reactions on in‐plane groundwater flow patterns and on contaminant uptake by matrix diffusion. The results of the simulations show that sparsely distributed fracture filling calcite patches provide a significant buffering capacity against the infiltration of acidic water. The advance of the geochemical perturbation is characterized by channeling, which is partly smeared out when low Peclet numbers are used. Mineral patches have a significant impact on groundwater flow patterns as shown by the computed distributions of groundwater travel time, which are characterized by long tails. The ability of the fracture to retain contaminants by uptake due to matrix diffusion is investigated by means of two metrics: the Eulerian quantity denoted as F‐factor and the Lagrangian variable known as transport resistance. F‐factor is shown to provide an average single estimate of the bulk retention capacity of the fracture whereas transport resistance is characterized by a distribution, whose very long tail is related to streamlines forced to travel through very narrow throats. A reduction of up to 70% in the mean travel time and 85% in the mean transport resistance is observed as a consequence of geochemical dissolution reactions, which also lead to an overall reduction of their variability.

Numerical Modeling of CO2 Sequestration within a Five-Spot Well Pattern in the Morrow B Sandstone of the Farnsworth Hydrocarbon Field: Comparison of the TOUGHREACT, STOMP-EOR, and GEM Simulators

The objectives of this study were (1) to assess the fate and impact of CO2 injected into the Morrow B Sandstone in the Farnsworth Unit (FWU) through numerical non-isothermal reactive transport modeling, and (2) to compare the performance of three major reactive solute transport simulators, TOUGHREACT, STOMP-EOR, and GEM, under the same input conditions. The models were based on a quarter of a five-spot well pattern where CO2 was injected on a water-alternating-gas schedule for the first 25 years of the 1000 year simulation. The reservoir pore fluid consisted of water with or without petroleum. The results of the models have numerous broad similarities, such as the pattern of reservoir cooling caused by the injected fluids, a large initial pH drop followed by gradual pH neutralization, the long-term persistence of an immiscible CO2 gas phase, the continuous dissolution of calcite, very small decreases in porosity, and the increasing importance over time of carbonate mineral CO2 sequestration. The models differed in their predicted fluid pressure evolutions; amounts of mineral precipitation and dissolution; and distribution of CO2 among immiscible gas, petroleum, formation water, and carbonate minerals. The results of the study show the usefulness of numerical simulations in identifying broad patterns of behavior associated with CO2 injection, but also point to significant uncertainties in the numerical values of many model output parameters.

Influence of Storage Period on the Geochemical Evolution of a Compressed Energy Storage System

Subsurface porous aquifers are being considered for use as reservoirs for compressed energy storage of renewable energy. In these systems, a gas is injected during times in which production exceeds demand and extracted for energy generation during periods of peak demand or scarcity in production. Current operational subsurface energy facilities use salt caverns for storage and air as the working gas. CO2 is potentially a more favorable choice of working gas where under storage conditions CO2 has high compressibility which can improve operational efficiency. However, the interaction of CO2 and brine at the boundary of the storage zone can produce a chemically active fluid which can result in mineral dissolution and precipitation reactions and alter the properties of the storage zone. This study seeks to understand the geochemical implications of utilization of CO2 as a working gas during injection, storage and extraction flow cycles. Here, reactive transport simulations are developed based on 7 h of injection, 11 h of withdrawal and 6 h of reservoir closure, corresponding to the schedule of the Pittsfield field test, for 15 years of operational life span to assess the geochemical evolution of the reservoir. The evolution in the storage system is compared to a continuously cyclic system of 12 h injection and extraction. The result of the study on operational schedule show that mineral reactivity occurs at the inlet of the domain. Furthermore, the porosity of the inner domain is preserved during the cycling of CO2 acidified brine for both systems.