Reactive Transport Simulations Research Articles

Avaliação numérica da eficiência da remediação de plumas de nitrato a partir de pulsos de injeção de etanol

O nitrato representa, em termos mundiais, o contaminante mais disseminado em aquíferos rasos, oferecendo riscos à saúde humana pela ingestão de água subterrânea contaminada. Deste modo, a descontaminação in situ do aquífero deve ser considerada. Embora testes de campo sejam escassos, testes laboratoriais têm indicado a forte eficiência do etanol na redução do nitrato. Com vistas a avaliar a viabilidade do emprego de técnicas de remediação baseadas na desnitrificação induzida pela injeção de solução com elevadas concentrações de etanol, foram utilizadas simulações numéricas de transporte reativo em área contaminada do Aquífero Rio Claro. Os resultados apontam que, nas condições técnicas impostas, a remediação foi eficiente para remoção do nitrato, restituindo a potabilidade da água subterrânea por um período superior a 500 dias. Contudo, para que essa solução seja eficiente em longo prazo deve ser combinada com a eliminação da fonte de contaminação, para evitar o aporte de novos contaminantes.

Applying the Rayleigh Approach for Stable Isotope-Based Analysis of VOC Biodegradation in Diffusion-Dominated Systems.

Compound-specific stable isotope analysis (CSIA) has become an established tool for assessing biodegradation in the subsurface. Diffusion-dominated vapor phase transport thereby is often excluded from quantitative assessments due to the problem of diffusive mixing of concentrations with different isotopic signatures for CSIA interpretation. In soils and other unsaturated porous media volatile organic compounds (VOCs) however, are mainly transported via gas-phase diffusion and may thus prohibit a CSIA-based quantitative assessment of the fate of VOCs. The present study presents and verifies a concept for the assessment of biodegradation-induced stable isotope fractionation along a diffusive transport path of VOCs in unsaturated porous media. For this purpose data from batch and column toluene biodegradation experiments in unsaturated porous media were combined with numerical reactive transport simulations; both addressing changes of concentration and stable isotope fractionation of toluene. The numerical simulations are in good agreement with the experiment data, and our results show that the presented analytically derived assessment concept allows using the slope of the Rayleigh plot to obtain reasonable estimates of effective in situ fractionation factors in spite of diffusion-dominated transport. This enlarges the application range of CSIA and provides a mean for a better understanding of VOC fate in the unsaturated subsurface.

Considering the utility of backward-in-time simulations of multi-component reactive transport in porous media

Abstract Indirect inversion has been the predominant method for matching models and data in geochemical systems, typically using observations of chemical concentrations as calibration targets. This reserves the data with the highest confidence (observed concentrations) for the final comparison but does little to constrain the initial state of the system on which the indirect inversion is based. An alternative approach to inverse modeling is to start with the observations to reconstruct a concentration field, but it is unclear if this is feasible for reactive transport in heterogeneous systems. The purpose of this article is to consider the applicability of backward-in-time (BIT) techniques as tools for simulating reactive transport in porous media. A multi-component reaction system is considered in a variety of systems of increasing complexity and we show that complex, non-linear systems can be simulated backward in time, given a sufficiently robust integration scheme. Recent advances in reactive random walk particle tracking are employed to investigate simple flow systems with spatially variable reactions, as well as 2-d heterogeneous flows, and we show that some level of time reversibility exists in both cases. Under a uniform injection scheme, the total masses generated in forward and backward simulations of the 2-d models were all within 3.5% of each other for all the species considered, indicating good overall agreement between the models. This suggests that BIT techniques may have yet unrealized applications to inverse modeling; however, further research on the sensitivity of the approach to measurement errors and on how to efficiently apply BIT methods to transient problems is needed.

The mechanisms, dynamics, and implications of self-sealing and CO2 resistance in wellbore cements

This study analyzes the dynamics and mechanisms of the interactions of carbonated brine with hydrated-Portland-cement; in particular, the study focuses on self-sealing, a process whereby hydrated-Portland cement reacts with carbonated brine to for silica and calcium carbonate in sufficient quantities to seal the flow pathway. The analysis is based on a comprehensive set of reactive-transport simulations that explore the complex coupled dynamics between the fluid flow and mineral reactions that underlie self-sealing, and it relies heavily on the synthesis of the extensive body of work on wellbore integrity that has been conducted over the past decade. The analysis explores a large chemical and mineralogical diversity and a wide range in physical conditions and flow regimes, attempting to assess the robustness of the analysis.Self-sealing conditions arise over a wide range in cement properties and reservoir conditions. Although some properties and conditions promote a stronger self-sealing response, self-sealing occurs for a wide range of Ca:Si ratios in cement and for various reservoir fluid compositions.Self-sealing conditions move along a wellbore proportional to the flux of the leaking carbonated brine, and the reaction zone spreads out proportional to the fluid velocity, where volumetric flux and velocity are related by porosity (flux = velocity * porosity). However, self-sealing conditions can be maintained in a specific section of a wellbore by controlling the pressure drive and/or effective wellbore permeability, which in turn can limit the flux and velocity of any leaking fluid.Finally, the phases produced by hydrating Portland cement represent a carbonic cement that will react with a carbonated brine to produce end products (calcium carbonate and silica) that can maintain integrity in the presence of carbonic acid. Moreover, the attributes that make hydrated Portland cement phases a carbonic cement are required for self-sealing.

Numerical Investigation into the Evolution of Groundwater Flow and Solute Transport in the Eastern Qaidam Basin since the Last Glacial Period

A complete understanding of groundwater circulation as well as the transport and distribution of solutes in arid-semiarid basin regions is a prerequisite for the safe use of groundwater resources. The distributions of the groundwater flow systema and solutes are affected by the basin morphology, lithology, and climate variations; therefore, they can change over geologic time. In this study, we performed a case study of the Qaidam Basin in the northeastern Tibetan Plateau, in which we utilized reactive solute transport simulations to build a numerical model in TOUGHREACT for a typical section of the eastern Qaidam Basin since the last glacial period. The results show that the groundwater in the eastern Qaidam Basin developed into a three-level groundwater flow system and that the seepage velocity of the local water flow system is significantly higher than that of the intermediate and regional water flow systems. Although groundwater in the discharge region has been continuously concentrated and enriched since the last glacial period, the distributions of the groundwater flow system and solutes have been greatly affected by climate variations. During warm periods, the centres of groundwater discharge and solute concentration shifted to areas with more groundwater recharge; in contrast, both centres shifted to the central basin during drought periods. The groundwater in the basin mainly contains Na+and Cl−ions, which vary significantly from the recharge region to the discharge region. Evaporation of groundwater results in increases in the concentrations of most of the components except HCO3−. The groundwater in the discharge region is currently in the stage of carbonate precipitation and is far from gypsum and halite precipitation.

Reactive transport modelling in dual porosity media

A pore-scale model is applied to investigate reactive mass transport in dual porosity media. We use a framework based on the Stokes-Brinkman flow to model reactive flow in porous media with multiscale features. The numerical method involves fluid flow in pore space as well as flow in the rock matrix with microporosity. Reactive transport simulations are performed in a simple fractured porous medium and carbonates. The results are compared against the predictions in regimes of Stokes flow where the microporosity of rocks is excluded. In the fractured porous medium, wormholing dissolution is observed while face dissolution occurs when microporosity is ignored for the same Péclet and Damköhler numbers. This is explained by the penetration of reactive fluid flow through the rock matrix by Stokes-Brinkman flow. Reactive transport is also simulated in a Ketton carbonate. We compare the simulation results with published dynamic micro-CT imaging experiments for dissolution. Inclusion of microporosity leads to more accurate predictions of the pore space structures. The effect of microporosity on pore structure and permeability-porosity relationships is also explored in the carbonate samples with a range of matrix permeability values. The results show that the impact of microporosity on rock permeability variations is dependent on the ratio of matrix permeability to the overall permeability of the porous medium. Greater errors in permeability predictions are observed in the cases with higher ratios. This paper illustrates the significance of microporosity in reactive transport and improves the understanding of reactive transport in porous domains with multiscale features.

Scale effect on the time dependence of mineral dissolution rates in physically heterogeneous porous media

Hydrologic heterogeneity complicates the time dependent behavior of mineral dissolution rates and introduces large uncertainty in effective surface area estimation. In order to resolve the coupled evolution of mineral dissolution rates and mineral surface area over time in various fluid flow conditions, this study presents multiple reactive transport simulations of anorthite dissolution over 3000 years with 2D latticed random permeability fields on small (1 m × 1 m) and large (400 m × 400 m) domains. Physical heterogeneity is controlled by varying integral scales (λmin and λmax) and variance in permeability distribution (σlnk2) conditions. Chemistry is simplified to a single dissolving primary mineral (anorthite) distributed homogeneously throughout the domain and a single secondary mineral (kaolinite) that is allowed to dissolve or precipitate. In a homogeneous domain, non-normalized effective mineral dissolution rate decreases over time as depletion of the dissolving mineral reduces reaction product concentration. When the reaction rate is normalized either by local or domain-averaged mineral surface area, the rate increases over time approaching the value of the predefined intrinsic rate constant as the effect of saturation state becomes negligible. In a small heterogeneous domain, the effect of physical heterogeneity is largely negligible regardless of the changes in control parameters. In large domains, however, large λ and σlnk2 contribute to develop low-reactive zones that preserve the reactive mineral from dissolution by limiting solute transport to slow diffusion. This preserved mineral fraction provides reactants in late time period resulting in gradual decrease of non-normalized effective mineral dissolution rate. If the rate is normalized by domain-averaged specific surface area, these remaining surface areas over-normalize effective dissolution rates resulting in ∼1.5 orders of magnitude reduction from the homogeneous case.

From mixed flow reactor to column experiments and modeling: Upscaling of calcite dissolution rate

The objective of this work was to assess the extent to which calcite dissolution rate (Rcalcite) measured at controlled saturation state in mixed flow set-ups could be upscaled to column experiments, where the solution composition was variable in space and time. The dissolution rate of the (104) calcite face was investigated in mixed flow reactor set-ups at room temperature and pH 8. Various saturation conditions were studied by changing the composition of the inlet solution, enabling the determination of an empirical relation between Rcalcite and the Gibbs free energy of calcite dissolution (∆G) by measuring the surface topography with vertical scanning interferometry (VSI). The prevalent dissolution mode (i.e., etch pit nucleation or homogeneous surface retreat) was assessed with the same method. A dramatic decrease of the dissolution rate was observed for −8 < ∆G < −3 kJ/mol, correlated with a switch in the dissolution regime (inhibition of etch pit nucleation). The resulting Rcalcite-∆G relation, which is at odds with that derived from transition state theory (TST), was successfully fitted using the stepwave model (SWM). These experiments were supplemented with plug-flow column experiments, consisting of a chemically inert porous medium and oriented rhombohedral calcite single-crystals (0.4 × 0.4 cm2) regularly distributed in the column. The dissolution rates of calcite single crystals all along the column were retrieved using VSI on the recovered crystals at the end of the experiments. These latter experiments were compared with 1D and 2D reactive transport simulations, using either the TST- or the SWM-derived rate equation determined from our mixed-flow reactor experiments. The reactive transport simulations revealed that (i) 1D-simulations overestimated the dissolution rate of upstream and downstream calcite faces by a ~3- and ~20-fold factor, (ii) 2D-simulations satisfactorily reproduced the dissolution rates of all faces without any fitting parameter, within experimental uncertainties and (iii) both models slightly overestimated the steady-state concentration of calcium at the outlet of the column. Overall, this study can be considered as one of the multiple intermediate steps between chemostatic experiments and field measurements to assess the validity of the classical bottom-up upscaling approach. It also provides insight into our ability to model fluid flow and solution composition in the vicinity of dissolving single crystals.

Physical heterogeneity control on effective mineral dissolution rates

Hydrologic heterogeneity may be an important factor contributing to the discrepancy in laboratory and field measured dissolution rates, but the governing factors influencing mineral dissolution rates among various representations of physical heterogeneity remain poorly understood. Here, we present multiple reactive transport simulations of anorthite dissolution in 2D latticed random permeability fields and link the information from local grid scale (1 cm or 4 m) dissolution rates to domain-scale (1m or 400 m) effective dissolution rates measured by the flux-weighted average of an ensemble of flow paths. We compare results of homogeneous models to heterogeneous models with different structure and layered permeability distributions within the model domain. Chemistry is simplified to a single dissolving primary mineral (anorthite) distributed homogeneously throughout the domain and a single secondary mineral (kaolinite) that is allowed to dissolve or precipitate. Results show that increasing size in correlation structure (i.e. long integral scales) and high variance in permeability distribution are two important factors inducing a reduction in effective mineral dissolution rates compared to homogeneous permeability domains. Larger correlation structures produce larger zones of low permeability where diffusion is an important transport mechanism. Due to the increased residence time under slow diffusive transport, the saturation state of a solute with respect to a reacting mineral approaches equilibrium and reduces the reaction rate. High variance in permeability distribution favorably develops large low permeability zones that intensifies the reduction in mixing and effective dissolution rate. However, the degree of reduction in effective dissolution rate observed in 1 m × 1 m domains is too small (<1% reduction from the corresponding homogeneous case) to explain several orders of magnitude reduction observed in many field studies. When multimodality in permeability distribution is approximated by high permeability variance in 400 m × 400 m domains, the reduction in effective dissolution rate increases due to the effect of long diffusion length scales through zones with very slow reaction rates. The observed scale dependence becomes complicated when pH dependent kinetics are compared to the results from pH independent rate constants. In small domains where the entire domain is reactive, faster anorthite dissolution rates and slower kaolinite precipitation rates relative to pH independent rates at far-from-equilibrium conditions reduce the effective dissolution rate by increasing the saturation state. However, in large domains where less- or non-reactive zones develop, higher kaolinite precipitation rates in less reactive zones increase the effective anorthite dissolution rates relative to the rates observed in pH independent cases.

Montmorillonite dissolution kinetics: Experimental and reactive transport modeling interpretation

The dissolution kinetics of K-montmorillonite was studied at 25 °C, acidic pH (2–4) and 0.01 M ionic strength by means of well-mixed flow-through experiments. The variations of Si, Al and Mg over time resulted in high releases of Si and Mg and Al deficit, which yielded long periods of incongruent dissolution before reaching stoichiometric steady state. This behavior was caused by simultaneous dissolution of nanoparticles and cation exchange between the interlayer K and released Ca, Mg and Al and H. Since Si was only involved in the dissolution reaction, it was used to calculate steady-state dissolution rates, RSi, over a wide solution saturation state (ΔGr ranged from −5 to −40 kcal mol−1).The effects of pH and the degree of undersaturation (ΔGr) on the K-montmorillonite dissolution rate were determined using RSi. Employing dissolution rates farthest from equilibrium, the catalytic pH effect on the K-montmorillonite dissolution rate was expressed as Rdiss = k·aH0.56±0.05 whereas using all dissolution rates, the ΔGr effect was expressed as a non-linear f(ΔGr) function Rdiss=k·1-exp-3.8×10-4·|ΔGr|RT2.13The functionality of this expression is similar to the equations reported for dissolution of Na-montmorillonite at pH 3 and 50 °C (Metz, 2001) and Na-K-Ca-montmorillonite at pH 9 and 80 °C (Cama et al., 2000; Marty et al., 2011), which lends support to the use of a single f(ΔGr) term to calculate the rate over the pH range 0–14. Thus, we propose a rate law that also accounts for the effect of pOH and temperature by using the pOH-rate dependence and the apparent activation energy proposed by Rozalén et al. (2008) and Amram and Ganor (2005), respectively, and normalizing the dissolution rate constant with the edge surface area of the K-montmorillonite.1D reactive transport simulations of the experimental data were performed using the Crunchflow code (Steefel et al., 2015) to quantitatively interpret the evolution of the released cations and to elucidate the stoichiometry of the reaction. After the implementation of (i) the obtained f(ΔGr) term in the K-montmorillonte dissolution rate law, (ii) a fraction of highly reactive particles and surfaces and (iii) the cation exchange reactions between the interlayer K+ and the released Al3+, Mg2+, Ca2+ and H+, the simulations agreed with the experimental concentrations at the outlet. This match indicates that fast dissolution of fine particles and highly reactive sites and exchange between the interlayer K and dissolved structural cations (Al and Mg) and protons are responsible for the temporary incongruency of the K-montmorillonite dissolution reaction. As long as dissolution of the bulk sample predominates, the reaction is stoichiometric.

Simulating vertical flow wetlands using filter media with different grain sizes with the HYDRUS Wetland Module

Abstract In this study, the simulation results of four vertical flow wetland systems using the HYDRUS Wetland Module are presented. The four wetland systems comprise three single-stage pilot scale systems and one full-scale two-stage system. The main difference between these systems is the filter media used, referred to as fine media, i.e., sand with a grain size distribution 0.063–4 mm, or coarse media, i.e., sand with grain size distributions between 1–4 mm, respectively. The water-flow simulation of each system is carried out using the single porosity van Genuchten-Mualem model. A good match between measured and simulated volumetric effluent flow rates could be achieved for all wetland systems. For reactive transport simulations, the CW2D biokinetic model was applied. First, simulations were run using the standard CW2D parameter set. For some systems, adjustments of the parameter set were needed in order to avoid unlimited bacteria growth. To better fit measured COD, NH4-N, and NO3-N effluent concentrations, adjustments of few parameters of the standard parameter set were required. The results show that for the VF wetlands with fine sand, no adjustments of the CW2D standard parameter set were needed, while for systems with coarser filter media as the main layer, the standard parameter set had to be adjusted to match simulated and measured effluent concentrations.

Relationships between CO2, thermodynamic limits on silicate weathering, and the strength of the silicate weathering feedback

Recent studies have suggested that thermodynamic limitations on chemical weathering rates exert a first-order control on riverine solute fluxes and by extension, global chemical weathering rates. As such, these limitations may play a prominent role in the regulation of carbon dioxide levels (pCO2) over geologic timescales by constraining the maximum global weathering flux. In this study, we develop a theoretical scaling relationship between equilibrium solute concentrations and pCO2 based on equilibrium constants and reaction stoichiometry relating primary mineral dissolution and secondary mineral precipitation. We test this theoretical scaling relationship against reactive transport simulations of chemical weathering profiles under open- and closed-system conditions, representing partially and fully water-saturated regolith, respectively. Under open-system conditions, equilibrium bicarbonate concentrations vary as a power-law function of pCO2(y=kxn) where n is dependent on reaction stoichiometry and k is dependent on both reaction stoichiometry and the equilibrium constant. Under closed-system conditions, bicarbonate concentrations vary linearly with pCO2 at low values and approach open-system scaling at high pCO2. To describe the potential role of thermodynamic limitations in the global silicate weathering feedback, we develop a new mathematical framework to assess weathering feedback strength in terms of both (1) steady-state atmospheric pCO2 concentrations, and (2) susceptibility to secular changes in degassing rates and transient carbon cycle perturbations, which we term 1st and 2nd order feedback strength, respectively. Finally, we discuss the implications of these results for the effects of vascular land plant evolution on feedback strength, the potential role of vegetation in controlling modern solute fluxes, and the application of these frameworks to a more complete functional description of the silicate weathering feedback. Most notably, the dependence of equilibrium solute concentrations on pCO2 may represent a direct weathering feedback largely independent of climate and modulated by belowground organic carbon respiration.

Simulating Oxygen Intrusion into Highly Heterogeneous Fractured Media Using High Performance Computing

Fractured crystalline rocks are under consideration by several countries as host formations for high-level nuclear waste repositories. The redox evolution in these host rock formations is an important issue for the stability and safety of these disposal sites. If, for example, during a glaciation/deglaciation event, oxygen-rich glacial meltwater penetrates to the depth of the planned repository, some of the engineered barriers would be adversely affected. Moreover, oxidizing conditions would increase the solubility and mobility of many radionuclides. Reactive transport simulations, which are typically used to assess the redox buffering capacity of these host rock formations, are computationally demanding, and thus, calculations for the evaluation of oxygen penetration are usually carried out over simplified geometries and the heterogeneity of the site, both physical (e.g., variability in the groundwater flow field and the kinematic porosity) and mineralogical (e.g., variability in the abundance of Fe(II)-bearing minerals), is usually represented in a simplified fashion. Here, it is shown how a recently developed numerical framework, combined with high performance computing technologies, allows the full geometrical, physical and mineralogical complexity of the site under study to be efficiently included in these types of reactive transport calculations. A synthetically generated realistic three-dimensional fractured medium is used to assess oxygen penetration patterns and their dependence on both the hydrogeological conditions and the availability of Fe(II)-bearing minerals. The results of the calculations point out the significant influence of both physical and mineralogical heterogeneity on the oxygen penetration patterns, thus highlighting the importance of a model parameterisation consistent with the site complexity.

Improvement of numerical approximation of coupled multiphase multicomponent flow with reactive geochemical transport in porous media

In this paper, we consider a parallel finite volume algorithm for modeling complex processes in porous media that include multiphase flow and geochemical interactions. Coupled flow and reactive transport phenomena often occur in a wide range of subsurface systems such as hydrocarbon reservoir production, groundwater management, carbon dioxide sequestration, nuclear waste repository or geothermal energy production. This work aims to develop and implement a parallel code coupling approach for non-isothermal multiphase multicomponent flow and reactive transport simulation in the framework of the parallel open-source platform DuMuX. Modeling such problems leads to a highly nonlinear coupled system of degenerate partial differential equations to algebraic or ordinary differential equations requiring special numerical treatment. We propose a sequential fully implicit scheme solving firstly a multiphase compositional flow problem and then a Direct Substitution Approach (DSA) is used to solve the reactive transport problem. Both subsystems are discretized by a fully implicit cell-centred finite volume scheme and then an efficient sequential coupling has been implemented in DuMuX. We focus on the stability and robustness of the coupling process and the numerical benefits of the DSA approach. Parallelization is carried out using the DUNE parallel library package based on MPI providing high parallel efficiency and allowing simulations with several tens of millions of degrees of freedom to be carried out, ideal for large-scale field applications involving multicomponent chemistry. As we deal with complex codes, we have tested and demonstrated the correctness of the implemented software by benchmarking, including the MoMaS reactive transport benchmark, and comparison to existing simulations in the literature. The accuracy and effectiveness of the approach is demonstrated through 2D and 3D numerical simulations. Parallel scalability is investigated for 3D simulations with different grid resolutions. Numerical results for long-term fate of injected CO2 for geological storage are presented. The numerical results have demonstrated that this approach yields physically realistic flow fields in highly heterogeneous media and showed that this approach performs significantly better than the Sequential Iterative Approach (SIA).

Accounting for the Decreasing Reaction Potential of Heterogeneous Aquifers in a Stochastic Framework of Aquifer‐Scale Reactive Transport

AbstractMany groundwater contaminants react with components of the aquifer matrix, causing a depletion of the aquifer's reactivity with time. We discuss conceptual simplifications of reactive transport that allow the implementation of a decreasing reaction potential in reactive‐transport simulations in chemically and hydraulically heterogeneous aquifers without relying on a fully explicit description. We replace spatial coordinates by travel‐times and use the concept of relative reactivity, which represents the reaction‐partner supply from the matrix relative to a reference. Microorganisms facilitating the reactions are not explicitly modeled. Solute mixing is neglected. Streamlines, obtained by particle tracking, are discretized in travel‐time increments with variable content of reaction partners in the matrix. As exemplary reactive system, we consider aerobic respiration and denitrification with simplified reaction equations: Dissolved oxygen undergoes conditional zero‐order decay, nitrate follows first‐order decay, which is inhibited in the presence of dissolved oxygen. Both reactions deplete the bioavailable organic carbon of the matrix, which in turn determines the relative reactivity. These simplifications reduce the computational effort, facilitating stochastic simulations of reactive transport on the aquifer scale. In a one‐dimensional test case with a more detailed description of the reactions, we derive a potential relationship between the bioavailable organic‐carbon content and the relative reactivity. In a three‐dimensional steady‐state test case, we use the simplified model to calculate the decreasing denitrification potential of an artificial aquifer over 200 years in an ensemble of 200 members. We demonstrate that the uncertainty in predicting the nitrate breakthrough in a heterogeneous aquifer decreases with increasing scale of observation.

An Embedded Grid-Free Approach for Near-Wellbore Streamline Simulation

SummaryReactive-transport phenomena, such as carbon dioxide sequestration and microbial enhanced oil recovery (EOR), have been of interest in streamline-based simulation (SLS). Tracing streamlines launched from a wellbore is important, especially for time-sensitive transport behaviors. However, discretized gridblocks are usually too large compared with the wellbore radius. Field-scale simulations with local-grid-refinement (LGR) models often consume huge computational time. An embedded grid-free approach to integrate near-wellbore transport behaviors into streamline simulations is developed, comprising two stages of development: tracing streamlines in a wellblock (a gridblock containing wells) and coupling streamlines with neighboring grids. The velocity field in a wellblock is produced using a gridless virtual-boundary-element method (VBEM), where streamlines are numerically traced with the fourth-order Runge-Kutta (RK4) method (Butcher 2008). The local streamline system is then connected with the global streamline system, which is produced with the Pollock (1988) algorithm. Finally, the reactive-transport equation will be solved along these streamlines.The algorithm presented for solving near-wellbore streamlines is verified by both a commercial finite-element simulator and a Pollock-algorithm-based 3D streamline simulator. A series of computational cases of reactive-transport simulation is studied to demonstrate the applicability, accuracy, and efficiency of the proposed method. Velocity field, time-of-flight (TOF), streamline pattern, and concentration distribution produced by different approaches are analyzed. Results show that the presented method can accurately perform near-wellbore streamline simulations in a time-effective manner. The algorithm can be directly applied to one grid containing multiple wells or to off-center wells. Furthermore, assuming streamlines are evenly launched from the gridblock boundary or ignoring transport in the wellblock is not always reasonable, and may lead to a significant error.This study provides a simple and grid-free solution, but is capable of capturing the flow field near the wellbore with significant accuracy and computational efficiency. The method is promising for reservoir SLS with time-sensitive transport, and other simulations requiring an accurate assessment of interactions between wells in one particular gridblock.

Evaluation of Macroscopic Porosity‐Permeability Relationships in Heterogeneous Mineral Dissolution and Precipitation Scenarios

AbstractMineral dissolution and precipitation reactions may alter porosity and permeability in porous media. While porosity generally increases with mineral dissolution and decreases with precipitation, corresponding permeability alterations are complex and predictive capabilities remain limited. The evolution of permeability depends on the spatial locations of geochemical reactions, both in individual pores and in the greater pore network and a range of reaction patterns have been observed in prior experimental investigations. Macroscopic relationships are needed in core‐to‐field scale reactive transport simulations that can accurately simulate permeability evolutions resulting from pore‐scale reactions. Currently, empirical porosity‐permeability relationships, such as the Kozeny‐Carman equation, are widely used to predict permeability evolution. However, these relationships only allow for a single permeability value for each porosity value and the validity of these relationships for the various spatial distributions of pore‐scale alterations is unknown. Using pore network model simulations, changes in permeability resulting from a range of dissolution and precipitation patterns (uniform, random, channelized, and size‐dependent) in a sandstone sample are investigated. The validity of macroscopic porosity‐permeability relationships for these reaction scenarios is evaluated by comparing computed permeability values with pore model simulations. Simulation results exhibit a large variation in porosity‐permeability relationships among reaction scenarios. Macroscopic porosity‐permeability relationships work well for uniform reaction scenarios where the extent of reaction is related to the pore size. The porosity and permeability relationships that result from more complex pore‐scale alterations, including pore size‐dependent reactions however, cannot be captured using these empirical relationships.

Modeling of Salt Sorption in Volcanic Ash Soil

ABSTRACTGroundwater pollution by nitrate from agricultural fields is a worldwide problem. To improve the understanding of nitrate transport processes through volcanic ash soils overlying groundwater aquifers, salt sorption experiments were performed. Salt sorption is the process that involves the simultaneous use of cation and anion adsorption mechanisms without changing the pH. Prepared six different concentrations of potassium chloride (KCl) solutions were mixed with soil samples. Anion and cation contents of the liquid and solid phases were determined. Experiments were performed in triplicate of six kinds of KCl solutions for three types of soils. Thus, total 54 of samples were analyzed. Relationships between ionic strength and sorption capacity increments of cations and anions were determined. The results show that if ionic strength of the infiltrating solution to the volcanic ash soil is known, the ECEC (effective cation exchange capacity) and EAEC (effective anion exchange capacity) can be reliably estimated. The resulting prediction equations can be included to improve reactive transport simulation models. Consequently, it will be possible to better understand solute transport with changing sorption capacity for the solid soil phase.

Effect of Anisotropy Structure on Plume Entropy and Reactive Mixing in Helical Flows

Plume dilution and reactive mixing can be considerably enhanced by helical flows occurring in three-dimensional anisotropic porous media. In this study, we perform conservative and reactive transport simulations considering different anisotropy structures of a single inclusion with the objective of exploring the effect of the inclusion’s geometry and orientation on the patterns of twisted streamlines and on the overall dilution and reaction of solute plumes. We analyzed 100 different scenarios by varying key parameters such as the angle of the anisotropic structures with respect to the average flow velocity, the spacing between alternated heterogeneous zones of coarse and fine materials, the permeability contrast between such matrices, and the magnitude of the seepage velocity. Entropy conservation equations and entropy-based metrics for both conservative and reactive species were adopted to quantify dilution, reactive mixing and their interactions with the helical flow patterns in the considered three-dimensional anisotropic setups. The results allowed identifying optimal anisotropic configurations maximizing mixing and reactions, and yielding enhancement factors up to 15 times the outcomes of analogous simulations in homogeneous media. Furthermore, the effects of compound-specific diffusive/dispersive properties of the transported species were found to be relevant for both plume dilution and reactive mixing in helical flows.

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...