Reactive Transport Model Research Articles

Microscale Chaotic Mixing as a Driver for Chemical Reactions in Porous Media.

Mixing-induced reactions play a key role in a large range of biogeochemical and contaminant transport processes in the subsurface. Fluid flow through porous media was recently shown to exhibit chaotic mixing dynamics at the pore scale, enhancing microscale concentration gradients and controlling mixing rates. While this phenomenon is likely ubiquitous in environmental systems, it is not known how it affects chemical reactions. Here, we use refractive index matching and laser-induced fluorescence imaging of a bimolecular redox reaction to investigate the consequence of pore scale chaotic mixing on the reaction rates. The overestimation of measured reaction rates by the classical macrodispersion model highlights the persistence of incomplete mixing on the pore scale. We show that the reaction product formation is controlled by microscale chaotic mixing, which induces an exponential increase of the mixing interface and of the reaction rates. We derive a reactive transport model that captures experimental results and predicts that chaotic mixing has a first order control on reaction rates across a large range of time scales and Péclet and Damköhler numbers. These findings provide a new framework for understanding, assessing, and predicting mixing-induced reactions and their role on the fate and mobility of environmental compounds in natural porous media.

Amended Vegetation Filters as Nature-Based Solutions for the Treatment of Pharmaceuticals: Infiltration Experiments Coupled to Reactive Transport Modelling.

In small populations and scattered communities, wastewater treatment through vegetation filters (VFs), a nature-based solution, has proved to be feasible, especially for nutrient and organic matter removal. However, the presence of pharmaceuticals in wastewater and their potential to infiltrate through the vadose zone and reach groundwater is a drawback in the evaluation of VF performances. Soil amended with readily labile carbon sources, such as woodchips, enhances microbial activity and sorption processes, which could improve pharmaceutical attenuation in VFs. The present study aims to assess if woodchip amendments to a VF's soil are able to abate concentrations of selected pharmaceuticals in the infiltrating water by quantitatively describing the occurring processes through reactive transport modelling. Thus, a column experiment using soil collected from an operating VF and poplar woodchips was conducted, alongside a column containing only soil used as reference. The pharmaceuticals acetaminophen, naproxen, atenolol, caffeine, carbamazepine, ketoprofen and sulfamethoxazole were applied daily to the column inlet, mimicking a real irrigation pattern and periodically measured in the effluent. Ketoprofen was the only injected pharmaceutical that reached the column outlet of both systems within the experimental timeframe. The absence of acetaminophen, atenolol, caffeine, carbamazepine, naproxen and sulfamethoxazole in both column outlets indicates that they were attenuated even without woodchips. However, the presence of 10,11-epoxy carbamazepine and atenolol acid as transformation products (TPs) suggests that incomplete degradation also occurs and that the effect of the amendment on the infiltration of TPs is compound-specific. Modelling allowed us to generate breakthrough curves of ketoprofen in both columns and to obtain transport parameters during infiltration. Woodchip-amended columns exhibited Kd and μw values from one to two orders of magnitude higher compared to soil column. This augmentation of sorption and biodegradation processes significantly enhanced the removal of ketoprofen to over 96%.

Influence of clay minerals on pH and major cation concentrations in acid-leached sands: Column experiments and reactive-transport modeling

A series of laboratory experiments are conducted to simulate the acidification and subsequent recovery of a sand aquifer exploited by in situ recovery (ISR) mining. A sulfuric acid solution (pH 2) is first injected into a column packed with sand from the Zoovch Ovoo uranium roll front deposit (Mongolia). Solutions representative of local groundwater or enriched in cations (Na+, Mg2+) are then circulated through the column to simulate the inflow of aquifer water. pH and major ion concentrations (Na+, Cl−, SO42−, Ca2+, Mg2+, K+) measured at the column outlet reproduce the overall evolution of porewater chemistry observed in the field. The presence of minor quantities of swelling clay minerals (≈6 wt% smectite) is shown to exert an important influence on the behavior of inorganic cations, particularly H+, via ion-exchange reactions. Numerical models that consider ion-exchange on smectite as the sole solid-solution interaction are able to reproduce variations in pH and cation concentrations in the column experiments. This highlights the importance of clay minerals in controlling H+ mobility and demonstrates that sand from the studied aquifer can be described to a first order as an ion-exchanger. The present study confirms the key role of clay minerals in controlling water chemistry in acidic environments through ion-exchange processes. In a context of managing the long-term environmental footprint of industrial and mining activities (ISR, acid mine drainage…), this work will bring insights for modeling choices and identification of key parameters to help operators to define their production and/or remediation strategies.

Towards Immobilized Proton-Coupled Electron Transfer Agents for Electrochemical Carbon Capture from Air and Seawater

Electrochemical CO2 separation has drawn attention as a promising strategy for using renewable energy to mitigate climate change. Redox-active compounds that undergo proton-coupled electron transfer (PCET) are an impetus for pH-swing-driven CO2 capture at low energetic costs. However, multiple barriers hinder this technology from maturing, including sensitivity to oxygen and the slow kinetics of CO2 capture. Here, we use vapor phase chemistry to construct a textile electrode comprising an immobilized PCET agent, poly(1-aminoanthraquinone) (PAAQ), and incorporate it into redox flow cells. This design contrasts with others that use dissolved PCET agents by confining proton-storage to the surface of an electrode kept separate from an aqueous, CO2-capturing phase. This system facilitates carbon capture from gaseous sources (a 1% CO2 feed and air), as well as seawater, with the latter at an energetic cost of 202 kJ/molCO2, and we find that quinone moieties embedded within the electrode are more stable to oxygen than dissolved counterparts. Simulations using a 1D reaction-transport model show that moderate energetic costs should be possible for air capture of CO2 with higher loadings of polymer-bound PCET moieties. The remarkable stability of this system sets the stage for producing textile-based electrodes that facilitate pH-swing-driven carbon capture in practical situations.

Adaptive mesh based efficient approximations for Darcy scale precipitation–dissolution models in porous media

AbstractIn this work, we consider the Darcy scale precipitation–dissolution reactive transport 1D and 2D models in a porous medium and provide the adaptive mesh based numerical approximations for solving them efficiently. These models consist of a convection‐diffusion‐reaction PDE with reactions being described by an ODE having a nonlinear, discontinuous, possibly multi‐valued right hand side describing precipitate concentration. The bulk concentration in the aqueous phase develops fronts and the precipitate concentration is described by a free and time‐dependent moving boundary. The time adaptive moving mesh strategy, based on equidistribution principle in space and governed by a moving mesh PDE, is utilized and modified in the context of present problem for finite difference set up in 1D and finite element set up in 2D. Moreover, we use a predictor corrector based algorithm to solve the nonlinear precipitation–dissolution models. For equidistribution approach, we choose an adaptive monitor function and smooth it based on a diffusive mechanism. Numerical tests are performed to demonstrate the accuracy and efficiency of the proposed method by examples through finite difference approach for 1D and finite element approach in 2D. The moving mesh refinement accurately resolves the front location of Darcy scale precipitation–dissolution reactive transport model and reduces the computational cost in comparison to numerical simulations using a fixed mesh.

Effects of the groundwater flowing and redox conditions on arsenic mobilization in aquifers of the Datong Basin, Northern China

A multi-level field site was built at the As-affected area of the Datong basin. The one-year monitoring results showed that the three aquifers exhibit different redox conditions and hydrogeochemical behaviors. In the shallow aquifer, groundwater As concentration is up to 220.5 μg/L, which is related to the dissimilatory reduction of poor-crystalline Fe minerals. In the middle aquifer, groundwater sulfate reduction shows two sides effects on As mobilization that the reduced sulfide on the one hand promotes the release of sediment As via abiotic reductive dissolution of Fe minerals and on the other hand immobilizes groundwater As via the adsorption on the newly formed Fe(II)-sulfide minerals. In the deep aquifer, the reductive dissolution of crystalline Fe minerals was the dominant process causing the release of sediment As into groundwater with the highest As concentration up to 341.1 μg/L. Based on the one-year monitored data, a coupled flow and reactive transport model was developed. The model results showed that only considering the groundwater flow underestimated the concentration of groundwater As. When adding the hydro-biogeochemical processes, the good agreement was achieved between the observed and simulated groundwater As. In the shallow and deep aquifers, microbially mediated reduction of poor-crystalline and crystalline Fe minerals contributed approximately 85 % of groundwater As. In the middle aquifer, abiotic reduction of Fe minerals driven is responsible for around 35 % of the release of sediment As, and around 17 % of released As was re-adsorbed or co-precipitated onto the neo-formed Fe(II) minerals. Under different redox conditions, the hydrogeochemical behavior of As was dominantly controlled by the Fe-S-As cycling in the groundwater system.

Effect of supplementary cementitious materials on the degradation of cement-based barriers in radioactive waste repository: A case study in Korea

This study focuses on investigating the chemical degradation characteristics of cementitious barriers used in low- and intermediate-level radioactive waste repository by reactive transport modeling. The impact of the blending with supplementary cementitious materials (SCMs) in the barriers on the chemical degradation was evaluated to find the optimum barrier design. A number of different barrier designs were examined by replacing ordinary Portland cement (OPC) by SCMs (i.e., fly ash, silica fume, and blast-furnace slag). The simulation results showed that silica fume blended barrier has better durability against chemical degradation by rainwater compared to fly ash or blast-furnace slag blended barriers. In addition, the chemical durability of silica fume-based barrier increased with increasing replacement level up to about 20 %. It seems that the amount of formed calcium silicate hydrate (CSH) in the initial cement-based barrier highly affects the overall chemical durability. The newly developed reactive transport model demonstrated its capability for understanding the barrier performance and investigating the optimal design of the barrier system.

Characteristics and numerical simulation of chromium transportation, migration and transformation in soil–groundwater system

Understanding chromium (Cr) migration and dispersion patterns in the soil-groundwater system is critical for the control and remediation of subsurface Cr contamination. In this study, a typical Cr-contaminated site from the Pearl River Delta (PRD) in China was simulated with a three-dimensional (3D) sandbox experiment to investigate the migration and transformation behavior of Cr. Results revealed that under the combined influence of rainfall and groundwater flow, a complex flow field favorable for 3D migration and solute dispersion was formed. The flow field characteristics were influenced by water-table depth, which in turn affected Cr behavior in the system. Moreover, downward flow field expansion under low water-table conditions led to Cr vertical migration range expansion, causing greater contamination in the deep soil. The migration process was accompanied with Cr(VI) reduction, during which approximately 75 % of the total Cr was immobilized in soils. The reactive transport model achieved a good fit for Cr retention and morphological distribution in the solid phase. The model indicates that Cr is more readily transported and dispersed with groundwater, and Cr migrated and spread downstream by 15 m during the eighth year. Therefore, managing water-table depth could be a strategy to minimize the Cr vertical migration and contamination.

Upscaling reactive transport models from pore-scale to continuum-scale using deep learning method

Reactive transport modeling of subsurface environments plays an important role in addressing critical problems of geochemical processes, such as dissolution and precipitation of minerals. Current transport models for porous media span various scales, ranging from pore-scale to continuum-scale. In this study, we established an upscaling method connecting pore-scale and continuum-scale models by employing a deep learning methodology of Convolutional Neural Networks (CNNs). We applied Darcy-Brinkmann-Stokes (DBS) method to simulate the fluid flow and reactive transport in pore-scale models, which would act as constituents of a continuum-scale model. The datasets of spatial pore distribution of subvolume samples were used as the input for the deep learning model, and the continuum (Darcy)-scale parameters such as permeability, effective surface area, and effective diffusion coefficient were figured out as outputs (i.e., labels). By applying the trained models of the subvolumes in the entire sample volume, we generated the initial field of porosity, permeability, effective diffusion coefficient, and effective surface area for continuum-scale simulation of a mineral dissolution problem. We took an acid dissolution case as an example to utilize the outcomes of trained deep learning models as input data in the continuum-scale simulation. This work presents a comprehensive upscaling workflow, as bridging the findings of microscale simulations to the continuum-scale simulations of a reactive transport problem.

State of the Art on Fe Precipitation in Porous Media: Hydrogeochemical Processes and Evolving Parameters

The mixing of terrestrial groundwater and seawater creates dynamic reaction zones in intertidal areas, where land-derived Fe(II) is oxidized to Fe(III) and then precipitates as Fe hydroxides at the groundwater–seawater interface. These hydrogeochemical processes contribute to the formation of iron bands at the saltwater wedge (SW) and beneath the upper saline plume (USP). This study provides a comprehensive review of physical and geochemical processes at field scale in coastal areas, explores the impact of mineral precipitation on pore structure at pore scale, and synthesizes reactive transport modeling (RTM) approaches for illustrating continuum-scale soil physio-chemical parameters during the evolution of porous media. Upon this review, knowledge gaps and research needs are identified. Additionally, challenges and opportunities are presented. Therefore, we reach the conclusion that the incorporation of observational data into a comprehensive physico-mathematical model becomes imperative for capturing the pore-scale processes in porous media and their influence on groundwater flow and solute transport at large scales. Additionally, a synergistic approach, integrating pore-scale modeling and non-invasive imaging, is equally essential for providing detailed insights into intricate fluid–pore–solid interactions for future studies, as well as facilitating the development of regional engineering-scale models and physio-chemical coupled models with diverse applications in marine science and engineering.

Model validation and interpretation of the interaction between glass and cement in an integrated glass dissolution experiment

This article presents the modeling results from the interaction between simulated nuclear waste glasses and ordinary Portland cement (OPC). In general, the modeling results are in good agreement with the experimental results. A diffusion-based glass dissolution model well describes the release of B and Li from the glass, and a reactive transport model well describes the profiles of dissolved glass elements in the cement. It is found that the effective diffusion coefficient of Li in the cement is much larger than that of B. Sorption of both Li and B onto the cement is required to better fit their measured diffusion profiles. For B profiles in the cement, one spatially uniform distribution coefficient is needed, while for Li, sorption needs to be linked to the gel-like secondary phase at the surface of the cement. The effective diffusion coefficients for both Li and B are relatively low and can be linked to the pore-filling effects of the gel-like secondary phase. Even though it is generally believed that nuclear waste glass will dissolve faster in cementitious environments, the results show that the gel-like secondary phase can both seal the cement and act as a strong sorbent for dissolved glass species, thus inhibiting the migration of dissolved glass species.

Process-Based and Probabilistic Quantification of Co and Ni Mobilization Risks Induced by Managed Aquifer Recharge.

Managed aquifer recharge (MAR) is an increasingly used water management technique that enhances water availability while commonly generating water quality benefits. However, MAR activities may also trigger adverse geochemical reactions, especially during the injection of oxidant-enriched waters into reducing aquifers. Where this occurs, the environmental risks and the viability of mitigating them must be well understood. Here, we develop a rigorous approach for assessing and managing the risks from MAR-induced metal mobilization. First, we develop a process-based reactive transport model to identify and quantify the main hydrogeochemical drivers that control the release of metals and their mobility. We then apply a probabilistic framework to interrogate the inherent uncertainty associated with adjustable model parameters and consider this uncertainty (i) in long-term predictions of groundwater quality changes and (ii) in scenarios that investigate the effectiveness of modifications in the water treatment process to mitigate metal release and mobility. The results suggested that Co, Ni, Zn, and Mn were comobilized during pyrite oxidation and that metal mobility was controlled (i) by the sediment pH buffering capacity and (ii) by the sorption capacity of the native aquifer sediments. Both tested mitigation strategies were shown to be effective at reducing the risk of elevated metal concentrations.

Reactive Transport Modeling for Exploring the Potential of Water Quality Sensors to Estimate Hydrocarbon Levels in Groundwater

AbstractPetroleum products have contaminated groundwater with harmful organic compounds, such as benzene, toluene, ethylbenzene, and xylenes (BTEX). Collecting and analyzing polluted groundwater samples is expensive and undertaken infrequently. However, quick remedial action in case of unexpected events requires continuous monitoring. In‐situ water quality sensors (pH, EC, DO, ORP) may show correlations with the components of dissolved petroleum hydrocarbon (PHC) such as aromatics and non‐volatile mobile fractions. Correlations are prerequisite to ultimately develop real‐time prediction models. Since suitable field data sets are limited, we simulated the fate of hydrocarbons in groundwater under various realistic conditions using a reactive transport model as novel approach to explore when, where, and why correlations occur. A stationary oil source zone continuously dissolved at the top of a heterogeneous and shallow sandy aquifer over a two‐dimensional cross‐section. Our model considered transient conditions (fluctuating water table) and spatially uniform hydrogeochemical composition. We observed a strong correlation between PHCs and water quality sensors (rolling Spearman's correlation > |0.8|) at varying periods. These correlations are strongly affected by the location of observation wells, the aquifer's hydraulic conductivity, and the availability of calcite and oxide minerals, and other electron acceptors. DO and ORP are significant for the early detection of hydrocarbon contamination, whereas pH and EC are important features for the long‐term monitoring of hydrocarbons. Our findings lay the foundation for the subsequent development of a data analysis model to detect and estimate in real time PHC levels in groundwater using in‐situ water quality sensors.

Antimony and arsenic migration in a heterogeneous subsurface at an abandoned antimony smelter under rainfall

While antimony (Sb) and arsenic (As) co-contamination in subsurface soil systems due to the legacy of Sb smelting wastes has been documented, the role of inherent heterogeneity on pollutant migration is largely overlooked. Herein this study investigated Sb and As migration in a slag impacted, vertically stratified subsurface at an abandoned Sb smelter. A 2-dimensional flume was assembled as a lab-scale analogue of the site and subject to rainfall and stop-rain events. Reactive transport modeling was then performed by matching the experimental observations to verify the key factors and processes controlling pollutant migration. Results showed that rainfall caused Sb and As release from the shallow slag layer and promoted their downward movement. Nevertheless, the less permeable deeper layers limited physical flow and transport, which led to Sb and As accumulation at the interface. The re-adsorption of Sb and As onto iron oxides in the deeper, more acidic layers further retarded their migration. Because of the large difference between Sb and As concentrations, Sb re-adsorption was much less effective, which led to higher mobility. Our findings overall highlight the necessity of understanding the degree and impacts of physicochemical heterogeneity for risk exposure assessment and remediation of abandoned Sb smelting sites.

A Critical Look at Colloid Generation, Stability, and Transport in Redox-Dynamic Environments: Challenges and Perspectives.

Colloid generation, stability, and transport are important processes that can significantly influence the fate and transport of nutrients and contaminants in environmental systems. Here, we critically review the existing literature on colloids in redox-dynamic environments and summarize the current state of knowledge regarding the mechanisms of colloid generation and the chemical controls over colloidal behavior in such environments. We also identify critical gaps, such as the lack of universally accepted cross-discipline definition and modeling infrastructure that hamper an in-depth understanding of colloid generation, behavior, and transport potential. We propose to go beyond a size-based operational definition of colloids and consider the functional differences between colloids and dissolved species. We argue that to predict colloidal transport in redox-dynamic environments, more empirical data are needed to parametrize and validate models. We propose that colloids are critical components of element budgets in redox-dynamic systems and must urgently be considered in field as well as lab experiments and reactive transport models. We intend to bring further clarity and openness in reporting colloidal measurements and fate to improve consistency. Additionally, we suggest a methodological toolbox for examining impacts of redox dynamics on colloids in field and lab experiments.

Technical note: A model of chemical transport in a wellbore–aquifer system

Abstract. Wellbore is proven to be the only effective way of delivering chemicals to a target aquifer during a tracer test or aquifer remediation. The volume of original water in the operational well is a critical parameter affecting the concentration of injected tracers or chemicals in the wellbore in the early stages. We found that the calculation of the wellbore water volume by previous numerical methods was correct when the wellbore penetrates an unconfined aquifer but incorrect when the wellbore penetrates a confined aquifer, further resulting in errors in describing the solute transport of injected chemicals in confined aquifers, such as MODFLOW/MT3DMS or FEFFLOW. Such errors caused by MODFLOW/MT3DMS and FEFFLOW increased with increasing wellbore water volume. This was because the groundwater in both the wellbore and aquifer was assumed to be confined where the water level was higher than the aquifer's top elevation and the groundwater thickness was assumed to be equal to the aquifer thickness. Actually, when the wellbore penetrated a confined aquifer, the groundwater was only confined in the aquifer, while it was unconfined in the wellbore. In this study, the solute transport model is revised based on the mass balance in a well–aquifer system, with special attention given to the wellbore water volume. The accuracy of the new model was tested against benchmark analytical solutions. The revised model could increase the accuracy of reactive transport modeling in aquifer remediation through the wellbore.

Enhancing in situ biodegradation in groundwater using pump and treat remediation: a proof of concept and modelling analysis of controlling variables.

A remediation approach which uses pump and treatment (PAT) to enhance the biodegradation of organic contaminants by increasing dispersive mixing between plumes and groundwater was evaluated for a phenol-contaminated aquifer, using a reactive transport model which simulates kinetic reactions between an electron donor (ED) in the plume and electron acceptor (EA) in the groundwater. The influence of system design and operation was examined in six modelling scenarios. Injection or extraction of groundwater increases biodegradation above no action and the location, pumping rate, and distance between well(s) are important variables which influence biodegradation. An increase in pumping rate, distance of the wells from the plume centreline, and changing the flow direction increase dispersive mixing between the plume and groundwater. This increases plume spreading and the plume fringe interface, providing a greater flux of dissolved EAs for biodegradation. In general, injection of groundwater containing natural EAs enhances biodegradation more than extraction. The enhancement of biodegradation is sensitive to the relative fluxes of ED and EA, as controlled by the arrangement of the wells. In the best performing scenario, biodegradation was enhanced by 128%, compared with no action.

Lagrangian modelling of reactive contaminant transport in the multi-component marine medium

The particle-tracking method for transporting radionuclides in multicomponent ocean medium (water and multifractional suspended and deposited sediments) is considered using a probabilistic approach for simulating interaction processes between several states of radioactivity. The state transformations as a result of reactions of the first order were described using the master equation for the probability of the particle being in the given state. Transition probabilities between all possible states can be obtained from the numerical solution of the matrix master equation that is derived in the paper. In the first approximation, the Euler method was used to obtain a solution for the next time step. This approach can be applied to any linear system of equations describing phase transitions with any number of states, but it requires small values of the transition probabilities to ensure only a single-phase change during one time step. The paper also focuses on deriving the Lagrangian interface conditions between the water column and bottom deposition. To apply the probabilistic approach, the boundary conditions were considered as the reaction terms in a thin near-bottom interface layer in which boundary conditions were converted into source terms. For this layer, the corresponding master equation was derived to obtain transitional probabilities for particle states. The developed approaches were tested on numerical and analytical solutions of two test cases. It was found that the optimal thickness of the interface layer must be larger than the maximum vertical displacement of the particle during the one-time step, but it must be small enough to approximate the condition of uniform distribution of concentration in this layer.

A computationally efficient approach to model reactive transport during CO2 storage in naturally fractured saline aquifers

Energy storage is critical to any decarbonization, energy sustainability, and energy security strategy. Natural gas, CO2, and hydrogen will compete for sizeable geological storage sites in the future, and society needs to consider all possible storage sites. Saline aquifers and depleted hydrocarbon reservoirs have emerged as key sites for storage. While carbonate rocks possess significant storage potential, as evidenced by successful CO2-enhanced oil recovery (CO2-EOR) projects, most operational and planned Carbon Capture and Storage (CCS) initiatives primarily focus on sandstones. Consequently, extensive fundamental and applied research is necessary to advance technologies for carbonate rock utilization and capitalize on the storage capacities of vast saline aquifers and infrastructure available in near-abandoned depleted carbonate reservoirs. However, the challenges posed by the fracture systems and the substantial heterogeneity inherent to such reservoirs necessitate a comprehensive understanding of rock integrity and the role of fracture networks in the overall storage process. Moreover, modeling CCS in carbonates introduces additional complexities due to reactive minerals, such as calcite and dolomite, which can undergo reactions with injected CO2. Consequently, a numerical formulation capable of producing accurate results within a reasonable computational time is required, considering computationally intensive processes like fracture flow and geochemical reactions. In this work, three approaches to modeling naturally fractured media were assessed, such as the explicit representation of fractures through local grid refinement (LGR), the traditional dual permeability/dual porosity model (DPDK), and the embedded discrete fracture model (EDFM). The combination of EDFM with dynamic gridding in the matrix gridblocks demonstrated superior computational efficiency compared to LGR and DPDK simulations. This novel approach was 75 times faster than LGR and 37 times faster than DPDK while yielding comparable results in terms of different CO2 retention forms (free supercritical fluid, dissolved in water, and residual trapped gas) in one of the geological models investigated. Therefore, it was proposed a novel combination of EDFM and dynamic gridding as a suitable and accurate numerical approach to simulate reactive CO2 transport in fractured saline aquifers. This approach offers a practical means of efficiently evaluating carbonate reservoir targets for CCS projects, enabling effective decision-making in energy storage endeavors.

Reactive transport modeling constraints on the complex genesis of a lacustrine dolomite reservoir: A case from the Eocene Qaidam Basin, China

Reactive transport modeling (RTM) is an emerging method used to address geological issues in diagenesis research. However, the extrapolation of RTM results to practical reservoir prediction is not sufficiently understood. This paper presents a case study of the Eocene Qaidam Basin that combines RTM results with petrological and mineralogical evidence. The results show that the Eocene Xiaganchaigou Formation is characterized by mixed siliciclastic-carbonate-evaporite sedimentation in a semiclosed saline lacustrine environment. Periodic evaporation and salinization processes during the syngenetic-penecontemporaneous stage gave rise to the replacive genesis of dolomites and the cyclic enrichment of dolomite in the middle-upper parts of the meter-scale depositional sequences. The successive change in mineral paragenesis from terrigenous clastics to carbonates to evaporites was reconstructed using RTM simulations. Parametric uncertainty analyses further suggest that the evaporation intensity (brine salinity) and particle size of sediments (reactive surface area) were important rate-determining factors in the dolomitization, as shown by the relatively higher reaction rates under conditions of higher brine salinity and fine-grained sediments. Combining the simulation results with measured mineralogical and reservoir physical property data indicates that the preservation of original intergranular pores and the generation of porosity via replacive dolomitization were the major formation mechanisms of the distinctive lacustrine dolomite reservoirs (widespread submicron intercrystalline micropores) in the Eocene Qaidam Basin. The results confirm that RTM can be effectively used in geological studies, can provide a better general understanding of the dolomitizing fluid-rock interactions, and can shed light on the spatiotemporal evolution of mineralogy and porosity during dolomitization and the formation of lacustrine dolomite reservoirs.