Reactive Mass Transport Model Research Articles

Effects of Paleoregolith and Fault Offset on the Formation of Unconformity-Type Uranium Deposits

Regional paleoregolith is found to exist immediately below unconformities separating basin fills from basement rocks in sedimentary basins. However, the controlling role of paleoregolith on unconformity-type uranium mineralization has not been quantitatively addressed before. Coupled hydrothermal fluid flow and reactive mass transport modeling are therefore performed in this study by using the software TOUGHREACT. The modeling results reveal that preferential flow occurs in the regolith due to its relatively high permeability in comparison with that of the host rocks. The thicker the regolith is, the more concentrated the fluids in the footwall of a fault zone are, leading to more compact and higher-grade deposits therein, and vice versa. Also, displacement of the regolith caused by fault offset plays an important role, as it appears to control the shape of uranium deposits. When the displacement is less than 30 m, the deposits are characterized by a more compact shape. When the displacement is over 60 m, the deposits extend more laterally and even exhibit a ‘discrete’ shape due to the expelling effect of downslope flow that occurs at the fault offset site.

Reactive mass transport in concrete including for gaseous constituents using a two-phase moisture transport approach

This article presents the further development of a tool for multi-phase reactive mass transport modeling for durability estimation of cement-based materials, by the addition of the gas phase, adopting a truly separate two-phase moisture transport approach. The governing system of equations are based on physically sound hybrid mixture based version of the Poisson-Nernst-Planck system of equations including gaseous transport in the air-filled space, ionic transport in the liquid phase, electro-migration of ionic species, a two-phase moisture transport model, and sorption. The addition of the gas phase and the two-phase moisture description enables the user of the model to investigate individual and combined effects of different degradation processes in unsaturated systems. The altering of hydrated cement under three different environments were studied representing an accelerated carbonation environment, a submerged marine environment and a cyclic drying-wetting zone in a marine environment, to illustrate some of the model’s capabilities.

CO2 Plume Migration and Fate at Sleipner, Norway: Calibration of Numerical Models, Uncertainty Analysis, and Reactive Transport Modelling of CO2 Trapping to 10,000 Years

The Sleipner Project in Norway is the world's first industrial-scale geological carbon dioxide storage project. Time-lapse seismic monitoring data have been collected, tracing CO2 plume development from 1996 to 2010. Therefore, the Sleipner Project provides a somewhat unique opportunity to simulate the dynamics of CO2 in a real geological system. The purpose of this study is to simulate CO2 plume migration dynamics and assess the impact of uncertain factors on short and long term migration and fate of CO2 for the uppermost layer (Layer 9) of the Utsira Sand.First, we applied a multi-phase compositional simulator to the Sleipner Benchmark model for Layer 9 and calibrated our model against the time-lapsed seismic monitoring data at the site from 1999 to 2008. By adjusting lateral permeability anisotropy, CH4 in the CO2 stream, and reservoir temperature, approximate match with the observed plume was achieved. Model-predicted gas saturation, thickness of the CO2 accumulation, and CO2 solubility in brine (none of them used as calibration metrics) were all comparable with interpretations of the seismic data in the literature.Second, hundreds of simulations of parameter sensitivity (pressure, temperature, feeders, spill rates, relative permeability curves, and CH4 content) were conducted for the plume migration, based on the calibrated model. The results showed that simulated plume extents are sensitive to permeability anisotropy, temperature, and CH4 content, but not sensitive to the other parameters. However, adjusting a single parameter within the reported range of values in the literature would not reproduce the north-south trending CO2 plume; it took a combination of permeability, CH4, and temperature adjustments to match simulated CO2 plume with seismic monitoring data. Although there is a range of uncertain parameters, the predicted fate of CO2 fell within a narrow band, ∼ 93±2% structural trapping and ∼ 7±2% solubility trapping. The calibrated model is not unique. Many combinations of permeability anisotropy, temperature, and CH4 would produce similar matches. Other possibilities that would have improved the development of an N–S elongated CO2 plume, such as a slight tilting of the surface of Utsira top to the south, were not experimented in this study, but are worthy of exploration for future studies.Finally, we used coupled reactive mass transport model to investigate the effects of rate laws and regional groundwater flow on long-term CO2 fate in Layer 9. The mineral composition and brine chemistry for the Utsira sand were adopted from the literature, and we modelled 100 year injection and continued water-rock interaction to 10,000 years. The results indicated that: (1) The predicted fraction of CO2 mineral trapping when using the linear rate law for feldspar dissolution is twice as much as when using the non-linear rate law. (2) Mineral trapping is more significant when regional groundwater flow is taken into consideration. Under the influence of regional groundwater flow, the replenishment of fresh brine from upstream continuously dissolves CO2 at the tail of CO2 plume, generating a larger acidified area where mineral trapping takes place. In a Sleipner like aquifer, the upstream replenishment of groundwater results in ∼ 22% mineral trapping at year 10,000, compared to the ∼ 4% when the effects of regional groundwater are ignored.

Redox Roll-Front Mobilization of Geogenic Uranium by Nitrate Input into Aquifers: Risks for Groundwater Resources.

Redox conditions are seen as the key to controlling aqueous uranium concentrations (cU(aq)). Groundwater data collected by a state-wide groundwater quality monitoring study in Mecklenburg-Western Pomerania (Germany) reveal peak cU(aq) up to 75 μg L-1 but low background uranium concentrations (median cU(aq) < 0.5 μg L-1). To characterize the hydrogeochemical processes causing such groundwater contamination by peak cU(aq), we reanalyzed measured redox potentials and total concentrations of aqueous uranium, nitrate, and sulfate species in groundwater together with their distribution across the aquifer depth and performed semigeneric 2D reactive mass transport modeling which is based on chemical thermodynamics. The combined interpretation of modeling results and measured data reveals that high cU(aq) and its depth-specific distribution depending on redox conditions is a result of a nitrate-triggered roll-front mobilization of geogenic uranium in the studied aquifers which are unaffected by nuclear activities. The modeling results show that groundwater recharge containing (fertilizer-derived) nitrate drives the redox shift from originally reducing toward oxidizing environments, when nitrate input has consumed the reducing capacity of the aquifers, which is present as pyrite, degradable organic carbon, and geogenic U(IV) minerals. This redox shift controls the uranium roll-front mobilization and results in high cU(aq) within the redoxcline. Moreover, the modeling results indicate that peak cU(aq) occurring at this redox front increase along with the temporal progress of such redox conversion within the aquifer.

Change in cap rock porosity triggered by pressure and temperature dependent CO2–water–rock interactions in CO2 storage systems

Abstract Carbon capture and storage in deep geological formations is a method to reduce greenhouse gas emissions. Supercritical CO 2 is injected into a reservoir and dissolves in the brine. Under the impact of pressure and temperature (P–T) the aqueous species of the CO 2 -acidified brine diffuse through the cap rock where they trigger CO 2 –water–rock interactions. These geochemical reactions result in mineral dissolution and precipitation along the CO 2 migration path and are responsible for a change in porosity and therefore for the sealing capacity of the cap rock. This study focuses on the diffusive mass transport of CO 2 along a gradient of decreasing P–T conditions. The process is retraced with a one-dimensional hydrogeochemical reactive mass transport model. The semi-generic hydrogeochemical model is based on chemical equilibrium thermodynamics. Based on a broad variety of scenarios, including different initial mineralogical, chemical and physical parameters, the hydrogeochemical parameters that are most sensitive for safe long-term CO 2 storage are identified. The results demonstrate that P–T conditions have the strongest effect on the change in porosity and the effect of both is stronger at high P–T conditions because the solubility of the mineral phases involved depends on P–T conditions. Furthermore, modeling results indicate that the change in porosity depends strongly on the initial mineralogical composition of the reservoir and cap rock as well as on the brine compositions. Nevertheless, a wide range of conditions for safe CO 2 storage is identified.

Hydrogen sulfide formation, fate, and behavior in anhydrite-sealed carbonate gas reservoirs: A three-dimensional reactive mass transport modeling approach

A novel hydrogeochemical modeling approach is developed to unravel thermochemical sulfate reduction (TSR) in hydrocarbon reservoirs. Our numerical model couples a web of interconnected hydrogeochemical reactions to three-dimensional (3-D) and reservoir-wide diffusive mass transport. Our modeling approach simulates a semigeneric gas reservoir sealed by anhydrite. The calculated diagenetic processes fit the observations in reservoirs affected by TSR: formation of water, precipitation of calcite, metal (di-)sulfides, and elemental sulfur as replacements of dissolved anhydrite at the expense of CH4(g), as well as formation of hydrogen sulfide (H2S). By varying input parameters, the crucial factors controlling TSR have been identified. Our results highlight that reservoir-wide diffusive mass transport is one prerequisite for TSR. An increase in the rate constant of abiotic sulfate reduction (ASR) and in diffusive mass fluxes, as well as lack of precursor minerals for metal (di-)sulfide precipitation, can increase the souring intensity and accelerate H2S outgassing. In contrast, precipitation of elemental sulfur, which is stable according to the chemical thermodynamics, weakens H2S formation. Our modeling shows that TSR is complex and cannot be represented by the single reaction ASR and by simple correlations between the rate constant of ASR and the H2S gas content. The application of 3-D reactive transport modeling presented here, despite its semigeneric nature, provides a good example of how such an approach can be used ahead of drilling. Our modeling helps to investigate TSR in time and space to quantify the mass conversion of all reactants involved within this web and to predict the souring level.

Impacts of Mineral Reaction Kinetics and Regional Groundwater Flow on Long-Term CO2 Fate at Sleipner

We conducted coupled reactive mass transport modeling of CO2 storage in a sandy aquifer resembling the uppermost layer in the Utsira Sand, Sleipner, North Sea, in order to investigate the general effects of rate laws and regional groundwater flow on long-term CO2 fate in saline aquifers. The temporal and spatial evolution of CO2 plume and the fate of injected CO2 were simulated with a series of scenarios with different rate law formulations for dissolution and precipitation reactions and different flow regimes. The results indicated the following: (1) Changing the dissolution rate laws of the main soluble silicate minerals can influence the silicate reactions and mineral trapping by impacting the sensitivity of the relevant coupled reaction’s rate to the acidification of brine. The steeper the slope of rate−ΔGr (Gibbs free energy of reaction) relationships, the more sensitive the coupled reaction rate and the mineral trapping are to the acidification of brine. The predicted fraction of CO2 mineral trappin...

Temporal and spatial development of scaling in reservoir aquifers triggered by seawater injection: Three-dimensional reactive mass transport modeling of water–rock–gas interactions

Abstract A new numerical modeling approach enables us to calculate the spatial and temporal development of chemical water–rock–gas interactions, including scaling in oil reservoirs which undergo seawater injection. This approach links such simultaneous hydrogeochemical interactions with the three-dimensional flow of pore water in a semi-generic reservoir aquifer. Zero-dimensional modeling and calculating saturation indices, which are based on just one seawater and one formation water analysis, are commonly used to evaluate the type and the intensity of wellbore scaling. This type of generalization about the fate and behavior of minerals (dissolution or precipitation) is incapable of correctly predicting scale formation. Our modeling results show that scaling and other water–rock–gas interactions are integrated in a complex web and are coupled to the flow of pore water. Even the same mineral shows different hydrogeochemical behaviors at different reservoir locations. In our case study, calcite dissolves near the injectors and is precipitated within the producers. The injection strategy determines hydraulics processes which lead to the mixing of seawater with formation water throughout the reservoir aquifer and within producers. Consequently, non-autoscales newly form (1) widely within the reservoir aquifer due to dispersion, and (2) intensively at the spots where the margins of seawater plumes approach to and converge close to the producers. On the other hand, water injection triggers the dissolution of primary minerals. Consequently, aqueous ions are released into pore water which later flows to producers. Such ions can be sequestrated as scale minerals in the reservoir aquifers or in the producers. Thus, coupled hydraulic and hydrogeochemical processes constantly alter the composition of seawater. Accordingly, original seawater will not reach the producer. In terms of equilibrium thermodynamics, scaling is an inevitable consequence of seawater injection. However, our modeling results reveal that several parameters that could be technically controlled can strongly affect the intensity of scaling processes as well as their spatial and temporal development, for example, the spatial arrangement of injectors and producers, decrease in total pressure, and CO 2 partial pressure in the pressure drop zone surrounding producers. Our study demonstrates that three-dimensional modeling is a useful tool for identifying the type of scale minerals and for quantifying their spatial and temporal distribution. It can help to predict the areas where the porosity and permeability properties of reservoirs strongly change due to mineral dissolution and/or precipitation induced by seawater injection. Different modeling scenarios can be calculated for case-specific hydrogeochemical and hydraulic conditions in oilfields of interest. The results gained about the distribution, the amount and the timing of scale formation help to optimize the water injection strategy in order to avoid the “worst case” of scale formation in reservoirs to extend the wellbore life.

Use of a multi-species reactive transport model to simulate chloride ingress in mortar exposed to NaCl solution or sea-water

Simulations of ion ingress in Portland cement mortar using a multi-species reactive mass transport model are compared with experimental test results. The model is an extended version of the Poisson–Nernst–Planck equations, accounting for chemical equilibrium. Saturated mortar samples were exposed after 8days of hydration to a NaCl-solution or natural sea-water in a laboratory controlled environment for the experimental part. The samples were analyzed for the total chloride content after 21, 90 and 180days of exposure, at varying depths from the exposed surface. The applicability of the reactive mass transport model is shown by comparing the simulation and experimental results. The tortuosity factor used in the simulations is adjusted to obtain the best reproduction of the experimental results. The model predicts the total chloride content satisfactorily, despite assumptions in the simulation like fixed hydration degree over time. Improvements and suggestions for further development of the model are discussed, e.g. extended hydration description, improved overall chemical description and a more strict use of the tortuosity factor.

Effect of graphite zone in the formation of unconformity-related uranium deposits: Insights from reactive mass transport modeling

In this study, reactive mass transport modeling is conducted for evaluating the role of faulted graphite zones in the formation of unconformity-related uranium (URU) deposits. For this aim, two different reducing mechanisms are examined for the precipitation of uraninite in a typical URU deposit. In the first mechanism, precipitation of the uraninite involves methane as a reducing agent which is produced by the alteration of the graphite zone. The second reducing mechanism does not incorporate methane as a reducing agent, and oxygen is used for formulating the redox reaction of uraninite precipitation. Numerical simulations show the following results. Firstly, by employing either reducing mechanisms, uraninite can precipitate in the basement close to the unconformity interface. Secondly, uraninite can precipitate below the unconformity interface away from the faulted graphite zone even if methane is not involved as a reducing agent. Physiochemical parameters such as oxygen fugacity and temperature play a significant role in localization of uraninite. Localization of uraninite below the unconformity interface is related to the decrease of oxygen fugacity, generally resulting from the interaction of oxidized uranium-bearing fluids with reductants. Through the second reducing mechanism, uraninite cannot precipitate around the faulted graphite zone. In comparison with the precipitation by the first reducing mechanism, it takes longer time for uraninite to precipitate through the second mechanism, and the volume fraction of uraninite precipitated by the second mechanism is lower than that by the first mechanism. Finally, faulted graphite zone has a major role in providing the pathway for transporting the fluid. The uranium bearing brines flow into the faulted graphite zone and interact with the basement lithology. Also, the basal fluids use faulted graphite zone as a conduit to mix with the basinal fluids. Maximum fluid flow rate happens along the faulted graphite zone because of the high permeability of this zone in comparison with that of other stratigraphic units present in the model.

Preliminary modeling of the long-term fate of CO2 following injection into deep geological formations

The injection of CO2 into deep saline aquifers is a potential option for greenhouse gas mitigation. However, several key issues, such as underground storage time and the fate of the injected CO2, must be studied before this option becomes economically and socially acceptable. In the current study, a one-dimensional reactive mass-transport model was used to predict the long-term chemical behavior of a deep saline aquifer following CO2 injection, far away from the injection site and representative of basin-scale migration and long-term fate. The dissolution of the injected CO2 into brine causes a sharp drop in pH, and consequently, the acidic brine aggressively reacts with aquifer minerals. Our model also predicts the dissolution of aluminosilicate minerals with the formation of secondary minerals and the precipitation and dissolution of carbonate minerals and is consistent with laboratory-scale CO2 core-flooding experiments. However, the extent and development of reaction fronts depend on the reaction rates used. For example, our modeling results indicate that the transport of carbon can be significantly retarded with respect to the flow of the brine itself, and a significant amount of injected CO2 is immobilized because of mineral trapping. The precise locations and patterns of the carbon reactive transport are sensitive to the reaction rates used, illustrating the need for improved knowledge of reaction kinetics, particularly the in-situ rates of dissolution and precipitation of aluminosilicate minerals, in evaluating mineral trapping of CO2 in deep geological formations.

Coprecipitation in the barite isostructural family: 2. Numerical simulations of reactions and mass transport

Coprecipitation of barite with trace constituents was simulated with consideration of aqueous speciation and complexation, mixing properties for the binary solid solutions (Zhu, this issue), precipitation and dissolution kinetics, and advective-dispersive transport. Speciation-solubility modeling was used to reproduce BaSO 4-RaSO 4 coprecipitation experimental results, and to calculate CrO 4 2− aqueous concentrations in equilibrium with a Ba(SO 4,CrO 4) solid solution. Kinetic reaction path modeling was used to simulate the coprecipitation of barite with RaSO 4 to form an onion-like chemically zoned solid upon the cooling of oil field brine. A one-dimensional coupled reactive mass transport model shows a strikingly different transport pattern for the tracer Ra 2+, when the dominant attenuation reaction is with solid solution (Ba, Ra) SO 4 as compared to the case when it is controlled by pure RaSO 4 and barite solids under local equilibrium conditions. A self-enrichment of Ra 2+ in the groundwater and aquifer solid matrix—higher concentrations of Ra 2+ downstream from the reaction front—results from the coprecipitation reaction and advective-dispersive transport. This self-enrichment process generates a secondary tracer source, which has tracer concentrations higher than that of the original source. On the other hand, coprecipitation reactions can reduce Ra 2+ concentrations in groundwater to a much lower level (below ppb) than that of pure RaSO 4(c) solubility (near ppm), which has been used to establish the Ra 2+ concentration limits in groundwater, soil, and nuclear waste repositories.

A case against Kd-based transport models: natural attenuation at a mill tailings site

This study compares numerical modeling results of contaminant transport using a multi-component coupled reactive mass transport model and a distribution coefficient ( K d)-based transport model. The study site is a contaminated groundwater aquifer underneath a uranium mill tailings pond in the western USA. Advective–dispersive–reactive transport is simulated for a 5-year period of intrusion of tailings fluid into the shallow aquifer, followed by flushing with uncontaminated upgradient groundwater for 1600 years. The coupled model shows that groundwater–sediment interactions result in multiple concentration waves, strong interactions among solutes, and chemical heterogeneity in both space and time. As a result, calculated K d values vary spatially and temporarily. None of these characteristics can be simulated with a K d-based model. These results illustrate the shortcomings of the K d approach, the usage of which is prevalent in the regulatory environment.

Mineralogical compositions of aquifer matrix as necessary initial conditions in reactive contaminant transport models

Mineralogical compositions and their spatial distributions are important initial conditions for reactive transport modeling. However, popular K d-based “reactive” transport models only require contaminant concentrations in the pore fluids as initial conditions, and minerals implicitly represent infinite sources and sinks in these models. That situation results in a general neglect of mineralogical characterization in site investigations. This study uses a coupled multi-component reactive mass transport model to predict the natural attenuation of a ground water plume at a uranium mill tailings site in western USA. Numerous ground water geochemistry data are available at this site, but mineralogical data are sketchy. Even given the well-defined pore fluid chemistry, variations of secondary mineral species and mineral abundances in the aquifer resulted in significantly different modeling outcomes. Results show that the amount of calcite in the aquifer determines the distances of plume migration. The possible presence of jurbanite, an aluminum sulfate phase, can store acidity temporarily but cause more severe contamination on a later date. The surfaces of iron oxyhydroxides can store significant amounts of sulfate and protons and serve as a second source for prolonged contamination. These simulations under field conditions illustrate that mineralogical compositions are an essential requirement for accurate prediction of contaminant fate and transport.