Long-term non-isothermal reactive transport model of compacted bentonite, concrete and corrosion products in a HLW repository in clay

Abstract. The geological disposal in deep bedrock repositories is the preferred option for the management of high-level radioactive waste. In some of these concepts, carbon steel is considered as potential canister material and bentonites are planned as backfill material to protect metal waste containers. Therefore, a 1D radial reactive transport model has been developed in order to better understand the processes occurring during the long-term iron–bentonite interaction. The conceptual model accounts for diffusion, chemistry of the porewater and aqueous complexation reactions, mineral dissolution/precipitation and absorption, at a constant temperature of 25 ∘C under anoxic conditions. The geometry of the axisymmetric model reflects the canister–bentonite interface and the bentonite. The primary phases considered are montmorillonitic smectite, quartz, muscovite, albite, illite, pyrite and calcite. We assume that carbon steel is composed only of iron. The potential secondary phases considered are from reported experiments, such as magnetite, nontronitic smectite, greenalite, cronstedtite and siderite. The numerical model results suggest that at the iron–bentonite interface, Fe is adsorbed at the smectite surface via ion exchange in the short term and it is consumed by formation of the secondary phases in the long term. Furthermore, calcite precipitates are due to cation exchange in the short term and due to montmorillonitic smectite dissolution in the long term. The numerical model predicts the precipitation of nontronitic smectite, magnetite and greenalite as corrosion products. Results further reveal a significant increase in pH in the long term, whereas dissolution/precipitation reactions result in limited variations of the porosity. Progressing bentonite dissolution owing to the rising pH and concomitantly increasing silicate concentrations in the porewater induce formation of Fe-silicates as corrosion products at the expense of magnetite. A sensitivity analysis has also been performed to study the effect of selected parameters, such as corrosion rate, diffusion coefficient and composition of the porewater, on the corrosion products. Overall, outcomes suggest that pH and concentration of dissolved Si play an important role in corrosion mechanisms. The predicted main secondary phases in the long term are Fe-silicate minerals. Thus, such phases deserve further attention as possible chemical barriers for radionuclide migration in the repository near-field.

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How Insoluble Inclusions and Intersecting Layers Affect the Leaching Process within Potash Seams

Applied Sciences ◽

10.3390/app11199314 ◽

2021 ◽

Vol 11 (19) ◽

pp. 9314

Author(s):

Svenja Steding ◽

Thomas Kempka ◽

Michael Kühn

Keyword(s):

Reactive Transport ◽

Mechanical Stability ◽

Transport Model ◽

Heterogeneous Systems ◽

Vertical Direction ◽

Risk Assessments ◽

Mining Operations ◽

Reactive Transport Model ◽

Zone Growth

Potash seams are a valuable resource containing several economically interesting, but also highly soluble minerals. In the presence of water, uncontrolled leaching can occur, endangering subsurface mining operations. In the present study, the influence of insoluble inclusions and intersecting layers on leaching zone evolution was examined by means of a reactive transport model. For that purpose, a scenario analysis was carried out, considering different rock distributions within a carnallite-bearing potash seam. The results show that reaction-dominated systems are not affected by heterogeneities at all, whereas transport-dominated systems exhibit a faster advance in homogeneous rock compositions. In return, the ratio of permeated rock in vertical direction is higher in heterogeneous systems. Literature data indicate that most natural potash systems are transport-dominated. Accordingly, insoluble inclusions and intersecting layers can usually be seen as beneficial with regard to reducing hazard potential as long as the mechanical stability of leaching zones is maintained. Thereby, the distribution of insoluble areas is of minor impact unless an inclined, intersecting layer occurs that accelerates leaching zone growth in one direction. Moreover, it is found that the saturation dependency of dissolution rates increases the growth rate in the long term, and therefore must be considered in risk assessments.

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A Streamlined Workflow From Experimental Analyses to Dynamic Geochemical Modelling

10.2118/205128-ms ◽

2021 ◽

Author(s):

Ahmed M. S. Elgendy ◽

Simone Ricci ◽

Elena I. Cojocariu ◽

Claudio Geloni

Keyword(s):

Reactive Transport ◽

Transport Model ◽

Geochemical Modelling ◽

Formation Water ◽

Geochemical Model ◽

Reactive Transport Model ◽

Mineral Alteration ◽

Experimental Analyses ◽

Water Vaporization

Abstract Dynamic-geochemical model is a powerful instrument to evaluate the geochemical effects on CO2storage capacity, injectivity and long-term containment. The study objective is to apply an integrated multi-step workflow to a carbon capture and storage (CCS) candidate field (offshore), namely hereinafter H field. From experimental analyses, a comprehensive real data-tailored reactive transport model (RTM) has been built to capture the dynamics and the geochemical phenomena (e.g., water vaporization, CO2solubility, mineral alteration) occurring during and after the CO2injection in sedimentary formations. The proposed integrated workflow couples lab activities and numerical simulations and it is developed according to the following steps: Mineralogical-chemical characterization (XRD, XRF and SEM-EDX experimental techniques) of field core samples; Data elaboration and integration to define the conceptual geochemical model; Synthetic brine reconstruction by means of 0D geochemical models; Numerical geochemical modelling at different complexity levels. Field rocks chosen for CO2injection have been experimentally characterized, showing a high content of Fe in clayey, micaceous and carbonate mineralogical phases. New-defined, site-specific minerals have been characterized, starting from real XRD, XRF and SEM-EDX data and by calculation of their thermochemical parameters with a proprietary procedure. They are used to reconstruct synthetic formation water chemical composition (at equilibrium with both rock mineralogy and gas phase), subsequently used in RTM. CO2injection is simulated using 2D radial reactive transport model(s) built in a commercial compositional reservoir simulator. The simulations follow a step-increase in the complexity of the model by adding CO2solubility, water vaporization and geochemical reactions. Geochemical processes impact on CO2storage capacity and injectivity is quantitatively analyzed. The results show that neglecting the CO2solubility in formation water may underestimate the max CO2storage capacity in H field by around 1%, maintaining the same pressure build-up profile. Sensitivities on the impact of formation water salinity on the CO2solubility are presented. In a one thousand years’ time-scale, changes in reservoir porosity due to mineral alteration, triggered by CO2-brine-rock interactions, seem to be minimal in the near wellbore and far field. However, it has been seen that water vaporization with the associated halite precipitation inclusion in the simulation models is recommended, especially at high-level of formation brine salinity, for a reliable evaluation of CO2injectivity related risks. The proposed workflow provides a new perspective in geochemical application for CCS studies, which relies on novel labs techniques (analyses automation), data digitalization, unification and integration with a direct connection to the numerical models. The presented procedure can be followed to assess the geochemical short-and long-term risks in carbon storage projects.

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Understanding of Long-Term CO2-Brine-Rock Geochemical Reactions Using Numerical Modeling and Natural Analogue Study

Geofluids ◽

10.1155/2019/1426061 ◽

2019 ◽

Vol 2019 ◽

pp. 1-16

Author(s):

Huixing Zhu ◽

Tianfu Xu ◽

Hailong Tian ◽

Guanhong Feng ◽

Zhijie Yang ◽

...

Keyword(s):

Reactive Transport ◽

Transport Model ◽

Measured Data ◽

Natural Analogue ◽

Reactive Transport Model ◽

Analogue Study ◽

Geochemical Reactions ◽

Acidic Conditions ◽

Simulation Results

To further understand the interactions of CO2-brine-rock at geological time scales, in this study, a 1D reactive transport model of CO2 intrusion into sandstone of the Longtan Formation (P2l) in the Huangqiao area, China, was constructed based on site-specific data. The simulation time is consistent with the retention time of CO2 in the Longtan sandstone Formation and is set to 20 Ma. The reactive transport model is calibrated and revised using the measured data for sandstone samples from Well X3 (i.e., the natural analogue). By comparing the simulation results with measured data for the natural analogue, the long-term geochemical reactions are investigated. The simulation results indicate that the brine-rock interactions induced by CO2 can be roughly divided into two stages. First, susceptible minerals (e.g., chlorite, ankerite, calcite, and feldspar minerals) dissolve rapidly under acidic conditions formed by the dissolution of CO2. The precipitation of siderite is facilitated by the dissolution of ankerite and chlorite. Smectite-Ca and dawsonite precipitate due to the dissolution of anorthite and albite, respectively. Dawsonite begins to convert into smectite-Na when albite is completely dissolved. As the reactions continue, intermediate products (i.e., illite, smectite-Na, and smectite-Ca) generated in the first stage become the reactants and subsequently react with CO2 and brine. These three clay minerals are not stable under acidic conditions and transform into kaolinite and paragenetic quartz in the later stage of reaction. Comparing the simulation results of the Base Case with the measured data for the natural analogue and inspired by previous studies, the scour of kaolinite is supposed to have occurred in this region and is considered in the revised model by introducing a coefficient of the scour of kaolinite (i.e., Case 2). The simulation results of Case 2 fit well with the measured data on mineral assemblage, and the trend of the sandstone porosity growth caused by the CO2-brine-rock reaction is captured by our simulation results. The combination of numerical simulation and natural analogue study indicates that the joint effects of long-term CO2-brine-rock reactions and scour of kaolinite increase the pore space of the host rock and result in an increase in quartz content in the sandstone.

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