HS8.1.5

Acidic crater lakes are common features of subaerial volcanic systems; indeed, research suggests the existence of over 700 volcanic lakes around the world. Their persistence requires a regular input of water (e.g., meteoric water) at a rate that exceeds the migration of fluid from the system—for example, due to evaporation or fluid flow through the porous edifice.  Flank aquifers and fumarole fields may similarly be strongly acidic environments.

In order to explore the evolution of the physical and mechanical properties of an andesite under these field-relevant chemical conditions, we performed batch reaction experiments over timescales from 1 day to 4 months. The experiments involved immersion of a suite of samples in a solution of sulfuric acid (0.125 M; pH ∼0.6). Periodically, samples were removed and their physical and mechanical properties measured. We observe a progressive loss of sample mass, along with a general increase in porosity. We attribute this to the dissolution of plagioclase,  accompanied by the generation of a microporous diktytaxitic groundmass due to glass dissolution.

Plagioclase phenocrysts are seen to undergo progressive pseudomorphic replacement by an amorphous phase enriched in silica and depleted in other, relatively more soluble, cations (Na, Ca, and Al). In the first phase of dissolution (i.e. between 1 and 10 days), this process appears to be confined to preexisting fractures within the plagioclase phenocrysts. Ultimately, however, these phenocrysts tend toward entire replacement by amorphous silica. We do not observe evidence of induced dissolution or alteration in the other mineral constituents of the material: pyroxene, cristobalite, and titanomagnetite, specifically.

Examining the required Klinkenberg corrections during permeability measurements, we quantitatively demonstrate that the relative aperture of flow pathways increases with progressive acid immersion, by as much as a factor of five. We propose that the dissolution process results in the widening of pore throats and the improvement of pore connectivity, with the effect of increasing permeability by over an order of magnitude relative to the initial measurements. Compressive strength of our samples was also decreased, insofar as porosity tends to increase.

We highlight broader implications of the observed permeability increase and strength reduction for volcanic systems including induced flank failure and related hazards, improved efficiency of volatile migration, and attendant eruption implications.

How to cite: Farquharson, J., Wild, B., Kushnir, A., Heap, M., Baud, P., and Kennedy, B.: Strength and permeability evolution of andesite during benchtop acid dissolution experiments: implications for volcanic systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-883, https://doi.org/10.5194/egusphere-egu21-883, 2021.

Salt deposits host an important industrial raw material and provide storage capacities for energy and nuclear waste. However, leaching zones can seriously endanger the development and utilisation of salt deposits for these purposes, especially if these occur in potash seams. Their increased solubility enables even NaCl-saturated solutions, if present, to deeply penetrate these seams. The resulting salt dissolution processes generate fluid flow paths and affect the mechanical rock integrity. To model the timely evolution of leaching zones and to assess their hazard potential, a reactive transport model has been developed, taking into account not only the complex dissolution and precipitation behaviour of potash salts, but also the resulting porosity and permeability changes as well as density-driven chemical species transport. Additionally, the model makes use of an approach to describe transport and chemical reactions at the interface between impermeable (dry) salt rocks and permeated leaching zones (Steding et al., 2021). In the present study, we focus on the effect of heterogeneity of the mineral distribution within potash seams and on the influence of mineral- and saturation-dependent dissolution rates.

The applied reactive transport model is based on a coupling of the geochemical module PHREEQC (Parkhurst & Appelo, 2013) with the TRANSport Simulation Environment (Kempka, 2020) as well as the newly developed extension of an interchange approach (Steding et al., 2021). A numerical model has been developed and applied to simulate the leaching process of a carnallite-bearing potash seam due to natural density-driven convection. The results show that both, the mineral composition and dissolution rate of the original salt rock, strongly influence the shape and evolution of the leaching zone (Steding et al., 2021).

In nature, strong variations of the mineralogy occur within potash seams with random or stratified distributions. Furthermore, dissolution rates depend on the mineral itself as well as on its saturation state. Both may considerably influence the growth rate of a leaching zone. Therefore, the reactive transport model has been extended by mineral- and saturation-dependent dissolution rates. A scenario analysis has been undertaken to compare the impact of homogeneous and heterogeneous rock compositions. For that purpose, the carnallite content in the potash seam was varied from 5 to 25 wt. % including different stratifications and random distributions. The simulations were classified by means of the Péclet and Damköhler numbers, and the long-term behaviour as well as hazard potential are discussed.

References:

Parkhurst, D.L.; Appelo, C.A.J. (2013). Description of Input and Examples for PHREEQC Version 3 - a Computer Program for Speciation, Batch-reaction, One-dimensional Transport, and Inverse Geochemical Calculations. In Techniques and Methods; Publisher: U.S. Geological Survey; Book 6, 497 pp

Kempka, T. (2020). Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Adv. Geosci. 54, 67–77.

Steding, S.; Kempka, T.; Zirkler, A.; Kühn, M. (2021). Spatial and temporal evolution of leaching zones within potash seams reproduced by reactive transport simulations. Water 13, 168.

How to cite: Steding, S., Kempka, T., Zirkler, A., and Kühn, M.: Inhomogeneous rock compositions and varying dissolution rates affect evolution and shape of leaching zones in potash seams, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1196, https://doi.org/10.5194/egusphere-egu21-1196, 2021.

Natural gas hydrates are considered as one of the most promising alternatives to conventional fossil energy sources, and are thus subject to world-wide research activities for decades. Hydrate formation from methane dissolved in brine is a geogenic process, resulting in the accumulation of gas hydrates in sedimentary formations below the seabed or overlain by permafrost. The LArge scale Reservoir Simulator (LARS) has been developed (Schicks et al., 2011, 2013; Spangenberg et al., 2015) to investigate the formation and dissociation of gas hydrates under simulated in-situ conditions of hydrate deposits. Experimental measurements of the temperatures and bulk saturation of methane hydrates by electrical resistivity tomography have been used to determine the key parameters, describing and characterising methane hydrate formation dynamics in LARS. In the present study, a framework of equations of state to simulate equilibrium methane hydrate formation in LARS has been developed and coupled with the TRANsport Simulation Environment (Kempka, 2020) to study the dynamics of methane hydrate formation and quantify changes in the porous medium properties in LARS. We present our model implementation, its validation against TOUGH-HYDRATE (Gamwo & Liu, 2010) and the findings of the model comparison against the hydrate formation experiments undertaken by Priegnitz et al. (2015). The latter demonstrates that our numerical model implementation is capable of reproducing the main processes of hydrate formation in LARS, and thus may be applied for experiment design as well as to investigate the process of hydrate formation at specific geological settings.

Key words: dissolved methane; hydrate formation; hydration; python; permeability.

References

Schicks, J. M., Spangenberg, E., Giese, R., Steinhauer, B., Klump, J., & Luzi, M. (2011). New approaches for the production of hydrocarbons from hydrate bearing sediments. Energies, 4(1), 151-172, https://doi.org/10.3390/en4010151

Schicks, J. M., Spangenberg, E., Giese, R., Luzi-Helbing, M., Priegnitz, M., & Beeskow-Strauch, B. (2013). A counter-current heat-exchange reactor for the thermal stimulation of hydrate-bearing sediments. Energies, 6(6), 3002-3016, https://doi.org/10.3390/en6063002

Spangenberg, E., Priegnitz, M., Heeschen, K., & Schicks, J. M. (2015). Are laboratory-formed hydrate-bearing systems analogous to those in nature?. Journal of Chemical & Engineering Data, 60(2), 258-268, https://doi.org/10.1021/je5005609

Kempka, T. (2020) Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Adv. Geosci., 54, 67–77, https://doi.org/10.5194/adgeo-54-67-2020

Gamwo, I. K., & Liu, Y. (2010). Mathematical modeling and numerical simulation of methane production in a hydrate reservoir. Industrial & Engineering Chemistry Research, 49(11), 5231-5245, https://doi.org/10.1021/ie901452v

Priegnitz, M., Thaler, J., Spangenberg, E., Schicks, J. M., Schrötter, J., & Abendroth, S. (2015). Characterizing electrical properties and permeability changes of hydrate bearing sediments using ERT data. Geophysical Journal International, 202(3), 1599-1612, https://doi.org/10.1093/gji/ggv245

How to cite: Li, Z., Kempka, T., Spangenberg, E., and Schicks, J.: Quantification of methane hydrate formation in the Large-scale Reservoir Laboratory Simulator (LARS) by numerical simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1312, https://doi.org/10.5194/egusphere-egu21-1312, 2021.

Quantifying trends in hydraulic and mechanical properties of reservoir sandstones has a wide practical importance for many applications related to geological subsurface utilization. In that regard, predicting macroscopic rock properties requires detailed information on their microstructure [1]. In order to fundamentally understand the pore-scale processes governing the rock behaviour, digital rock physics represents a powerful and flexible approach to investigate essential rock property relations [2]. This was shown, e.g., for hydraulic effects of anhydrite cement in the Bentheim sandstone in relation to an unsuccessful drilling campaign at the geothermal well Allermöhe, Germany [3]. Rock weakening due to decreasing calcite mineral content was also demonstrated by application of numerical simulations [4]. 

In the present study, a process-based method is used for reconstructing the full 3D microstructure of three typical reservoir reference rocks: the Fontainebleau, Berea and Bentheim sandstones. For that purpose, grains are initially deposited under the influence of gravity and afterwards diagenetically consolidated. The resulting evolution in porosity, permeability and rock stiffness is examined and compared to the respective micro-CT scans of the sandstones. The presented approach enables to efficiently generate synthetic sandstone samples over a broad range of porosities, comprising the microstructural complexity of natural rocks and considering any desired size, sorting and shape of grains. In view of a virtual laboratory, these synthetic samples can be further altered to examine the impact of mineral dissolution and/or precipitation as well as fracturing on various petrophysical correlations, what is of particular relevance for a sustainable exploration and utilisation of the geological subsurface.

[1] Wetzel M., Kempka T., Kühn M. (2017): Predicting macroscopic elastic rock properties requires detailed information on microstructure. Energy Procedia, 125, 561-570. DOI: 10.1016/j.egypro.2017.08.195 
[2] Wetzel M., Kempka T., Kühn M. (2020): Hydraulic and mechanical impacts of pore space alterations within a sandstone quantified by a flow velocity-dependent precipitation approach. Materials, 13, 4, 3100. DOI: 10.3390/ma13143100
[3] Wetzel M., Kempka T., Kühn M. (2020): Digital rock physics approach to simulate hydraulic effects of anhydrite cement in Bentheim sandstone. Advances in Geosciences, 54, 33-39. DOI: 10.5194/adgeo-54-33-2020 
[4] Wetzel M., Kempka T., Kühn M. (2018): Quantifying rock weakening due to decreasing calcite mineral content by numerical simulations. Materials, 11, 542. DOI: 10.3390/ma11040542 

How to cite: Wetzel, M., Kempka, T., and Kühn, M.: Digital reconstruction of reservoir sandstones to predict hydraulic and mechanical rock properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1512, https://doi.org/10.5194/egusphere-egu21-1512, 2021.

The modern advances in computing and experimental capabilities in the research of water-rock-interactions require geoscientists to routinely combine laboratory data and models to produce knowledge in order to solve pressing societal challenges connected to subsurface utilization. Data science is hence a more and more pervasive instrument also for  geochemists, which in turn demands flexible and easy to learn software adaptable to their specific needs. 
In this contribution we showcase geochemical and reactive transport modelling with our RedModRphree [1] extension package for the GNU R environment and programming language. The new version of the package leverages the R interface to the established PHREEQC geochemical simulator maintained by its original authors [2]. R has established itself as de facto standard language for statistics and machine learning. It enjoys increasing diffusion in many applied scientific fields such as bioinformatics, chemometrics and ecological modelling. The availability of excellent third party extensions such as the thermodynamic package CHNOSZ [3], which extends the functionalities of SUPCRT92, as well as its advanced graphical and numerical capabilities, make R an attractive platform for comprehensive geochemical data analysis, experiment evaluation and modelling. 
The aim of RedModRphree is to provide the user with an easy-to-use, high-level interface to program algorithms involving geochemical models, which are then solved using the PHREEQC engine: parameter calibration, error and sensitivity analysis, visualization, up to CPU-intensive parallel coupled reactive transport models. Among the out-of-the-box features included in RedModRphree, we highlight the computation and visualization of Pourbaix (Eh-pH) diagrams and the implementation of 1D advective reactive transport supporting the use of surrogate models replacing expensive PHREEQC calculations [4]. RedModRphree is open source and can be installed from https://git.gfz-potsdam.de/delucia/RedModRphree.

[1] De Lucia, M. and Kühn, M.: Coupling R and PHREEQC: Efficient Programming of Geochemical Models, Energy Procedia, 40, 464–471, doi.org/10.1016/j.egypro.2013.08.053, 2013.

[2] Charlton, S.R. and Parkhurst, D.L.: Modules based on the geochemical model PHREEQC for use in scripting and programming languages, Computers & Geosciences 37, 10, 1653–1663, doi.org/10.1016/j.cageo.2011.02.005, 2011.

[3] Dick, J.M.: CHNOSZ: Thermodynamic Calculations and Diagrams for Geochemistry, Frontiers in Earth Science, 7, https://doi.org/10.3389/feart.2019.00180, 2019.

[4] Jatnieks, J., De Lucia, M., Dransch, D., and Sips, M.: Data-driven Surrogate Model Approach for Improving the Performance of Reactive Transport Simulations, Energy Procedia, 97, 447–453, doi.org/10.1016/j.egypro.2016.10.047, 2016.

How to cite: De Lucia, M. and Kühn, M.: Geochemical and reactive transport modelling in R with the RedModRphree package, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2636, https://doi.org/10.5194/egusphere-egu21-2636, 2021.

Understanding the geochemistry of water resources is a prerequisite in the development of sustainable water resource management strategies. The Pra Basin is one of the few basins in Ghana with economic importance. The Basin is constituted by three river systems (Birim, Offin and Pra) and covers a total land size of approximately 2,300 km². It traverses several towns and serves as the main water supply for communities and industry. Currently, the quality of water resources in the Pra Basin especially surfacewaters have been affected negatively as a result of activities such as illegal mining (e.g., the use of mercury for the extraction of gold), indiscriminate waste disposal, and poor farm management practices (e.g., inappropriate application of fertilizers and pesticides). Specific contaminants include mercury (Hg), arsenic (As), lead (Pb), iron (Fe), manganese (Mn), cadmium (Cd), selenium (Se), and nitrate (NO₃). The Pra Basin is underlain by three rock formations, the Birimian Supergroup, the Tarkwain Formation and the granitoids. The mineral composition of the Birimian Supergroup comprises argillitic/pellitic sediment (plus or minus kerogen), sericite schist, and quartz-sericite schist. The granitoids comprise biotite (hornblende, muscovite), biotite gneiss, biotite schist, amphibolite partly of contact metamorphism, K-feldspar rich granitoid, two-mica or muscovite granite and monzonite, serecite schist, quartz-serecite, and garnet. The Tarkwaian rocks mineralogy also includes basaltic flow/subvolcanic rock and minor interbedded volcaniclastics, detrital sediment mainly sandstone and conglomerate ultramafic and minor mafic igneous rock. Samples of groundwater were collected from shallow (mainly hand-dug wells of depths < 10 m) and deep (mainly boreholes of depths >30 m) aquifers across the Pra Basin. Surfacewaters were collected from rivers and stream networks.  The samples were analysed for major ions, trace metals and stable isotopes (oxygen-18 and deuterium) using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Ion Chromatography (IC), and Picarro L-2140i Ringdown Spectrometer at the GFZ laboratories. Multivariate statistical analysis and inverse geochemical modelling have been applied to around 100 water samples sourced from boreholes, hand-dug wells, and rivers of the Pra Basin to determine the chemical state of the waters. Specifically, the study seeks to (1) determine the origin and evolution of the geochemistry of both surfacewater and groundwater, (2) identify recharge and discharge areas, and (3) study sources and sinks of minerals including sulphates, carbonates, and silicates. The abundance of cations and anions are in the order of Na>Ca>K>Mg and HCO₃>Cl>SO₄>NO₃ (surfacewater), Na>Ca>Mg>K and HCO₃>Cl>NO₃>SO₄ (hand-dug well), and Na>Ca>Mg>K, and HCO₃>Cl>NO₃>SO₄ (boreholes). Our research findings demonstrate that geochemistry of water resources in the Pra Basin is mainly controlled by rock-water interaction. With the application of hydrogeochemical modelling, including silicate mineral weathering and ion exchange, significant processes controlling the basin’s hydrochemistry variations are quantified. The presented results will support the development of sustainable water resources management strategies and contribute to mitigating future contamination.

How to cite: Manu, E., Kühn, M., Kempka, T., Goldberg, T., Vieth-Hillebrand, A., and Rach, O.: Hydrogeochemical modelling of origin, evolution and mechanisms controlling water resources quality in the Pra Basin (Ghana), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7800, https://doi.org/10.5194/egusphere-egu21-7800, 2021.

To analyze the influence of a precipitation mineralization reaction between dissolved CO₂ and calcium ions on the convective transfer of CO₂ towards an aqueous phase, the convective dissolution of CO₂ into aqueous solutions of calcium hydroxyde (Ca(OH)₂) and calcium chloride (CaCl₂) of various concentrations is studied experimentally. We show that different precipitation patterns develop in the aqueous solution depending on the nature and concentration of the reactant in the host phase. In the case of Ca(OH)₂, precipitation coupled to convection leads to vigorous convective mixing in the host phase and sedimentation of solid particles of calcium carbonate (CaCO₃) down to the bulk of the reservoir. Conversely, dissolution of CO₂ in buffered CaCl₂ solutions leads to a stabilisation of the buoyancy-driven convection due to a decrease in density and the adherence of the precipitate to the cell walls.

How to cite: De Wit, A., Thomas, C., and Dehaeck, S.: Effect of precipitation mineralization reactions on convective dissolution of CO2 : an experimental study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8026, https://doi.org/10.5194/egusphere-egu21-8026, 2021.

The weathering front, the interface beneath Earth’s surface where unweathered bedrock is converted into weathered rock, is a zone where chemical disequilibrium results in some of the most intense mineralogical transformations. These are focused into a narrow zone; yet its depth is poorly known due to its inaccessible nature deep beneath the Earth’s surface. Studies in humid and temperate climate suggest a maximum depth of 20 m for the weathering front in granitoid rock (Hayes et al., 2020).

To explore whether this depth is unique to humid climate we drilled into fractured rock in the semi-arid climate zone of the Coastal Cordillera of Chile. We found deep weathering down to 76 m below the surface which represents one of the deepest weathering fronts ever found. To characterise and quantify rock weathering, we investigated mineralogical and geochemical transformations. Iron (Fe) oxidation and related porosity formation is the first weathering process taking place and hence an indicator for the onset of weathering (Buss et al., 2008). Elemental (τ) and bulk loss (chemical depletion fraction, CDF) calculated from the chemical composition reveal multiple zones with more intense weathering compared to bedrock, and where the specific surface area also increases due to formation of secondary solids. Fracturing and the related increase in macro-porosity thus induce these mineralogical and chemical transformations. Below 76 m, bedrock is devoid of weathering features. We suggest that tectonic pre-fracturing in this geologically active region provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. This porosity was preserved throughout the weathering process, as secondary minerals that might fill pores were not formed due to the low fluid flow.

Hayes, N. R., Buss, H. L., Moore, O. W., Krám, P. and Pancost, R. D. (2020): Controls on granitic weathering fronts in contrasting climates. Chemical Geology, 535, 119450.

Buss, H.L., Sak, P. B., Webb, S. M. and Brantley, S. L. (2008): Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing. Geochimica et Cosmochimica Acta, 72 (18), 4488-4507.

How to cite: Krone, L. and von Blanckenburg, F.: Detecting Deep Rock Weathering, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9541, https://doi.org/10.5194/egusphere-egu21-9541, 2021.

Barite formation is of concern for many sustainable utilisations of the geological subsurface, ranging from oil and gas extraction to geothermal reservoirs, and also acts as a scavenger mineral for the retention of radium for nuclear waste disposal. The surface reaction-controlled nature of its formation in these dynamic systems entails a strong sensitivity of the host rock's permeability towards heterogeneities and boundary conditions. The impact of precipitation on effective flow properties can vary by many orders of magnitude as shown by barite scale formation and injectivity loss models for geothermal systems [1], emphasising the need for robust prediction models.

A relevant example case is the replacement of celestite (SrSO4) with barite (BaSO4), which was investigated for various barite supersaturations with flow-through experiments on the core-scale [2]. Three distinct cases were observed for supersaturations from high to low: (1) quick overgrowth and passivation of soluble celestite grains, (2) partial replacement of celestite with barite, (3) slow moving reaction front with complete mineral replacement. The authors presented heuristic approaches that include linking reactive surface area development to molar fractions to model the results. We provide a comprehensive, full-physics geochemical modelling approach using precipitation and dissolution kinetics as well as nucleation and crystal growth [3] for a more flexible representation of the problem. Additionally, the generation of a digital rock representation based on CT-scans of the granular sample is utilised to derive its inner surface area [4]. The experiments were modelled using core-scale reactive transport simulations. The three observed cases at varying supersaturations were reproduced with regard to evolution of sample rock composition and porosity.

In a next step, the characteristic values taken from the calibrated reactive transport models can be further integrated into the existing digital rock physics model [4], thus enabling the development of up-scaled relationships such as reactive surface area as a function of mineral fractions and porosity. The resulting models can then be applied to reservoir-scale simulations for various applications related to subsurface utilisation. 

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[1] Tranter, M., De Lucia, M., Wolfgramm, M., Kühn, M., 2020. Barite Scale Formation and Injectivity Loss Models for Geothermal Systems. Water 12, 3078. https://doi.org/10/ghntzk
[2] Poonoosamy, J., Klinkenberg, M., Deissmann, G., Brandt, F., Bosbach, D., Mäder, U., Kosakowski, G., 2020. Effects of solution supersaturation on barite precipitation in porous media and consequences on permeability: Experiments and modelling. Geochimica et Cosmochimica Acta 270, 43–60. https://doi.org/10/ghntxn
[3] Tranter, M., De Lucia, M., Kühn, M., 2021. Numerical investigation of barite scaling kinetics in fractures. Geothermics 91, 102027. https://doi.org/10/ghr89n
[4] Wetzel, M., Kempka, T., Kühn, M., 2020. Hydraulic and Mechanical Impacts of Pore Space Alterations within a Sandstone Quantified by a Flow Velocity-Dependent Precipitation Approach. Materials 13, 3100. https://doi.org/10/ghsp42

How to cite: Tranter, M., Wetzel, M., De Lucia, M., and Kühn, M.: Reactive transport model of kinetically controlled celestite to barite replacement, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9832, https://doi.org/10.5194/egusphere-egu21-9832, 2021.

The Bohemian Cretaceous Basin (BCB) is the most important hydrogeological structure in the Czech Republic, with large sources of groundwater. The origin of high-transmissivity zones is poorly understood in many BCB areas. The doyen of Czech hydrogeology prof. Hynie described some of the largest springs to be of karst origin and he attributed the most permeable areas to facies transition between shallow-water sandstones and deep-water marlstones. In many BCB areas with large springs we can find thin sandstones and siltstones layers with high carbonate content even in stratigraphical levels corresponding with aquifers.

Research is focused on Vysoké Mýto and Ustí synclines in BCB, 125 km east of Prague in the Czech Republic. Overall 167 rock samples were taken from borehole cores and rock outcrops in this area, the most from Jizera and Bílá Hora formations. Cores were taken from intervals where: (i) high carbonate content was expected, (ii) conduits and enlarged fractures were observed at outcrops and in wells, (iii) inflows to boreholes were determined by well logging. Calcium carbonate content was determined by calcimetry in cores. Cores were leached in 10 % hydrochloric acid to observe the degree of subsequent disintegration. Polished sections were prepared from selected cores and Ca, Si, Na, K, Al content was automatically mapped by SEM-EDS to visualize the calcium, silica, feldspar and clay mineral distribution in cores.

Leaching in hydrochloric acid is an accelerated simulation of natural processes of dissolution by acidic solutions (Kůrková et al. 2019). In many aquifers in BCB there are thin calcite-rich layers with quartz sand which disintegrates after leaching calcite. Leaching of the samples in acid results in the decrease of sample strength, sometimes to their disintegration. Leaching experiments showed that the carbonate content is not the only controlling factor in the karstification process.

In sediments with detrital quartz admixture in central or western parts of the BCB the total disintegration mostly occurs between 35-50% CaCO₃ content depending on insoluble material content (Kůrková et al. 2020). In contrast, in the eastern part of the BCB, a degree of disintegration above 10% is documented in only 7% of the studied samples. In sediments with diagenetically precipitated cement from marine sponges even calcite content as high as 80% may not be sufficient for material to disintegrate after leaching. Disintegration occurs mainly along fractured zones where rock is heavily fractured.  

It seems that the increased content of microcrystalline silica cementy in sandy limestones and calcareous sandstones (spongolites) of the studied area has a fundamental influence on the higher cohesion and resistance of rocks to dissolution. Cause for increased cohesion is the specific spatial distribution of  microcrystalline silica, which bound the quartz grains together or formed a foam-like supporting structure in fine calcite-rich deposits.

The research was financially supported by the GA ČR 19-14082S.

References:

Kůrková I., Bruthans J., Balák F., Slavík M., Schweigstillová J., Bruthansová J., Mikuš P., Grundloch J. (2019): Factors controlling evolution of karst conduits in sandy limestone and calcareous sandstone (Turnov area, Czech Republic). Journal of Hydrology: 574: 1062-1073.

How to cite: Stary, J., Schweigstillova, J., and Bruthans, J.: Formation of high-transmisivity zones in the facies transition in the Bohemian Cretaceous Basin (Czech Republic), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10138, https://doi.org/10.5194/egusphere-egu21-10138, 2021.

Laterite nickel-ore formation in New Caledonia is classically assumed to be governed by supergene processes, and downward migration of waters with Ni-enrichment at the basis of the laterite profile. However, Ni-ore distribution's heterogeneity seems to have been favoured by secondary processes controlled by the combined effects of inherited tectonics, geomorphological evolution and hydrologic systems since the primary laterite formation. Fluid flow and mass transfer processes are not purely downward at low-temperature conditions, but can also be related to lateral fluid circulations, and local drainage along damaged zones in the vicinity of faults (Cathelineau et al., 2016a; 2016b; Myagkiy et al., 2019). This study aims to investigate through reactive transport modelling the impact of discrete fracture on the Ni distribution.

We simulate the dissolution of olivine profile where fractures are the main channels of the fluid-flow. Olivine dissolution is assumed to be kinetically controlled whereas the precipitation of secondary weathering products is considered to occur according to local equilibrium. Results from two different numerical approaches are presented and discussed. The first one is based on a 1D dual-porosity model of a vertically oriented column of serpentinized olivine using PhreeqC associated with the llnl.dat thermodynamic database. The second one is a 2D modelling of hydro-chemical processes in fractured porous media based on the coupling of PhreeqC and Comsol Multiphysics through ICP. While the 1D model aims to describe the general trend of the progression of the weathering front and the global mineral redistribution, the 2D model focuses on particular fracture geometry and hotspot moments of the dissolution process to highlight crucial transition and redistribution of the different mineral phases in relation with the spatial distribution of fractures.

In the 1D dual-porosity model, the fractures are modelled as advective cells connected to a diffusive cell containing the main part of olivine. Two different geochemical models are thus designed. The first one describes the fracture and the advective area's geochemical behaviour, while the second one focuses on the matrix in the diffusive area. The 2D model extends the work initiated by Myagkiy et al. (2019) on simple configurations. The fractures are modelled herein as 1D discrete surfaces interacting with a porous matrix of olivine. Different fracture configurations are studied to assess their impact on mineral redistribution.

Results from both modellings are then compared with observed field data from New Caledonia and previous modelling of an olivine profile without fractures (Myagkiy et al., 2017) to validate the models and highlight the differences induced by the fracture network.

How to cite: Favier, S., Teitler, Y., Golfier, F., and Cathelineau, M.: Reactive Transport Modelling applied to Ni laterite ore deposits in New Caledonia : Impact of discrete fractures on Ni mineralization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12661, https://doi.org/10.5194/egusphere-egu21-12661, 2021.

Magnetite commonly forms during serpentinization of mantle peridotite, involving the hydrogen generation within the oceanic lithosphere. Although magnetite is concentrated in veins, the mobility of iron during serpentinization is still poorly understood. The completely serpentinized ultramafic rocks (originally dunite) within the Taishir massif in the Khantaishir ophiolite, western Mongolia, include abundant magnetite + antigorite veins, which manifest novel distribution of magnetite. The serpentinite records the multi-stage serpentinization, in order of (1) Al-rich antigorite + lizardite mixture with hourglass texture (Al₂O₃ = 0.46-0.69 wt%; Atg+Lz), (2) Al-poor antigorite composed of thick veins and their branches (Atg), and (3) chrysotile that cut all previous textures. The Mg# (= Mg/ (Mg + Fe_total)) of Atg+Lz (0.94-0.96) is lower than Atg (0.99) and chrysotile (0.98). In the region of Atg+Lz, magnetite occurs as the arrays of fine grains (<50 μm) around the hourglass texture. In the Atg veins replacing Atg+Lz, magnetite disappears and re-precipitated as coarse grains (100-250 μm) in the center of some veins. As the extent of replacement of Atg+Lz by Atg veins increases, both modal abundance of magnetite and the bulk Fe content decrease. These characteristics indicate that hydrogen generation mainly occurred at the stage of Atg+Lz formation, and magnetite distribution was largely modified via dissolution and precipitation in response to later fluid infiltration associated with the Atg veins. This also indicates the high iron mobility within the serpentinized peridotites even after the primary stage of magnetite formation.

How to cite: Dandar, O., Okamoto, A., Uno, M., and Tsuchiya, N.: Magnetite Redistribution during Multi-stage Serpentinization: Evidence from the Taishir massif, the Khantaishir Ophiolite, Western Mongolia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13773, https://doi.org/10.5194/egusphere-egu21-13773, 2021.

In the simulation of many porous media applications, such as geothermal energy, CO2 and nuclear waste storage and groundwater management, transport and reaction processes are fundamental to make accurate and reliable predictions. Moreover, the fractures present in the undeground, with their physical properties (high/low conductivity) and complex geometry are of paramount importance, on one side, but extremely complex to handle on the other. We propose two models based on a geometrical reduction procedure that approximate fractures as objects of lower dimension forming thus a system of mixed-dimensional partial differential equation. The first model describes a thermal reactive flow problem in both the fractures and porous media, coupled with suitable interface conditions. Chemical reactions can change porosity and fracture aperture with dissolution and precipitation of minerals. To obtain the solution for each time step, we consider a splitting strategy so that each physical process is solved sequentially ensuring, at the discrete level, mass conservation.  The second model in addition accounts for a thin layer of porous media surrounding the fractures where the effect of chemical reactions is prevalent, approximating it with a line/surface coupled with the fracture. Its thickness, which may change in time, is estimated with a mono-dimensional equation solved in the direction normal to the fracture. Comparison with the equi-dimensional problem ensure the quality of the proposed models.

How to cite: Fumagalli, A. and Scotti, A.: Two reduced models for reactive flows in fractured porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14872, https://doi.org/10.5194/egusphere-egu21-14872, 2021.

The study of mercury receipt within volcanic activity zones and large hydrothermal systems recently causes the big interest connected with attempts of an estimation of volumes of natural mercury receipt on a daily surface.

The hydrothermal system connected with volcanic massif Big Semyachik is one of the largest on the territory of Kamchatka peninsula. On the surface, the hydrothermal system is manifested by three large hydrothermal fields - the Verhnee Field, the parychay Dolina, and the Northern Crater of the Central Semyachik, the heat export from which is estimated at 300 MW (Vakin, 1976). On the surface of the thermal fields hot thermal waters and powerful steam-gas jets are unloaded.  At the same time, due to the inaccessibility of thermal fields remain poorly studied, and in particular, there is no information on the concentrations of mercury in hydrothermal solutions.

During fieldwork in 2020 all types of thermal waters were sampled, chemical types of waters were established, concentrations of mercury in hydrothermal solutions: for hot thermal waters the average value of mercury was - 0.44 mcg / L, and in steam-gas jets - the average value of mercury was - 4.60 mcg / L.

Thus, in the course of the work the data on concentrations of mercury in hydrothermal solutions of one of the largest hydrothermal systems of Kamchatka were received for the first time.

How to cite: Nuzhdaev, A.: Mercury concentrations in thermal waters of the Bolshoi Semyachik hydrothermal system, Russia, Kamchatka., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15094, https://doi.org/10.5194/egusphere-egu21-15094, 2021.