Carbonate rock formations constitute common hydrocarbon reservoirs and are often considered as candidates for geological CO₂ storage, where acid-driven carbonate dissolution may occur in the near-wellbore region. Calcite dissolution can substantially reconfigure pore networks, altering permeability and influencing storage efficiency and long-term containment integrity. While carbonate dissolution has been extensively studied experimentally and numerically, its detection and characterization using non-invasive monitoring tools remain challenging. Nuclear magnetic resonance (NMR) is a particularly promising tool as it is sensitive to pore geometry and fluid distribution. However, a quantitative framework that links regime-dependent pore-scale dissolution patterns to NMR observables remains underdeveloped. In this work, we establish such structure–signal mapping by coupling pore scale reactive transport simulations of calcite dissolution with forward modeling of low-field NMR responses, generating synthetic observables from dynamically evolving pore geometries due to calcite dissolution. By varying the relative timescales governing advection, diffusion, and surface reaction rates, we analyze the evolution of three representative dissolution patterns: uniform face dissolution, conical channeling dissolution, and wormholing dissolution. To capture the spatial heterogeneity of these features, we segment the pore geometry along the flow axis and derive an NMR T₂ relaxation time distribution for each section, constructing flow-direction T₂ profiles. In contrast, bulk T₂ distributions derived from the entire pore volume tend to average out the spatial heterogeneity of dissolution patterns. Furthermore, to capture the propagation of reaction fronts and characterize the permeability of emerging channels, we formulate specific NMR-based metrics: a pore-enlargement index E_i(t), a heterogeneity index H(t), and a connectivity index C(t). Dissolution breakthrough, defined by k/k₀ ≥ 10, occurs at PV₁₀ ≈ 314 for face dissolution, 138 for channeling, and 144 for wormholing. While H(t) consistently evolves non-monotonically, breakthrough is governed by the emergence and strengthening of an inlet-to-outlet pathway. Accordingly, C(t) closely tracks breakthrough during channeling, whereas in wormholing it indicates early connectivity without an immediate permeability increase. Our weighted pore network connectivity by cumulative enlargement yields a single metric that correlates with permeability growth across regimes. This structure–signal framework provides a workflow for using spatially distributed NMR signals to identify pathway formation and provide an early indication of permeability surges. The framework for mapping structures to signals enhances the interpretation of NMR signals in dissolution reactive settings and provides a quantitative foundation for interpreting NMR monitoring signals and informing risk assessment for geological CO₂ storage in settings where carbonate dissolution may alter flow pathways.

How to cite: Wang, B., Zhou, J., Zhou, S., Yang, Y., Bate, B., and Zhang, C.: NMR T2 Profile Reveals Connectivity-Controlled Permeability Breakthrough during Pore-Scale Carbonate Dissolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2244, https://doi.org/10.5194/egusphere-egu26-2244, 2026.

EGU26-2244

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Natural gas hydrates are crystalline, ice-like compounds formed by water molecules arranging into cage-like lattices that encapsulate gas molecules under low-temperature and high-pressure conditions typical of continental margins. Within these environments, free gas migrating upwards is generally expected to be trapped upon entering the hydrate stability zone (HSZ) through hydrate formation. Nevertheless, extensive geological and geophysical observations indicate that free gas can traverse the HSZ and escape at the seafloor, suggesting the presence of dynamic leakage mechanisms that are not yet fully understood.

In this study, we develop a fully coupled thermal–hydro–mechanical–chemical (THMC) framework [1] that explicitly incorporates salt transport and hydrate generation and apply it to a three-dimensional subsea geological model. The model is used to investigate gas migration and leakage through the HSZ under realistic pressure–temperature conditions. Simulation results reveal that gas leakage is governed by a transient, fracture-controlled process. Initial hydrate formation locally reduces permeability, acting as a temporary barrier that traps migrating gas and promotes progressive pore pressure build-up beneath HSZ. Continued pressurization compromises sediment mechanical stability, triggering fracture initiation and propagation.

Following fracture development, gas preferentially migrates through these newly formed high-permeability pathways, bypassing the surrounding low-permeability hydrate-bearing sediments. Within the fractured zones, rapid gas invasion promotes local hydrate formation, which is inherently self-limiting. Hydrate growth results in a progressive reduction in local water saturation, while salt is excluded from the hydrate phase and accumulates in the remaining pore fluid. The combined effects of water depletion and salinity increase thermodynamically suppress further hydrate formation, even under favourable pressure–temperature conditions. At the margins of the fractured zones, hydrate saturation becomes locally elevated, forming low-permeability hydrate-rich barrier that effectively restrict lateral water supply and salt diffusion into the fractured zone. This spatial heterogeneity in hydrate distribution reinforces the persistence of gas-conductive pathways within fractures zone. In contrast, the central parts of fractured zone remain characterised by high gas saturation and limited hydrate accumulation, preserving high gas relative permeability and enabling sustained gas flow through the hydrate stability zone.

As gas continues to be supplied, pore pressure progressively increases within and beneath the existing fracture network. This renewed pressurisation promotes further mechanical weakening of the surrounding sediments, leading to the second and more fractured zones. Ultimately, the development of interconnected fracture networks allows free gas to breach the hydrate stability zone and reach the seafloor, resulting in gas leakage into the overlying water column. Once these fractures connect to the seafloor, natural gas is released, causing leakage into the overlying water column.

Therefore, the limited water availability and salinity effects on hydrate formation are fundamental controls on gas leakage through the HSZ, as they restrict further hydrate growth and accelerate more generation of fractures, thereby maintaining highly permeable pathways for gas migration. This highlights the importance of fully coupled THMC processes with considerating salt transport in assessing subsea gas escape and associated geohazards.

[1] L. Zhang, B. Wu, Q. Li, Q. Hao, H. Zhang, Y. Nie, A fully coupled thermal–hydro–mechanical–chemical model for simulating gas hydrate dissociation, Applied Mathematical Modelling, 129 (2024) 88-111.

How to cite: Zhang, L., Gupta, S., and Berndt, C.: Fracture-Controlled Gas Leakage through the Hydrate Stability Zone under Coupled THMC and Salinity Effects in Subsea Sediments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3195, https://doi.org/10.5194/egusphere-egu26-3195, 2026.

EGU26-3195

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Uranium behavior during water–rock interactions is strongly influenced by geochemical conditions relevant to geological disposal environments. This study investigated how variations in pH, redox conditions, carbonate availability, and temperature regulate uranium behavior through a series of batch experiments. Uranium-bearing coaly slate was collected from a natural analogue site in Boeun-gun within the Okcheon Metamorphic Belt, Korea. The coaly slate contains approximately 99.6 ppm of uranium, primarily hosted in uranium-bearing minerals such as uraninite and ekanite. Five batch experiments were conducted using artificial groundwater designed to represent the groundwater chemistry of the study site. The experimental design isolated the effects of pH, carbonate buffering, temperature, and uranium spiking. Batch 1 and Batch 2 were conducted under initially acidic (pH 5) and alkaline (pH 9) conditions, respectively. Batch 3 involved uranium-spiked artificial groundwater (2 mg L^-1), while Batch 4 and Batch 5 were conducted under carbonate-buffered, near-neutral pH conditions at 15 °C and 30 °C, respectively. In Batches 1–3, pH decreased rapidly immediately after the reaction began, resulting in acidic and high Eh conditions driven by pyrite oxidation in the coaly slate. This process promoted the formation of secondary Fe(III) (oxyhydr)oxides and Fe-bearing secondary phases. In Batch 1 and Batch 2, uranium concentrations increased rapidly, reaching approximately 60 and 30 µg L^-1 within 72 hours, respectively, and approached near-equilibrium, indicating limited uranium release under acidic conditions. In contrast, despite similarly acidic conditions, Batch 3 exhibited a gradual decrease in aqueous uranium concentration over time, suggesting uranium removal through adsorption or surface complexation onto newly formed Fe(III) (oxyhydr)oxides. In carbonate-buffered systems (Batch 4 and Batch 5), pH remained near neutral throughout the experiments, and uranium concentrations increased continuously with time, reaching levels of up to ~20 µg L⁻¹, which were lower than those observed under acidic conditions. Uranium speciation was dominated by aqueous carbonate complexes, with Ca₂UO₂(CO₃)₃ prevailing at 15 °C and UO₂(CO₃)₂²⁻ dominating at 30 °C. This sustained increase under neutral conditions contrasts with the rapid but limited uranium release observed in acidic systems, highlighting the role of carbonate complexation in regulating uranium mobility in groundwater.

How to cite: Cho, H., Lim, S., Choi, M., and Jeen, S.-W.: Geochemical Controls on Uranium Behavior During Water–Rock Interactions at a Natural Analogue Site in Korea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3207, https://doi.org/10.5194/egusphere-egu26-3207, 2026.

EGU26-3207

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Hydrothermal dolomitization is a critical process in carbonate diagenesis, capable of nonlinearly and heterogeneously restructuring pore networks, thereby fundamentally affecting permeability and fluid pathways in carbonate-hosted geothermal systems. Reaction rates and mechanisms in natural rocks remain poorly constrained, as few experimental setups permit direct observation of the process. Here, we present early analyses of operando (4D) µCT data acquired at the PSICHÉ beamline of Synchrotron SOLEIL (France) that document hydrothermal dolomitization in a fractured limestone from the Terwagne Formation of the Lower Carboniferous Kohlenkalk sequence (North Rhine-Westphalia, Germany). Our data provide mechanistic insights that cannot be obtained from conventional experimental approaches.

The fine-grained oosparitic limestone contains microstylolites, which are likely to be diagenetic. A cylindrical core (20.08 × 9.76 mm) was drilled sub-parallel to bedding and axially fractured ex situ (UCS = 98.1 MPa) to promote fluid flow in an otherwise low-porosity (<2%) rock. The initial permeability at experimental conditions was 1.1–2.9 × 10^-10 m². The experiment was conducted using the X-ray transparent Heitt Mjölnir triaxial flow-through rig (Freitas et al., 2024), with continuous injection of a 2.05 M NaCl–MgCl₂–CaCl₂ brine at 1.5 µL min^-1, at 260 °C, 20 MPa confining pressure, and 15 MPa pore fluid pressure. Reaction progress was documented in 62 three-dimensional volumes at a 5.8 µm voxel size over 128 h. Each tomography volume is based on 1,400 projections acquired over 180° using a pink beam with a peak energy of ~81 keV. Fluid samples collected after 49, 79, 105, and 128 h were analysed by ICP-OES for Na, Ca, and Mg concentrations, and post-mortem SEM/EDX analyses corroborated the µCT-based interpretations.

Our 4DµCT data resolve the spatiotemporal evolution of reaction products, allowing observation of phase formation sequences, quantification of local reaction rates, and identification of rate-limiting transport mechanisms controlling phase growth within a fractured carbonate rock. Early analyses show that calcite reacts with brine and forms several distinct phases nearly simultaneously, including magnesite, dolomite-type carbonate, and locally brucite where carbonate availability is limited. Post-mortem SEM/EDX reveals that the dolomite-type phase comprises both Ca-dolomite and stoichiometric dolomite, which cannot be distinguished in our 4DµCT data. Magnesite and brucite remain largely confined to the inlet region, whereas dolomite-type carbonate nucleates preferentially along hydraulically active fractures and stylolites with apertures exceeding ~32 µm, reflecting the evolving fluid pathways during reaction. Our observations indicate that magnesite precipitation generates macro-porosity (10–100 µm), facilitating advective fluid transport, whereas dolomite-type carbonate develops sub-micron to micron-scale porosity, likely resulting in transport dominated by grain-boundary diffusion. Brucite locally reduces porosity, but its metastable nature likely limits its impact on bulk fluid flow. Porosity generation associated with dolomite-type replacement enhances fracture and stylolite connectivity, establishing preferential fluid pathways in the process. These spatially and temporally heterogeneous transport regimes reflect local chemical-hydraulic feedbacks, producing differential growth rates among phases and exerting first-order control on the overall rate of dolomitization. ICP-OES data are consistent with bulk mineralogical evolution, while 4DµCT uniquely resolves a spatiotemporal coupling between fluid flow and reaction progress.

How to cite: Ng, A.: Operando 4D synchrotron tomography resolves multiphase hydrothermal dolomitization in a natural carbonate rock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10300, https://doi.org/10.5194/egusphere-egu26-10300, 2026.

EGU26-10300

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Carbonate dissolution by CO₂-rich brine (carbonic acid) can strongly modify pore structure and flow pathways in subsurface systems relevant to geological CO₂ storage. However, predicting the resulting dissolution regimes remains challenging, as widely used transport–reaction scaling approaches based on Péclet and Damköhler numbers often fail to reproduce experimentally observed dissolution patterns. Here, we present a new set of core-scale dissolution experiments designed to directly observe the coupled evolution of pore structure, flow, and reaction-front migration during CO₂-rich water injection. Experiments were performed on cylindrical limestone cores with a diameter of 12 mm and a length of 36 mm from two formations exhibiting contrasting pore-scale heterogeneity: (1) Ketton limestone, representing a relatively homogeneous system, and (2) Estaillades limestone, representing a heterogeneous system. Carbonated water with an initial pH of 3 was injected into three samples of each limestone at ambient temperature and a pore pressure of 50 bar under constant flow rates of 0.1, 1, and 10 ml/min. Dissolution processes were monitored using time-lapse X-ray microcomputed tomography at approximately 6 µm spatial resolution. Scans were acquired under initial dry conditions, fully water-saturated conditions, and after successive intervals of 100 injected pore volumes of CO₂-rich water, enabling four-dimensional visualization of dissolution pattern development. Across both lithologies, systematic transitions in dissolution behaviour are observed with increasing flow rate: compact or inlet-localized dissolution at low flow rate, dominant wormhole formation at intermediate flow rate, and increasingly distributed, multi-branch, or ramified wormholing (nearly uniform) at the highest flow rate. While pore-scale heterogeneity influences the geometry and symmetry of the resulting dissolution structures, the overall regime transitions remain consistent across both carbonate systems. We observe that dissolution patterns cannot be solely explained by classical Pe-Da scaling based on initial flow and kinetic conditions. Instead, the results demonstrate that the spatial persistence of fluid reactivity governs both the extent and morphology of dissolution across flow rates and lithologies with contrasting heterogeneity. These experiments show that accounting for the evolution of fluid reactivity and reaction-front migration is essential for more accurate prediction of carbonate dissolution during CO₂ injection.

How to cite: Vafaie, A., Rahimzadeh Kivi, I., and Krevor, S.: Unraveling dissolution regime transitions in carbonates during CO2-rich water injection, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12691, https://doi.org/10.5194/egusphere-egu26-12691, 2026.

EGU26-12691

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Enhanced Geothermal Systems (EGS) rely on heat extraction from deep crystalline rocks, whose inherently low permeability requires reservoir stimulation to establish effective fluid circulation. Current stimulation strategies are largely limited to hydraulic methods, while chemical approaches remain underexplored in crystalline lithologies, even though natural hydrothermal analogues demonstrate that fluid–rock reactions can substantially modify pore structure and flow properties. 

Here, we investigate the reaction-driven evolution of porosity and permeability in low-porosity granite using controlled reactive flow-through experiments conducted under conditions relevant to chemical stimulation of EGS. Reactive fluids with modified regular mud acid (RMA), are continuously circulated through saw-cut granite cores, enabling direct monitoring of hydraulic property evolution during fluid flow. These measurements are complemented by post-experimental mineralogical and microstructural characterisation using electron microprobe analyses (EMPA), scanning electron microscopy (SEM), surface profilometry, and X-ray micro-computed tomography (µCT), conducted via the EXCITE network at the Ghent University Centre for X-ray Tomography. Previous batch experiments, presented separately at this conference (see Rüdiger et al., EGU2026), demonstrate that the used modified RMA fluid reacts preferentially with feldspar and mica, resulting in increased porosity. Building on these findings, the flow-through experiments examine how such mineral reactions progress under dynamic conditions and assess whether the newly formed porosity contributes to connected flow pathways and enhance permeability. In addition, the experiments further address the formation and stability of secondary phases and quantify the advance of reaction fronts into the granite matrix as function of time and flow. Together, these data allow to assess whether the substantial porosity increases observed in batch experiments are sustained under flow-through conditions, and how these changes affect both the magnitude and long-term stability of permeability enhancement.

How to cite: Kummerow, J., Rüdiger, G., Schröer, L., Winardhi, C. W., Cnudde, V., and John, T.: Reactive Flow Experiments on Granite: Implications for Chemical Stimulation of Enhanced Geothermal Systems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14201, https://doi.org/10.5194/egusphere-egu26-14201, 2026.

EGU26-14201

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Cement-based material has great potential to store carbon dioxide (CO₂) as carbonate minerals (mainly calcite, CaCO₃), through aqueous carbonation, driven by their alkaline nature and high portlandite (Ca(OH)₂) content. The carbonation capacity is influenced by many variables, such as cement mass, particle size, and water volume. However, the mechanistic understanding of how these parameters collectively control carbonation kinetics and long-term CO₂ uptake under dynamically evolving conditions remains underexplored. In this study, we developed a geochemical model using PHREEQC that integrates thermodynamic descriptions of aqueous speciation and mineral equilibria with kinetic rate laws to simulate simultaneous reactions in dynamically evolving systems. Portlandite dissolution releases Ca²⁺ into solution, which subsequently reacts with dissolved CO₂ to form CaCO₃ over time. By tracking phase assemblages involving Ca(OH)₂ dissolution, CaCO₃ precipitation, and pore-solution evolution, the progression of carbonation can be quantitatively resolved. Model results under experimentally relevant conditions indicate that CO₂ dissolution is the rate-limiting step of the overall process. Elevated pH is sustained for a finite duration, which depends on key controlling factors such as cement mass and particle size. This modeling framework provides a mechanistic foundation for upscaling laboratory observations and evaluating the potential performance of cement-based carbonation processes in natural environments, supporting the development and optimization of mineral-based carbon sequestration strategies under environmentally relevant conditions.

How to cite: Tai, Y.-H., Wu, W., Smith, S., and Van Cappellen, P.: Evaluation of Potential Carbon Storage of Cement-based Material in Aqueous Media Using PHREEQC , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15203, https://doi.org/10.5194/egusphere-egu26-15203, 2026.

EGU26-15203

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Pore-scale processes govern the emergence of macroscopic patterns in porous media. Direct experimental access to these coupled processes at the pore scale, however, remains limited by the opacity and structural heterogeneity of natural geomaterials. Microfluidic porous media offer real-time visualization of flow, interfacial phenomena, and chemical reactions within well-defined pore networks.

Here, we present a microfluidic platform that bridges pore-scale physical chemistry and biologically mediated precipitation process. The device architecture was originally developed to quantify multiple-contact miscibility in CO₂-enhanced oil recovery, providing direct measurements of phase behaviour and interfacial dynamics in a controlled pore network. We now extend this same framework to investigate microbial biomineralization as a precipitation-driven reactive process in porous media.

Using an optically transparent microfluidic porous medium, we resolve microbial transport, attachment, and growth, together with spatially localized mineral precipitation within individual pores and throats. This enables quantitative analysis of nucleation sites, precipitation kinetics, and pore-scale clogging. We apply the platform to study fungal-induced calcium carbonate precipitation, a biologically mediated mineralization pathway relevant to soil stabilization and the development of bio-based construction materials.

Our results demonstrate that a single microfluidic porous medium can be used to transition from physicochemical multiphase flow studies to biologically driven dissolution–precipitation processes. This approach provides a versatile experimental framework for reactive transport research, with implications for biomineralization, subsurface engineering, and biomaterial design based on microbially controlled mineral formation.

How to cite: Zou, H., Pla-Ferriol, M., van Velzen, S., Floudas, D., and Hammer, E.: From miscibility development to microbial biomineralization: visualization of pore scale process in microfluidic porous medium. , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17522, https://doi.org/10.5194/egusphere-egu26-17522, 2026.

EGU26-17522

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Fluid-mineral interactions taking place within the natural reactor at the Earth’s surface, the Critical Zone (CZ), are fundamental processes that regulates Earth’s surface conditions and terrestrial weathering fluxes across multiple spatiotemporal scales. Dissolution, precipitation and chemical reaction networks established through fluid-mineral interactions generally take place in the subsurface, and their extent is largely dictated by both the pathways of infiltrating water and the timescales of fluid transport. Deriving a quantitative understanding of how subsurface fluid residence times relate to weathering reaction rates remains a key challenge. This study seeks to better address this unknown by applying advanced geochemical tracers of weathering (silicon stable isotopes, δ³⁰Si) and groundwater ages tracers, along with reactive transport modeling approaches to a well-characterized natural system.

Our work focuses on Sagehen Creek basin, a small (27 km²) montane catchment situated in the Central Sierra Nevada of Northern California, USA. Sagehen Creek hosts robust, multi-decadal hydrologic and geochemical records of groundwater sourced from 12 naturally occurring springs. Over the course of a water year, > 80 spring water samples were collected at a bi-weekly frequency to develop a comprehensive geochemical (δ³⁰Si and dissolved solutes) and groundwater age tracer (CFCs, SF₆) dataset. Preliminary results from the field data show spring δ³⁰Si signatures to exhibit a strong correlation with groundwater ages over decadal timescales where the oldest springs have the lowest δ³⁰Si (+0.16 ± 0.08 ‰) and the youngest, the most elevated δ³⁰Si (+1.45 ± 0.07 ‰). This result suggests that weathering reaction progress varies as a function of mean groundwater ages and evolving transit time distributions (TTDs). A series of 1D isotope-enabled reactive transport models (RTMs) were developed to identify the major hydrogeochemical factors underlying the observed relationship between δ³⁰Si and groundwater ages. The leading framework generated from our preliminary RTM efforts centers on secondary mineral precipitation reactions and stable isotope equilibration. Younger groundwaters reflect early reaction progress dominated by active secondary mineral precipitation, which produce elevated δ³⁰Si due to kinetic effects. Older groundwaters on the other hand, reflect late stage, (near)equilibrium conditions for secondary mineral reactions, facilitating continued isotope exchange between minerals and the surrounding fluids, and thereby producing low δ³⁰Si values. Together, these preliminary results provide new constraints on the links between subsurface fluid residence times, weathering reaction progress, and solute generation in catchment-scale CZ systems.

How to cite: Fernandez, N., Jamison, H., López-Urzúa, S., Meyers, Z., Rademacher, L., Harpold, A., and Derry, L.: Connecting groundwater age to subsurface weathering reactions at the catchment scale using silicon isotopes and reactive transport modeling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17861, https://doi.org/10.5194/egusphere-egu26-17861, 2026.

EGU26-17861

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Dynamic interactions between surface water and groundwater induce pronounced temporal and spatial variability in redox conditions and substance concentrations within hyporheic zones, giving rise to highly complex nitrogen transformation dynamics. However, under environmentally heterogeneous and data-limited conditions, the level of kinetic complexity required to adequately represent nitrogen processes remains poorly constrained. In this study, we use soil microcosm experiments representative of hyporheic environments to systematically evaluate the applicability and modeling performance of first-order and Monod-type kinetics for simulating nitrogen transformations. Time-series measurements of ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), nitrite nitrogen (NO₂^--N) and dissolved organic carbon (DOC) were used to constrain nitrogen transformation rates, while functional gene abundances quantified by quantitative PCR served as indicators of microbial functional potential. Two kinetic frameworks, consisting of parsimonious first-order kinetics and Monod-type kinetics that explicitly incorporate substrate limitation, were independently calibrated to the experimental observations.
Our results indicate that both kinetic frameworks reproduced the overall temporal evolution of nitrogen species, including the general trends of ammonium oxidation and nitrate reduction. However, only the Monod-type kinetics captured substrate-dependent process controls and reactions associated with anoxic microenvironments, even when overall concentration variability was limited. While the first-order kinetics provide an efficient representation of net nitrogen turnover, the Monod-type kinetics offer a more mechanistic description of pathway sensitivity and environmental regulation that is essential for interpreting nitrogen transformation processes in hyporheic zones. The derived kinetic parameters therefore provide scenario-dependent priors for reactive biogeochemical modeling and highlight the importance of explicitly representing substrate limitation and redox regulation using Monod-type kinetics when coupling biogeochemical dynamics with hydrologic variability. 

How to cite: Xing, J., Cai, Y., and Zhou, N.: Modeling Nitrogen Cycling in Hyporheic Zones: A Comparison of First-Order and Monod-Type Kinetics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12091, https://doi.org/10.5194/egusphere-egu26-12091, 2026.

EGU26-12091

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Water-rock and aqueous reactions affect CO₂ geological storage in several ways, including through carbonate mineralization, dissolution and reprecipitation, silicate dissolution, and acid-base buffering. However, complex water chemistry compositions and multiple rock mineral types make the quantitative characterization of these reactions difficult. Here, predictive models of geochemical reactions were developed taking place within strong to moderately reactive storage formations where pH-sensitive silicate dissolution and carbonate precipitation dominate. To do this, the TOUGHREACT thermal-hydrological-chemical multiphysics subsurface reactive transport simulator was used to develop well-calibrated models based on field monitoring data.

This study first benchmarked a model against a single-well CO₂ push-pull field test conducted in a pH 11.02, shallow peridotite formation in Oman, described in Matter et al. (2025). The simulation included the 13.7-hour carbonated water injection, the subsequent 45-day shut-in, and then 11.2 days of pumping. Calibration of the porosity, permeability and formation mineral assemblage primarily occurs against recorded ion concentrations during the pumping period. The model suggests calcite precipitation dominated at the margins of the 6.4 m radius mineralization zone, with dolomite at intermediate distances and magnesite in the immediate vicinity of the well. Magnesite precipitation is associated with lower pH conditions near the well where there is sufficient available Mg²⁺ dissolved from the host rock, whereas dolomite and calcite are deposited at higher pH and sufficient available Ca²⁺. During the storage period, our model underpredicts mineralization (52%) compared to that inferred by Matter et al. (88%), likely due to underprediction of dolomite or magnesite. The precipitated carbonates remain stable upon re-equilibration of the groundwater.

The model was then applied to a hypothetical doublet storage operation in a CO₂-rich hydrothermal system at Ohaaki, New Zealand. The goal was to predict CO₂ phase evolution subject to long-term geochemical reactions as well as boiling of the fluid phase. Simulations show that ions primarily controlled by a single mineral (Ca²⁺, Na⁺, K⁺, and Fe²⁺) all reach their peak concentrations within five years, whereas subsequent geochemical evolution is influenced by the dynamic equilibrium of aqueous complexes, such as CaSO₄(aq), NaCl, MgHCO₃⁺, and FeCl⁺. Driven by the injection of aqueous solutions with high carbonic acid concentrations, the mineral volume fraction at the injection well changes at a rate 2–11 times greater than that observed in the rest of the simulation domain. Under high-temperature and low-pressure conditions of the production well, a CO₂ boiling zone forms in the reservoir, with the peak gas saturation of CO₂ exsolved from the liquid phase reaching 7.6 wt% over the simulation period. This research shows that the geochemical reaction simulation holds significant scientific value for CO₂ storage applications in strong to moderately reactive storage formations.

How to cite: Hu, T., Dempsey, D., Zhao, Z., Dong, J., and Rui, Z.: Geochemical Modelling for Carbon Dioxide Removal Applications, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3306, https://doi.org/10.5194/egusphere-egu26-3306, 2026.

EGU26-3306

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The Ediacaran dolomites of the Tarim Basin constitute a strategic frontier in global ultra-deep hydrocarbon exploration, yet their complex diagenetic evolution and porosity preservation mechanisms remain pivotal challenges for predicting reservoir "sweet spots." To decipher this history, our study employs an integrated approach—combining detailed petrography with in-situ U-Pb geochronology, trace element analysis, and  in-situ C-O isotopic data-to reconstruct a high-precision, multi-stage diagenetic fluid sequence for the Qigebrak Formation dolomites.

This work not only clarifies the primary origin of the Ediacaran microbial dolomites (Md1) but also delineates six key diagenetic phases: four dolomite cement generations (Cd1-Cd4), one episode of hydrothermal saddle dolomite (Sd), and late-stage calcite veins (Cd5). The evolutionary trajectory is defined as follows: (1) Penecontemporaneous Stage (~583-538 Ma): The microbial matrix (Md1) yields U-Pb ages of 583–559 Ma, consistent with deposition. Its seawater-like REE signatures (high Y/Ho, LREE depletion) and C-O isotopes confirm penecontemporaneous dolomitization in an evaporative setting. Subsequent fibrous/bladed cements (Cd1, Cd2), dated to ~541–538 Ma, display high Mg and inherited seawater chemistry, marking the end of early marine cementation. (2) Shallow-to-Intermediate Burial Stage (~466–409 Ma): Cement Cd3 (~466 Ma) shows negative Ce anomalies and elevated BSI, reflecting mildly reducing modified seawater. A significant fluid shift is recorded by Cd4 (~409 Ma), which exhibits marked MREE enrichment ("bell-shaped" REE patterns) and sharply increased BSI, indicating influence from deep, reducing connate brines during the Late Caledonian to Hercynian. (3) Deep Burial and Tectonic-Hydrothermal Stage (~215 Ma): Saddle dolomite (Sd) is dated to ~215 Ma (Indosinian). Coupled with strong positive Eu anomalies and hydrothermal mineralogy, it unequivocally records tectonically driven, fault-focused hydrothermal fluid influx. Late calcite veins (Cd5) represent final fracture-fill during deep burial.

By establishing an absolute geochronological diagenetic framework, this study precisely pins the timings of fundamental fluid-property shifts. Our results demonstrate that the early rigid framework of penecontemporaneous dolomite (Md1) and marine cements (Cd1/Cd2) was essential for preserving primary porosity against deep burial compaction. In contrast, mid-to-late diagenetic fluids were governed by the basin's tectonic rhythm; the Indosinian hydrothermal event (Sd) underscores the critical role of deep-seated faults in superimposing reservoir modification. These findings deliver a temporally calibrated evolutionary model for ancient cratonic dolomites and provide seminal geological evidence to guide the prediction of ultra-deep hydrocarbon "sweet spots."

How to cite: chen, X., xu, Q., and Hao, F.: From Penecontemporaneous Seawater to Deep Hydrothermal Fluids: Records of Multi-Stage Superimposed Fluid Evolution in Ediacaran Dolomites, Tarim Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7825, https://doi.org/10.5194/egusphere-egu26-7825, 2026.

EGU26-7825

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