HS8.1.1

Mineral dissolution/precipitation reactions are critical in several contexts (e.g., geologic sequestration of CO2 or reservoir hydraulic fracturing). High-resolution imaging techniques such as Atomic Force Microscopy (AFM) allow direct observation of the (nanometer-scale) evolution of the crystal topography during the reaction, thus enhancing our knowledge on the various dissolution mechanisms occurring at the liquid-solid interface. These mechanisms are imprinted onto the highly heterogeneous patterns observed and are originated from the presence of inhomogeneities and defects in the mineral lattice, resulting in a broad range of local reaction rate values. We rely on a multimodal Gaussian model to capture the spatial heterogeneity of dissolution rates from AFM (in-situ and in real time) topography measurements collected on calcite samples subject to dissolution at far from equilibrium conditions. We resort to an imaging segmentation technique to cluster reaction rate data into regions associated with diverse dissolution mechanisms and relate each region to a component of the Gaussian mixture. We analyze the temporal trend of model parameters to provide quantitative insights on the dynamic evolution of the spatial heterogeneity of dissolution rate.

How to cite: Recalcati, C., Siena, M., Riva, M., and Guadagnini, A.: Stochastic modeling of the multimodal behavior of spatially heterogeneous calcite dissolution rate at the nanoscale, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1129, https://doi.org/10.5194/egusphere-egu22-1129, 2022.

Bank filtration is widely used for drinking water production around the world. Due to the general composition of river water and the possibility of direct anthropogenic inputs, it is of great importance to understand the interaction between river and groundwater as well as the subsurface flow conditions. These can alter with increased runoff during flood events. Higher gradients between the surface water and groundwater, enlarged infiltration zones or the removal of colmation layers can lead to elevated infiltration rates and thus to changes in the river aquifer system. Due to often high hydraulic permeabilities and the potential spatial proximity between the river and the extraction system, short dwell times may therefore occur. Additionally, associated with stormwater runoff of wastewater treatment plants, higher contamination risks can therefore be expected in the extraction system during flood events.

In our investigations, we use a regional three-dimensional numerical groundwater model to help prevent changes in the quality of the extracted water through optimized operational management. However, the need to predict the flow paths between the river and the water work with maximum precision makes it necessary to complement the regional model with high resolution local models. To better capture vertical heterogeneities constraining local flow paths, a two-dimensional vertical model following the direction of maximum contribution of bank filtration to the water work was additionally created using FEFLOW 7.5 (DHI). Along this transect, the X-ray contrast agent gadolinium was sampled for use as a conservative anthropogenic tracer at different depths.

The sampling of gadolinium every 12 hours during a minor flood event showed a weekly, wastewater-influenced signal in the surface water, which could also be followed in the transect. This signal, together with ²²²Rn tracer ages, complements the time-resolved observations of groundwater levels to calibrate the vertical distribution of hydraulic parameters of the two-dimensional mass transport model. These are supposed to improve the conceptual regional model to ultimately optimize the operation of the waterworks and allow the extraction of the groundwater with the best possible quality.

How to cite: Erlmeier, K., Marazuela, M. A., Formentin, G., Bruenjes, R., and Hofmann, T.: Determination of the vertical distribution of hydraulic parameters using gadolinium as an anthropogenic tracer and inverse modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7279, https://doi.org/10.5194/egusphere-egu22-7279, 2022.

Image-based pore-scale modeling is an important method to study multiphase flow in permeable rocks. However, many rocks have pore size distribution that are so wide that they cannot be resolved in a single pore-space image. The accurate identification and characterization of this sub-resolution porosity have received extensive attention, due to its crucial impact on porosity and permeability estimation. To date, several modeling methods incorporate this information to improve the simulation of multiphase flow in complex rocks. The challenge is that the microporosity’s flow properties are difficult to characterize in images, and to include in models, of representative volumes. In this study, a novel sub-rock typing method was proposed to better characterize and classify microporous regions in rock samples, and to reduce the uncertainty of image-based pore-scale modeling of such samples. To this end, we performed capillary drainage experiments with brine and decane on two water-wet rock samples (Estaillades limestone and Luxembourg sandstone). Laboratory-based Micro-CT was used to image intricate pore structures and fluid occupancy changes at controlled capillary pressure steps during drainage. We proposed a novel workflow to generate 3D, micrometer-scale capillary pressure maps, which we combined with porosity maps to classify zones of microporosity types (sub-rock types). This was used to extract multi-scale pore network models and perform multiphase flow simulations. We found that the new approach yielded a good match with macroscopic experimental measurements, and significantly improved the prediction accuracy of the fluid distributions on a pore-by-pore basis. The results illustrate the importance of characterizing microporosity for simulations in heterogeneous rocks. The workflow can be applied to other complex geological porous rocks to improve modeling and simulation of subsurface multiphase flow.

How to cite: Wang, S., Ruspini, L. C., Øren, P.-E., Van Offenwert, S., and Bultreys, T.: Sub-rock typing and its influence on pore-scale, image-based simulations of multiphase flow in complex geological rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8053, https://doi.org/10.5194/egusphere-egu22-8053, 2022.

A variety of algorithms have been proposed to cope with issues associated with local refinements of numerical grids typically employed to cope with subsurface flow driven by sources acting on diverse scales (e.g., pumping wells, channels or ditches or abrupt changes in hydraulic conductivity distributions). In this context, here we focus on grid refinement associated with nonmatching grids, which still pose significant challenges in terms of accuracy. We propose a numerical modeling scheme based on a new algorithm that has an improved accuracy when compared against approaches that are typically used in conjunction with nonmatching grids. Our approach is based on the vertex-centred finite-volume method (VCFVM), the key feature of the algorithm resting on setting all unknowns on vertices of the mesh elements while the flux crossing a lateral surface of the control volume centred around a mesh vertex is expressed as a function of the hydraulic heads at the vertices of the element containing the lateral surface. A given row of the stiffness matrix of the system includes the entries associated with mass conservation formulated for the control volume associated with a corresponding grid node. Since the algorithm sets all unknowns on element vertices and a control volume can be defined for each vertex, our scheme readily embeds treatment of nonmatching grids in the presence of local grid refinement. Hydraulic heads evaluated through our algorithm are benchmarked against (a) results obtained from the widely used and tested MODFLOW and MODFLOW6 groundwater modeling suites in the presence of a variety of boundary conditions and considering high resolution matching (for MODFLOW) and nonmatching (for MODFLOW6) grids, and (b) a test analytical solution. Our results show that the average value of relative root mean square error (RRMSE) resulting from comparing our approach against the analytical solution and the MODFLOW simulations performed on the highly resolved grid (which we consider as reference) was always lower than 0.50%, thus imbuing us with confidence with respect to the accuracy of our proposed scheme. Additionally, while the requirement of CPU time associated with our algorithm is of the same order (on average) as the one associated with MODFLOW6 (benefits being noted mainly for settings involving flow in confined groundwater scenarios), our scheme is highly flexible in terms of spatial discretization and is characterized by higher accuracy for a given discretization.

How to cite: Qian, Y., Zhu, Y., and Guadagnini, A.: An improved Local Grid-Refined Numerical Groundwater Model Based on the Vertex-centred Finite-Volume Method, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8082, https://doi.org/10.5194/egusphere-egu22-8082, 2022.

The characterization of the inherent heterogeneity of aquifer systems on a regional scale represents one of the main challenges in the study of groundwater, especially when there are uncertainties associated with the scarcity of hydrogeological information (Maliva, 2016).

Based on the implementation of a numerical hydrogeological model in a regional scale, the objective of this study was to characterize the most recent geological formations which have the greatest hydrogeological potential in the Middle Magdalena Valley (VMM) in Colombia. The VMM is bounded by the Eastern Cordillera of the Colombian Andes and the Bucaramanga-Santa Marta fault to the east and the San Lucas Mountain range and the Central Cordillera to the west. This is considered one of the main hydrogeological basins in the country, in which a high potential of hydrocarbon production from the exploitation of unconventional deposits has also been identified (Sarmiento et al., 2015; Londoño, 2019).  For these reasons, the groundwater management, based on the characterization of this hydro system, has a national interest.

In this study the characterization of the heterogeneity was approached from the estimation of hydraulic parameters by solving the inverse problem by means of a hydrogeological model on a regional scale (Carrera et al., 2005; Zhou et al., 2014). For this, 153 pumping tests were carried out and interpreted, allowing to parameterize the model in seven iso-conductivity zones. Also 12,383 discrete static level data were measured and collected in 19 years -time window and they were used in the calibration process. Furthermore, three field campaigns for measuring in-situ parameters (electrical conductivity, pH, dissolved oxygen, and temperature) were performed they served to enhance the conceptual model.

Based on this, variations in hydraulic conductivities were identified on a regional scale for each iso-conductivity zone, fluctuating by four (4) orders of magnitude. The main flow direction was in the south-north, parallel to the Andes cordillera and therefore to the Magdalena River, and with some minor local flows perpendicular to them, producing important outflows from the system in permanent lentic water bodies in the north. The results of the research are encouraging, but at a regional scale they still do not allow to have a high resolution of the heterogeneity of the hydro system models for decision making, so it is suggested to implement stochastic models at a regional scale and to construct multi-purpose groundwater monitoring networks in this basin.

Acknowledgments
The researcher thanks the MEGIA Research Project, Contingent Recovery Contract FP44842-157-2018 funded by Minciencias and the National Hydrocarbons Agency.

How to cite: Lora, B., Silva, L., Castro, E., and Donado, L.: Characterization of spatial heterogeneity in sedimentary aquifer systems at regional scale by using hydrogeological modeling: A case study of the Middle Magdalena Valley basin - Colombia., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10680, https://doi.org/10.5194/egusphere-egu22-10680, 2022.

Immiscible displacement of fluids across fractures with spatially variable aperture is key in several subsurface processes, including enhanced oil recovery and geological CO₂ sequestration. We illustrate the results of an experimental investigation campaign focused on qualitative and quantitative assessment of main features associated with single and multiphase flow in a single fracture, including, e.g., an appraisal of the geometrical parameters of the fracture and the distribution and dynamic characteristics of fluids.
Experiments are performed on fracture replicas reproduced from natural rock blocks, that are artificially split by steer wedges under normal load. Two fracture replicas are then considered, corresponding to (a) a sample molded with synthetic material after the rock sample and (b) the actual rock sample, respectively. This enables one to provide a first appraisal of the impact of the material constituting the wall of the fracture on the multiphase flow system behavior. A transparent upper wall is set in place to enable visualization.
Surface profiles of the fracture are collected, comprising a set of more than 400,000 data of aperture distribution to create a digital twin of the system. These data are first subject to detailed statistical characterization, including standard geostatistical and fractal-based approaches. Experimental data of flow are quantitatively correlated to the key statistical features characterizing fracture geometry under various flow rates and normal loading conditions. Temporal monitoring of fluid saturation distribution enable us to provide a preliminary assessment of the impact of the material constituting the fracture wall (e.g., in terms of its wettability) on fluid saturation distribution.

How to cite: Wang, P., Huang, Y., Guadagnini, A., and Zhao, X.: Visualization and quantification of single and multiphase flow in a rough fracture, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11193, https://doi.org/10.5194/egusphere-egu22-11193, 2022.

Darcy scale transport in porous media ranges between Fickian and non-Fickian according to the medium conductivity layout, which ranges between homogenous and heterogeneous. Yet, evidence shows that preferential flows that funnel and bypass even areas with high conductivity occur in heterogeneous and homogenous cases. We model the Darcy scale transport using a 2D conductivity field ranging from homogenous to heterogeneous and find that these preferential flow bifurcate, leaving voids where particles do not invade while forming a tortuous path. The fraction of bifurcations decreases downflow and reaches an asymptotical value, which scales as a power-law with the heterogeneity level. We show that the same power-law scaling holds for the void fraction, tortuosity, and fractal dimension analysis. We conclude that the scaling with the heterogeneity is the dominant feature in the preferential flow geometry, which will lead to variations in weighting times for the transport and eventually to anomalous transport.

How to cite: Edery, Y. and Dagan, A.: Bifurcating-Paths: the relation between preferential flow bifurcations, void, and tortuosity on the Darcy scale., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11340, https://doi.org/10.5194/egusphere-egu22-11340, 2022.

Biologically mediated degradation of organic compounds is heavily non-linear. When an organic compound is degraded part of the carbon is present in the form of metabolite while a fraction of it is used to increase the biomass, capable then to enhance the degradation process. The rate of biomass growth is usually modeled with the experimentally derived Monod equation, so that it is proportional to the actual existing biomass multiplied by a non-linear factor in terms of available organic matter. The non-linearity in the degradation equation implies a strong difficulty in directly implementing a numerical solution within a Lagrangian framework. Thus, numerical solutions have traditionally been sought in an Eulerian framework.

Here we pursue a fully Lagrangian solution to the problem. First, the Monod empirical equation is derived from a two-step reaction ( B+C → ^k1 BC → ^k2 B +  ΔB + P); while the approach is less general to other derivations existing in the literature, it allows two things: (1) providing some physical meaning to the actual parameters in Monod equation, and more interestingly (2) formulate a methodology for the solution of the degradation equation incorporating Monod kinetics by means of a particle tracking formulation. For the latter purpose, reactants and biomass are represented by particles, and their location at any given time is represented by a kernel that includes the uncertainty in the actual physical location. By solving the reaction equation in a kernel framework, we can reproduce the Monod kinetics and, as a particular result in the case of no biomass growth is allowed, the Michaelis–Menten kinetics. We show how the method is successfully applied to reproduce two studies of microbially induced degradation. First, the observed kinetics of Pseudomonas putida F1 in batch reactors while growing on benzene, toluene and phenol, and second, the column study of carbon tetrachloride biodegradation by the denitrifying bacterium Pseudomonas Stutzeri KC.

How to cite: Dawi, M. and Sanchez-Vila, X.: Modeling biodegradation and growth of microorganisms via particle tracking, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11821, https://doi.org/10.5194/egusphere-egu22-11821, 2022.

Recent studies have shown that chaotic advection is spontaneously produced by laminar flows through granular media such as bead packs, strongly impacting solute mixing rates. This has strong implications for many reactive and biological processes in the subsurface. Chaotic dynamics could also be key in a wide range of environmental and industrial applications driven by mixing. Beside granular media, there is still no evidence that chaos broadly arises in the large variety of porous architectures that exist. In particular, it is unknown how the pore structure and topology can control chaotic dynamics.
In this study, we numerically investigate the mixing behavior of solute for a wide range of natural and engineered porous material that goes from carbonates and sandstones to beadpacks. We quantify chaotic advection by measuring Lagrangian stretching statistics (Lyapunov exponent) and its impact on mixing by estimating the decay of solute concentration variance. We find that stretching and mixing rates vary significantly between the different classes of porous architectures. A simple topological model is proposed to explain this behavior.

How to cite: Puyguiraud, A., Le Borgne, T., and Heyman, J.: Chaotic mixing in laminar flows through rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13055, https://doi.org/10.5194/egusphere-egu22-13055, 2022.