Convective dissolution of CO2 is a well-known mechanism in geological storage of CO2 . It is triggered by gravitational instability which leads to the onset of free convection. The phenomenon is well studied in porous media, such as saline aquifers, and the literature provides substantial evidence that onset times and effective flux rates can be estimated based on a characterization of instabilities that uses the Darcy velocity. This work extends the study of convective dissolution to open water-filled fractures, where non-Darcy regimes govern the induced flow processes. Numerical simulations using a Navier-Stokes model with fluid density dependent on dissolved CO2 concentration were used to compute scenario-specific results for effective CO2 entry rates into an idealized fracture with varying aperture, temperature, and CO2 concentration at the gas-water interface. The results were analyzed in terms of dimensionless quantities. They revealed a Rayleigh invariance of the effective CO2 flux after the complete formation of a quasi-stationary velocity profile, i.e. after a certain entry length. Hence, this invariance can be exploited to estimate the effective CO2 entry rates, which can then be used, in perspective, in upscaled models. We have studied convective CO2 dissolution for two different fracture settings; the first one relates to karstification scenarios, where CO2 is the dominant driving force, and were stagnant-water conditions in fractures have not yet received attention to date. The second setting is inspired from geological CO2 storage, where the literature provides only studies on convective CO2 dissolution for porous-media flow with Darcy regimes.

Enzymatically Induced Calcite Precipitation (EICP) in porous media can be used as an engineering option to achieve targeted precipitation in the pore space, e.g. with the aim to seal flow paths. This is accomplished through an alteration of porosity and, consequently, permeability. A major source of uncertainty in modelling EICP is in the quantitative description of permeability alteration due to precipitation. This study investigates experimentally the time-resolved effects of growing precipitates on porosity and permeability on the pore scale in a PDMS-based micro-fluidic flow cell. The experimental methods are explained; these include the design and construction of the micro-fluidic cells, the preparation and usage of the chemical solutions, including the injection strategy, and the monitoring of pressure drops at given flux rates to conclude on permeability. Imaging methods are explained with application to EICP, including optical microscopy and X-Ray micro-Computed Tomography (XRCT) and the corresponding image processing and analysis. We present and discuss detailed experimental results for one particular micro-fluidic set-up as well as the general perspectives for further experimental and numerical simulation studies on induced calcite precipitation. The results of the study show the enormous benefits and insights of combining both light microscopy and XRCT with hydraulic measurements in micro-fluidic devices. This allows for a quantitative analysis of the evolution of precipitates with respect to their size and shape, while monitoring the influence on permeability. We can demonstrate that we improved the interpretation of monitored flow data dependent on changes in pore morphology.

Density-driven dissolution of carbon dioxide in water is a well-known and much described mechanism in geological sequestration of this greenhouse gas. It is remarkable that such enhanced dissolution does not receive much attention in karst hydrology and speleology. Models and hypotheses on karst development are complex and consider many different processes. We focus here on the influence of CO2 partial gas pressures at the interface between atmosphere and karst water on the dynamics of dissolved CO2 concentrations below the water table. Seasonal variation of microbial soil activity and root respiration or barometric-pressure changes cause fluctuations in CO2 partial pressures. Dependent on the existence and strength of a karst-water background flow, fingering regimes might be triggered causing enhanced dissolution of CO2 This allows replenishment of CO2, and, thus, dissolutional power even deep in the water body without the need for percolating water to transport dissolved CO2. We present and discuss simplified and generic experimental and computational scenarios to strengthen our claim, and we try to give answers to: how much? and under which circumstances? The applied numerical model solves the Navier-Stokes equation with water density dependent on CO2 concentration and temperature. We show that calculated CO2 mass fluxes into the water bodies are dependent on the ratio of P\’{e}clet to Rayleigh numbers (Pe/Ra) and show a local minimum around Pe/Ra=1, i.e. when natural and forced convection are about equal. Concluding, we claim there is sufficient reason to consider density-driven dissolution as a process of relevance in karstification if circumstances are given.

Employing kinetic interface sensitive (KIS) tracers, we investigate three different types of glass-bead materials and two natural porous media systems to quantitatively characterize the influence of the porous-medium grain-, pore-size, and texture on the “mobile” interfacial area between an organic liquid and water. By interpreting the breakthrough curves (BTCs) of the reaction product of the KIS tracer hydrolysis we obtain a relationship for the specific interfacial area (IFA) and wetting saturation. The immiscible displacement process coupled with the reactive tracer transport across the fluid-fluid interface is simulated with a Darcy-scale numerical model. The results show that the current reactive transport model is not always capable to reproduce the breakthrough curves of tracer experiments and that a new theoretical framework is required. Total solid surface area of the grains, i.e., grain surface roughness, is shown to have an important influence on the capillary-associated IFA by comparing results obtained from experiments with spherical glass beads having very small or even no surface roughness and those obtained from experiments with the natural sand. Furthermore, a linear relationship between the mobile capillary associated IFA and the inverse mean grain diameter can be established. The results are compared with the data collected from literature measured with high-resolution microtomography and partitioning tracer methods. The capillary associated IFA values are consistently smaller because KIS tracers measure the mobile part of the interface. Through this study, the applicability range of the KIS tracers is considerably expanded and the confidence in the robustness of the method is improved.