Experimental and numerical study of fluid flow, solute transport, and mineral precipitation in fractured porous media

Geothermal energy can replace fossil fuels, thus reducing CO$_2$ emissions and mitigating greenhouse effects. Similar to other subsurface reservoirs, geothermal reservoirs often consist of fractures,i.e. high-permeable conduits, and porous rock materials, i.e. low-permeable matrices. In such a fractured porous medium, mass and energy are considered to be transported mainly through the open fracture networks, while the rock matrices often serve as the source of mass and energy. With the aim of filling the knowledge gap in fluid flow, solute transport, and mineral precipitation, this thesis reports on an integrated experimental-numerical study of pore-scale flow and transport properties in a fractured porous medium. For our experimental studies, we use a well-defined, reproducible, 3D-printed medium which consists of two (i.e., high- and low-permeability) matrices, each containing one flow-through and one dead-end fracture. To conduct experimental measurements of fluid flow and solute transport, we employ Particle Image Velocimetry (PIV), Laser-induced Fluorescence (LIF), and Magnetic Resonance Imaging (MRI). In this context, an image analysis framework and a novel PIV method, i.e., temporo-ensemble method, have been developed to substantially increase the spatial resolution of velocity vectors per unit area/volume. We compare the velocity measurements obtained with PIV and MRI, and use the PIV measurements to calculate fluid exchange, and the dimensionless Beavers-Joseph velocity-slip ($\alpha$) and Whitaker stress-jump ($\beta$) coefficients at the fracture-matrix interfaces. Under the current definition of the boundaries at the physical fracture-matrix interfaces, the coefficients $\alpha$ and $\beta$ are incapable of explaining and predicting the fluid exchange between fractures and matrices. Consequently, a new quantity is proposed to relate the shear rates inside the fractures and inside the matrices around the fracture-matrix interfaces. LIF experiments are conducted to monitor the transport of tracer dyes during the progression and depletion processes. Based on the LIF measurements, the temporal and spatial evolution of the displacement front, moments of the concentration field, and solute mass fractions are elucidated. We explore the non-Fickian behavior of tracer dyes and associate it to the evolution of relative concentration in different regions of the medium. In our numerical work, we use Lattice-Boltzmann Methods (LBMs) to simulate fluid flow, solute transport, and mineral precipitation in the 3D-printed porous medium. We focus on the feedback loop of fluid flow, solute transport, mineral precipitation, pore-space geometry changes, and permeability. Our simulations are carried out over a wide range of species diffusivity and reaction rates from advection- to diffusion-dominated, and from transport- to reaction-limited, respectively. Using the ratio of Damköhler (Da) and the Peclet number (Pe) , the numerical results exhibit four distinct precipitation patterns, namely (1) no precipitation (Da/Pe $100$), (3) fracture isolation ($11$), and (4) diffusive precipitation ($1<$ Da/Pe $<100$ and Pe $<0.1$). Finally, this thesis establish a general relationship among mineral precipitation pattern, porosity, and permeability using statistical and spatial distribution. This doctoral thesis deepens the understanding of pore-scale processes using numerical simulations and experimental measurements in fractured porous media. In particular, this thesis has developed a novel experimental approach for validation of numerical and theoretical models. These results are of upmost importance to a wide range of scientific and industrial applications such as, but not limited to, geological CO$_2$ sequestration, hydrogeology, geothermal energy utilization, geochemistry, groundwater supply, subsurface contaminant migration, hydrometallurgical recovery, and evaporation from soil matrices

Flow characterization in fractured porous media using the temporo-ensemble PIV method

ISSN:1029-7006ISSN:1607-796

The Role of High-Permeability Inclusion on Solute Transport in a 3D-Printed Fractured Porous Medium: An LIF-PIV Integrated Study

It is well-known that the presence of geometry heterogeneity in porous media enhances solute mass mixing due to fluid velocity heterogeneity. However, laboratory measurements are still sparse on characterization of the role of high-permeability inclusions on solute transport, in particularly concerning fractured porous media. In this study, the transport of solutes is quantified after a pulse-like injection of soluble fluorescent dye into a 3D-printed fractured porous medium with distinct high-permeability (H-k) inclusions. The solute concentration and the pore-scale fluid velocity are determined using laser-induced fluorescence and particle image velocimetry techniques. The migration of solute is delineated with its breakthrough curve (BC), temporal and spatial moments, and mixing metrics (including the scalar dissipation rate, the volumetric dilution index, and the flux-related dilution index) in different regions of the medium. With the same H-k inclusions, compared to a H-k matrix, the low-permeability (L-k) matrix displays a higher peak in its BC, less solute mass retention, a higher peak solute velocity, a smaller peak dispersion coefficient, a lower mixing rate, and a smaller pore volume being occupied by the solute. The flux-related dilution index clearly captures the striated solute plume tails following the streamlines along dead-end fractures and along the interface between the H-k and L-k matrices. We propose a normalization of the scalar dissipation rate and the volumetric dilution index with respect to the maximum regional total solute mass, which offers a generalized examination of solute mixing for an open region with a varying total solute mass. Our study presents insights into the interplay between the geometric features of the fractured porous medium and the solute transport behaviors at the pore scale.ISSN:0169-3913ISSN:1573-163

High-resolution temporo-ensemble PIV to resolve pore-scale flow in 3D-printed fractured porous media

ISSN:0169-3913ISSN:1573-163

Quantification of mineral accessible surface area and flow-dependent fluid-mineral reactivity at the pore scale

Accessible surface areas (ASAs) of individual rock-forming minerals exert a fundamental control on the maximum mineral reactivity with formation fluids. Notably, ASA efficiency during fluid-rock reactions can vary by orders of magnitude, depending on the inflow fluid chemistry and the velocity field. Due to the lack of adequate quantification methods, determining the mineral-specific ASAs and their reaction efficiency still remain extremely difficult. Here, we first present a novel joint method that appropriately calculates ASAs of individual minerals in a multi-mineral sandstone. This joint method combines SEM-image processing results and Brunauer-Emmett-Teller (BET) surface area measurements by a Monte-Carlo algorithm to derive scaling factors and ASAs for individual minerals at the resolution of BET measurements. Using these atomic-scale ASAs, we then investigate the impact of flow rate on the ASA efficiency in mineral dissolution reactions during the injection of CO2-enriched brine. This is done by conducting a series of pore-scale reactive transport simulations, using a two-dimensional (2D) scanning electron microscopy (SEM) image of this sandstone. The ASA efficiency is determined employing a domain-averaged dissolution rate and the effective surface area of the most reactive phase in the sandstone (dolomite). As expected, the dolomite reactivity is found to increase with the flow rate, due to the on average high fluid reactivity. The surface efficiency increases slightly with the fluid flow rate, and reaches a relatively stable value of about 1%. The domain averaged method is then compared with the in-out averaged method (i.e the “Black-box” approach), which is often used to analyzed the experimental observations. The in-out averaged method yields a considerable overestimation of the fluid reactivity, a small underestimation of the dolomite reactivity, and a considerable underestimation of the ASA efficiency. The discrepancy between the two methods is becoming smaller when the injection rate increases. Our comparison suggests that the result interpretation of the in-out averaged method should be contemplated, in particular, when the flow rate is small. Nonetheless, our proposed ASA determination method should facilitate accurate calculations of fluid-mineral reactivity in large-scale reactive transport simulations, and we advise that an upscaling of the ASA efficiency needs to be carefully considered, due to the low surface efficiency.ISSN:0009-2541ISSN:1872-683

Shear induced flow path evolution in rough-wall fractures: A Particle Image Velocimetry examination

Rough-walled fractures in rock masses, as preferential pathways, largely influence fluid flow, solute and energy transport. Previous studies indicate that fracture aperture fields could be significantly modified due to shear displacement along fractures. We report experimental observations and quantitative analyses of flow path evolution within a single fracture, induced by shear displacement. Particle image velocimetry and refractive index matching tecques were utilized to determine fluid velocity fields inside a transparent 3D-printed shear-able rough fracture. Our analysis indicate that aperture variability and correlation length increase with the increasing shear displacement, and they are the two key parameters, which govern the increases in velocity variability, velocity longitudinal correlation length, streamline tortuosity, and variability of streamline spacing. The increase in aperture heterogeneity significantly impacts fluid flow behaviors, whilst changes in aperture correlation length further refine these impacts. To our best knowledge, our study is the first direct measurements of fluid velocity fields and provides insights into the impact of fracture shear on flow behavior.ISSN:0022-1694ISSN:1879-270