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

Abstract

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 $<1$), (2) near-inlet clogging (Da/Pe $>100$), (3) fracture isolation ($1<$ Da/Pe $<100$ and Pe $>1$), 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.