The Elder problem is one of the well-known examples of an unstable density-driven flow (DDF) and solute transport in porous media. The goal of this research is to investigate the influence of fracture networks on this benchmark problem due to the great importance of the fractured heterogeneity effect on unstable DDF. For this aim, the fractured Elder problem is solved using COMSOL Multiphysics, which is a finite element method simulator. Uniform and orthogonal fracture networks are embedded to analyze free convective flow and development of unstable salt plumes. The results indicate that the mesh sensitivity of the fractured Elder problem is greater than the homogeneous case. Furthermore, it has been shown that in the fractured cases, the onset of instability and free convection occur with lower critical Rayleigh number, which means that fracture networks have a destabilizing effect. Also, we examined the structural properties of fracture networks that control convective flow patterns, and the simulation results show that the strength of convection and instability at the beginning of the intrusion is proportional to the aperture size of the fractures. Moreover, the increase of the fracture’s density leads different modes of transient convective modes, until a specific fracture density after which the transient convective modes become similar to the homogenous case.
<p>Flow and mixing processes in porous media control many natural and industrial systems, such as microbial clogging, oil extraction, and effluent disposal. In many systems, the porosity may evolve during mineral precipitation, such as in rocks, and control fluid mixing and fluid transport properties. Here, we use three-dimensional in situ dynamic neutron and X-ray micro-tomography imaging to explore fluid transport into Berea sandstone core samples during in-situ carbonate precipitation. Neutron imaging can track fluid flow inside the rock, whereas X-ray imaging illuminates the regions where mineral precipitation occurs. We control the precipitation of calcium carbonate in the rock through reactive-mixing between solutions containing CaCl<sub>2</sub> and Na<sub>2</sub>CO<sub>3</sub>. By solving the advection-diffusion equation using the contrast in neutron attenuation from time-lapse images, we derive the 3D velocity field of the injected fluids and characterize the evolution of the permeability field into the rock during mineral precipitation. We also investigate the mixing between heavy water and a cadmium solution under the influence of mineral precipitation. Results show that, under the effect of mineral precipitation, a wide range of local flow velocities develop in the sample, under the same fluid injection rate, and we quantify the distribution of flow velocities in the sample. Moreover, we observe more efficient mixing between heavy water and a cadmium solution after mineral precipitation. The finding of this experimental study is useful in progressing the knowledge in the domain of reactive solute and contaminant transport in the subsurface.</p>
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