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Convective-Reactive Transport and Unstable Density-Driven Flow in Fractured Porous Media

Shafabakhsh, Paiman | 2020

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53020 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Ataie-Ashtiani, Behzad
  7. Abstract:
  8. The goal of this study is to explore the density-driven flow and study the effect of fracture as well as chemical processes and reactions on convective transport. Convective flow is used in connection with the density-driven flow where the flow is driven by density differences in the fluid, which can be affected by the ambient rocks. Several studies of density-driven flow in porous media have focused on the effect of heterogeneity on the mixing convection; however, they neglect the key processes of geochemical reactions in fractured porous media. This study aims to address this gap by investigating the combined effect of heterogeneity (as fractures) and the existing geochemical reactions on convective mixing to enhance our awareness of reactive-convective flow mechanism in the geological formations. For this aim, we have studied two models: First, the Elder problem, which is the well-known example of an unstable density-driven flow and solute transport in porous media for practical and environmental purposes. Second, the hypothetical model of carbon dioxide (CO2) injected into the subsurface reservoir as a part of carbon storage to examine the impact of fractures and chemical reactions on the convective-reactive transport of dissolved CO2, as understanding the underlying mechanisms of CO2¬ within the reservoirs after injection is the prime importance for reducing greenhouse gas emissions. The methodology is based on numerical simulations with COMSOL Multiphysics, which is a commercial finite-element simulation package. In each model, we have explored the effect of fracture properties and topology on the mixing convection. The results of the Elder problem 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 occurs with lower critical Rayleigh number, which means that fracture networks have a destabilizing effect. Also, we have 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. The results of the model of CO2 storage show that depending on the fracture’s characteristics, the fractures not only can help the mixing convection and reaction process in the domain but also may play a restrictive role in entering of dissolved CO2 and hinder the plume fingers to growth. For more representative geology, we have investigated the migration-dissolution of buoyant CO2 on a large-scale outcrop of a volcanic basalt rock formation. It is found that neglecting thin fractures, to reduce model complexity, can significantly affect the predicted amount of trapped CO2. The storage capacity is more sensitive to heterogeneity at low dissolution rates. The findings are likely to be useful for the management of CO2 sequestration in fractured domains
  9. Keywords:
  10. Coastal Aquifer ; Fracture ; Carbon Dioxide Separation ; Density Current ; Fingering ; Geochemical Reaction ; COMSOL Software

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