As concern about the adverse consequences of anthropogenic climate change has grown, so too has research into methods to reduce the emissions of greenhouse gases that will drive future climatic change. Carbon dioxide emissions arising from use of fossil-fuels are likely to be the dominant drivers of climate change over the corning century. The use of carbon dioxide and geologic storage (or sequestration) offers the possibility of maintaining access to fossil energy while reducing emissions of carbon dioxide to the atmosphere. One of the essential concerns in geologic storage is the risk of leakage of CO₂ from the injection sites. Carbon dioxide injected into saline aquifers, dissolves in the resident brines, increasing their density potentially leading to convective mixing. Convective mixing increases the rate of dissolution, and therefore decreases the time-scale over which leakage is possible. Understanding the factors that drive convective mixing and accurate estimation of the rate of dissolution in saline aquifers is important for assessing geological CO₂ storage sites.

This dissertation has three components, which includes linear stability analysis, prediction of CO₂-brine PVT, and numerical modeling. A hydrodynamic stability analysis is performed for non-linear, transient concentration fields in a saturated, hornogenous and isotropic porous medium under various initial and boundary conditions. The role of the natural flow of aquifers and associated dispersion on the onset of convection in the saline aquifers is also investigated. A fugacity and an activity models are combined to develop an accurate thermodynamic module appropriate for geological CO₂ storage application. A three-dimensional, two-phase and two-component numerical model for simulation of CO₂ storage in saline aquifers is also developed. The numerical model employs higher order and total-variation-diminishing schemes, capillary pressure, relative permeability hysteresis, and full dispersion tensor formulation. The model also takes into account an accurate representation of a CO₂-brine mixture thermodynamic and transport properties. The model is validated for a number of problems against one- and two-dimensional standard analytical and numerical solutions.

The theoretical analysis and numerical model are used to investigate the role of convective mixing on CO₂ storage in homogenous and isotropic saline aquifers. Scaling analysis of the convective mixing of CO₂ in saline aquifers is presented. The convective mixing of CO₂ in aquifers is characterized, and three mixing periods are identified. It is found that mixing achieved can be approximated by a scaling relationship for Sherwood number as a measure of mixing. Furthermore, the onset of natural convection and the wavelengths of the initial convective instabilities are determined. A criterion is also developed that provides the appropriate numerical mesh resolution required for accurate modeling of convective mixing of CO₂ in deep saline aquifers. In addition, using the model developed, a method to accelerate CO₂ dissolution in brines is also suggested. The acceleration of dissolution by brine pumping increases the rate of solubility trapping in saline aquifers and therefore increases the security of storage. Results of this dissertation give insight into appropriate implementation of large scale geological CO₂ storage in deep saline aquifers.