Mathematical modeling of convective mixing in porous media for geological carbon dioxide storage

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 coming 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 CO 2 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 CO 2 storage sites.;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 CO 2 in saline aquifers is presented. The convective mixing of CO 2 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 CO 2 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.;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, homogenous 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.