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Abstract
Storage of CO2 into geological formations is a reasonable technical choice for decreasing carbon dioxide emissions to the atmosphere. The dissolution of supercritical CO2 in formation water is one of the main long term trapping mechanisms for CO2 storage into saline aquifers. Convective mixing is predicted to occur, which accelerates the dissolution of carbon dioxide in the saline formation water. This unusual phenomenon arises from the increase in the density of brine when saturated with carbon dioxide. Several factors influence the performance and long-term fate of CO2 injection into deep saline formations. The efficiency of mixing in density-driven natural convection is largely governed by aquifer permeability which is heterogeneous in practice. For deep injection of CO2 in deep saline formations, the movement of both free gas and dissolved CO2 are sensitive to variations in permeability. In this paper, the effects of anisotropy and different kinds of heterogeneity like horizontal and vertical layers and also existence of barriers between layers on the CO2 dissolution in a saturated porous media with brine are investigated using simulation methods. Also the performance of naturally fractured systems and the effective dissolution mechanisms in CO2 storage in such these systems are investigated. Following to simulation results it can be said that the permeability of the system and the permeability anisotropy ratio should be considered as the most important parameters in convective mixing process. In the barrier systems, the geometry of the barriers has a large effect on the density-driven natural convection while in layers systems, the vertical and horizontal location of the layers and also the degree of heterogeneity can be so important. In the case of natural fractured systems and based on the simulation studies on a single block fractured model, it can be said that density-driven natural convection is an effective dissolution mechanism in naturally fractured aquifers.

1. Introduction
Disposal of CO2 in geologic formations represents one of the most promising solutions for the purpose of reducing greenhouse gas emissions. The geological formations such as coal beds, depleted oil and gas reservoirs and deep saline aquifers are widely available with large capacity. In particular, the saline aquifers have an estimated capacity of 320 to 10,000 GT (1 GT = 109 Tonne) of CO2 worldwide and they can be considered as one of the major types of geological formations for CO2 storage (Bachu, 2002).