The research conducted here is an attempt to develop a simulation framework of CO2 sequestration in geologic formations. One of the very important parts of this framework is centered on phase equilibrium computations between CO2 and Brines for a wide range of temperature and pressure. Besides accuracy of the models, time efficiency is extremely important to save computational expenses. Therefore, thorough investigations have been carried out to model the phase equilibrium of CO2 and Brines system over the temperature range 20 - 300 °C, and pressure range 1 - 600 bar from different perspectives, like, vapor state Equation of State (EoS), liquid state EoS, and Statistical Associating Fluid Theory (SAFT), in time efficient manner. First, a non intuitive scheme and a new EoS for CO2 has been proposed which gives more than 1000 times speed up after integrating with the simulator compared with other EoS's. To model the phase equilibrium for super critical CO2, currently available liquid state models such as UNIQUAC, LSG, NRTL, and GEM-RS have been modified which reproduce literature data within fair deviation. SAFT, which is a theoretically sound model, has also been modified to apply in the simulator. A comprehensive study is carried out to model the viscosity of CO2 and Brines applicable for the geologic environment. While modeling the viscosity, the effect of CO2 dissolution is taken into consideration. Double diffusive natural convection of CO2 in brine saturated porous media is investigated to show how CO2 dissolves over the time after injection.