Reservoir simulation studies were performed to investigate compositional effects between aquifer fluid (brine) and injected supercritical CO2 during the sequestration process in the Frio brine formation. Accurate data calibrations of CO2 solubility and density, as well as brine density and viscosity, were performed. Hysteresis relative permeability was taken into consideration to account for the effect of trapped gas in the aquifer. In addition, real aquifer data obtained from the test site were used in order to characterize the Frio aquifer. The Frio Brine Storage Project is a study in USA that was conducted by the Bureau of Economic Geology (BEG) at the University of Texas at Austin as a pilot to investigate the CO2 sequestration process in an aquifer as the first closely monitored case study. The objective of monitoring this pilot study, which was conducted by a collaboration of several research organizations including BEG, Lawrence Berkley National Laboratory (LBNL), Sandia Technologies LLC, US Geological Survey (USGS), and Schlumberger, and funded by the DOE's National Energy Technology Laboratory (NETL), was to observe the behavior of supercritical CO2 in the subsurface, investigate the physics of the process, and obtain data for further validation of simulation results. This simulation study was carried out before the pilot began, and the results were verified against the early results of the field experiment. Different scenarios were simulated to investigate the lateral and vertical spread of the CO2 plume in the aquifer. Local grid refinement around both injection and monitoring wells helped to obtain accurate results. According to our studies, at the end of the simulation period (10 years from the injection date), a significant part of the injected CO2 will be in the form of trapped gas or will be dissolved in the aqueous phase. Considering the proposed injection plan, and since only a small portion of the injected CO2 will be in the form of free gas, the possibility of CO2 escape through the shale cap rock will be very low. (C) 2008 Elsevier Ltd. All rights reserved.