In geological formations, migration of CO2 plume is very complex and irregular. To make CO2 capture and storage technology feasible, it is important to quantify CO2 amount associated with possible leakage through natural occurring faults and fractures in geologic medium. Present work examines the fracture aperture effect on CO2 migration due to free convection. Numerical results reveal that fracture with larger-aperture intensify CO2 leakage. Mathematical formulation and equations of state for the mixture are implemented within the object-oriented finite element code OpenGeoSys developed by the authors. The volume translated Peng-Robinson equation of state is used for material properties of CO2 and water.