Diffusion is an influential mechanism in oil recovery, which accelerates miscibility achievement, and usually has difficulties in modeling by commercial software. Previous studies have illustrated that compositional software could not calculate diffusion flux properly between two neighboring grids, in which oil saturation has a sudden drop from 1 to 0. Therefore, a dummy zone with medium oil saturation has to be defined at the oil/gas interface; however, this would affect the material balance and fluid flow. Furthermore, it is impossible to use such a method in a dual-pore fracture model and it would impose severe restrictions for full field simulation. In the first part of this work, a comprehensive review has been done on estimation of diffusion coefficients. Also, simulation of miscible CO2 displacement in fractured models has been reviewed and simulation difficulties have been widely discussed. Then, to analyze diffusion flux specifically, a 1-D horizontal model with neither viscose nor gravity force is simulated using two of the most famous compositional simulators, and their results are compared with analytical solution of the problem. Next, experimental results of miscible CO2 injection in a fractured system have been used to examine the new approach on simulation of a more complicated case.


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