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Abstract
Evaluation and improvement of sweep efficiency are important issues of miscible displacement process. Miscibility is controlled by physical dispersion during miscible processes. It comprises of two parameters: diffusion and mechanical dispersion. Unfortunately this parameter is usually ignored during simulating the miscible processes which would be considered as an enormous source of error. Results show the existence of three regions at which each component of physical dispersion controls the miscible CO2 injection. For dispersivities between 0.123m to 0.175m the miscible process is controlled by physical dispersion mechanism while dispersivities lower than 0.123m specifies a region controlled by diffusion in the field case. The third region is the region for dispersivities greater than 0.175m and the main parameter affecting the miscible process in this region is mechanical dispersion. This study discusses the the effect of this parameter on recovery, mixing zone length and breakthrough time of injecting CO2 in a natural fracture reservoir in Iran.

Introduction
Mixing of fluids in porous media during miscible injection process is due to convection, diffusion and mechanical dispersion. Diffusion is the result of concentration gradient while mechanical dispersion is due to velocity variation. [1, 2]

The term physical dispersion which is usually used in miscible processes is the summation of diffusion and mechanical dispersion. Physical dispersion in porous media is a function of miscible fluids, structure of the medium, particle size of the porous media and the velocity of displacement.

Although diffusion and physical dispersion are essential parameters to simulate a miscible injection process, they are mostly neglected in simulations. This study shows the effect of such simplification on performance of the process. This simplification is mostly due to have scarce correlations at high pressure and temperature. Knowing the appropriate methods to calculate diffusion and mechanical dispersion, leaves no doubt in using these parameters in simulating the miscible processes. [3]

The effect of neglecting physical dispersion on performance of the system has been discussed in this Study. Effect of physical dispersion on breakthrough time and recovery of the process has been considered.