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.The study of carbon dioxide injection into fractured reservoirs is of great importance, because carbon dioxide is the most effective greenhouse gas in the global warming phenomenon; moreover, being the residual oil in the matrix of fractured reservoirs is significant, with the injection of carbon dioxide into the fractured reservoirs, various mechanisms are involved which reduce the amount of residual oil, among which, the most important are: gravity drainage, molecular diffusion, oil swelling, viscosity reduction, evaporation and extraction. In the present study, we have two goals. The first one is the extension of previous studies of CO2 molecular diffusion effects on single block modeling of fractured reservoirs considering three porosities, 11%, 26% and 44%. For the porosity effect study, it is observed that, at a higher porosity, the viscosity decreases later, and the duration of viscosity reduction as well as increase in viscosity, or oil swelling and oil evaporation mechanism, is longer. Also, the higher the porosity of the matrix, the greater the effect of the molecular diffusion on the oil recovery. The second goal is to present a new model which calculates the oil swelling factor. In this regard, Fick's second law is applied to a constant pressure diffusion cell considering the corresponding initial and boundary conditions. Then, taking into account the works by Kendall Marra et al. (1988), swelling model, by modifying the gridding and initial conditions, a new moving boundary swelling factor numerical model is presented. The model results are compared with measured experimental swelling factor data, which show good compatibility.