Selective diffusion inclusion for a reservoir simulation for hydrocarbon recovery

A method includes creating a diffusion model for a simulation of hydrocarbon recovery from a reservoir having a plurality of fractures during injection of an injected gas into the plurality of fractures, wherein the reservoir is partitioned into a plurality of grid cells in the diffusion model. Creating the diffusion model for a grid cell of the plurality of grid cells comprises, determining a flux ratio of a convective flux to an estimated diffusive flux for the grid cell, determining whether the flux ratio is less than a threshold, and in response to determining that the flux ratio is less than the threshold, determining a full diffusive flux for the grid cell for inclusion in the diffusion model. The method also includes performing the simulation of the hydrocarbon recovery from the reservoir based on the diffusion model.