Estimating diffusion coefficients of dense fluids

Gas injection (hydrocarbon, nitrogen, or carbon dioxide) into oil and condensate reservoirs may be attempted to recover more in situ hydrocarbons. In some cases, particularly in naturally fractured reservoirs, there has been a need for information on the rate and amount of mass transfer by diffusion. The most important property, to obtain such information, is the diffusion coefficient at reservoir conditions. A simple and generalized correlation in terms of viscosity and molar density is proposed to estimate diffusion coefficients for hydrocarbon systems. The correlation can be used for both gases and liquids up to a pressure of about 400 bar (6,000 psia). It has been shown that the proposed method may also be used to estimate effective diffusion coefficients in multicomponent systems with a reasonable degree of accuracy. Although the proposed correlation is based on experimental data in hydrocarbon systems, preliminary evaluations have shown that it is also satisfactory for non-hydrocarbon systems as well. The proposed equation predicts diffusion coefficients in gases with an absolute average deviation of 8% and in liquid systems with an absolute average deviation of 15%. The input parameters for the correlation are molecular weight, critical properties, and acentric factor of components in the system; mixture molar density; low-pressure gas viscosity; and actual viscosity. The last three properties may be predicted from appropriate correlations.