A Mixing Rule of Self-Diffusivities in Methane Hydrocarbon Mixtures and the Determination of GOR and Oil Viscosities from NMR Log Data

摘要

A new mixing rule is described for the prediction of the self-diffusivities of gas and liquid hydrocarbon molecules in methane-oil mixtures. Unlike macroscopic fluid properties such as density and viscosity, molecule self-diffusivity is a microscopic parameter associated with individual molecule species. Thus, a self-diffusivity of a binary mixture of gas and oil is non-existent. Instead, the self-diffusivity of each species in the binary system is affected by presence of the other species. For that reason, the commonly used log-mean based mixing rule that applies to macroscopic properties of the mixtures is unsuitable for self-diffusivity when the reference states are pure Component 1 and pure Component 2. We found that it is necessary to introduce two new reference states: one is the infinite dilution of Component 1 in Component 2; the other is the infinite dilution of Component 2 in Component 1. The component can be a single-molecule species or a mixture; as long as its self-diffusivity or self-diffusivity distribution can be measured. Using this approach, the self-diffusivity of each component in the mixture follows the log-mean based mixing rule. This new mixing rule is verified with literature data of methane-hexane, ethane-hexane, methane-octane, ethane-octane, methane-decane, and ethane-decane mixtures over a wide range of temperatures, pressures, and solution gas concentrations.