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

Abstract

Summary 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, molecular self diffusivity is a microscopic parameter associated with individual molecular species. Thus, a self diffusivity of a binary mixture of gas and oil is a misnomer. Instead, the self diffusivity of each species in the binary system is affected by the 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 Components 1 and 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 solutiongas concentrations. The new mixing rule is applied, and a detailed procedure is developed, to determine the gas/oil ratio (GOR) and the live-oil viscosity for in-situ volatile oils from nuclear-magnetic-resonance (NMR) log data. First, the self diffusivities and proton fraction of the methane and oil mixture are determined by fitting the NMR-measured diffusivity distribution with a bimodal distribution. Then, the self diffusivities for the four reference states are calculated using the mixing rule. Finally, the GOR is calculated from the proton ratio, while the live-oil viscosity is calculated from the self diffusivity of pure oil and the GOR. The calculated GOR and oil viscosities are compared to the pressure/volume/temerature (PVT) measurement of the oil sample taken from downhole and show good agreement. GOR and oil viscosities from this new technique can be used for optimizing testing and sampling programs, for reservoir-simulation studies, and for the design of surface production facilities.