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 solution-gas
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/temperature (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.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
7 July 2008
- Meeting paper published:
21 September 2008
- Revised manuscript received:
14 December 2008
- Manuscript approved:
9 April 2009
- Published online:
11 March 2010
- Version of record:
20 April 2010
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