The Viscosity of Natural Gases

Abstract Experimental viscosity and density data of four natural gases are presented for temperatures from 100 to 340 F and pressures from 100 to 8,000 psia. A correlation is discussed and results reported. Introduction This investigation is one of several efforts by the authors to provide viscosity data for pure hydrocarbons and mixtures. Results of some pure hydrocarbons and their mixtures have been presented. Several predictive methods and correlations have also been reported. This paper presents the experimental viscosity and density data of four natural gases and confirms a correlation in a previous study. APPARATUS The viscometer was described previously. A bank of stainless steel pycnometers was included to determine density in conjunction with the viscosity measurements (Fig. 1). DATA AND MATERIALS The natural gases were furnished by the Atlantic Richfield Co. (Samples 1 and 2), the Continental Oil Co. (Sample 3) and the Pan American Petroleum Corp. (Sample 4). Table 1 shows the composition of these gases obtained by mass spectrometer analysis. The pure component viscosity data used in the correlation have been published. Experimental and calculated viscosity data for the natural gases are presented in Tables 2 through 5. CORRELATION Starling and Ellington have reported several semi-empirical expressions based to a certain extent on the theory of viscosity advanced by Born and Green. The final expression presented by Starling and Ellington is: (micropoise)= o expX(T) pY(T)....... (1) Eq.1 was modified by Lee et al. to represent mixture and pure component data simultaneously. This equation has the form: ..........(2) where K= ..........(3) X= ..........(4) Y= ..........(5) Over the pressure and temperature range studied in this investigation, this equation represents the data on methane, ethane, propane, n-butane and four methane-n-butane mixtures with a standard deviation* of 1.89 per cent. Density data by Sage and Lacey were used to fit the equations. Eq. 2 gives reliable values of the viscosity of light hydrocarbons without prior knowledge of experimental viscosity values, but accurate density data must be available. Eq. 2 was used to predict viscosity values for the natural gases studied in this paper. The particular set of parameters contained in Eqs. 3 through 5 gave values that reproduced experimental data within 5 per cent. Needless to say, in using Eq. 2 better density values will give better viscosity results. However, those engineers who do not have accurate density values at hand or the time, or such facilities as a library. computer, etc. readily available, may feel uneasy in using Eq. 2. Therefore, the authors sought the easiest but by no means best density prediction method, which was reported by Kay 30 years ago. A set of density values, which were calculated based on Kay's method, and a generalized compressibility factor chart were used to fit Eq. 2. JPT P. 997ˆ