The viscosity of natural gas depends on the temperature, pressure, and composition of the gas. It has units of centipoise (cp). It is not commonly measured in the laboratory because it can be estimated with good precision. Carr, Kobayashi, and Burrows have developed correlation charts, Figs. 2.6 and 2.7, for estimating the viscosity of natural gas from the pseudoreduced temperature and pressure.17 The pseudoreduced temperature and pressure may be estimated from the gas specific gravity or calculated from the composition of the gas. The viscosity at 1 atm and reservoir temperature (Fig. 2.6) is multiplied by the viscosity ratio (Fig. 2.7) to obtain the viscosity at reservoir temperature and pressure. The inserts of Fig. 2.6 are corrections to be added to the atmospheric viscosity when the gas contains nitrogen, carbon dioxide, and/or hydrogen sulfide. Example 2.6 illustrates the use of the estimation charts.
Figure 2.6 The viscosity of hydrocarbon gases at 1 atm and reservoir temperature, with corrections for nitrogen, carbon dioxide, and hydrogen sulfide (after Carr, Kobayashi, and Burrows, trans. AlME).17
Figure 2.7 Viscosity ratio as a function of pseudoreduced temperature and pressure (after Carr, Kobayashi, and Burrows, trans. AlME).17
Example 2.6 Using Correlation Charts to Estimate Reservoir Gas Viscosity
Given
Reservoir pressure = 2680 psia
Reservoir temperature = 212°F
Well fluid specific gravity = 0.90 (Air = 1.00)
Pseudocritical temperature = 420°R
Pseudocritical pressure = 670 psia
Carbon dioxide content = 5 mol %
Solution
μ1 = 0.0117 cp at 1 atm (Fig. 2.6)
Correction for CO2 = 0.0003 cp (Fig. 2.6, insert)
μ1 = 0.0117 + 0.0003 = 0.0120 cp (corrected for CO2)
μ/μ1 = 1.60 (Fig. 2.7)
μ = 1.60 × 0.0120 = 0.0192 cp at 212°F and 2608 psia
Lee, Gonzalez, and Eakin developed a semiempirical method that gives an accurate estimate of gas viscosity for most natural gases having specific gravities less than 0.77 if the z-factor has been calculated to include the effect of contaminants.18 For the data from which the correlation was developed, the standard deviation in the calculated gas viscosity was 2.69%. The ranges of variables used in the correlation were 100 < p (psia) < 8000, 100 < T (°F) < 340, 0.55 < N2 (mol %) < 4.8, and 0.90 < CO2 (mol %) < 3.20. In addition to the gas temperature and pressure, the method requires the z-factor and molecular weight of the gas. The following equations are used in the calculation for the gas viscosity in cp:
where
where
ρg = gas density from Eq. (2.5), g/cc
p = pressure, psia
T = temperature, °R
Mw = gas molecular weight
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