There are two commonly used correlations for determining the viscosity of natural gases. These are the Carr, Kobayashi and Burrows (CKB) and the Lee, Gonzalez and Eakin (LGE). The CKB applies to both sweet and sour gas, but the LGE was derived purely for sweet gas. The CKB is a graphical correlation, and consists of two sets of graphs with temperature as the correlating parameter (not easily adaptable for computer applications). On the other hand, the LGE is a set of simple equations, and therefore can be easily programmed in to a computer. The LGE is used in numerous computer programs, with no regard to the fact that it was not derived for sour gas.
This study compares the LGE equation to the CKB graphs for both sweet and sour gases. A detailed comparison of the two methods using various gas mixture compositions, temperatures and pressures was conducted. The results illustrate that for sweet gases, the comparison is acceptable, and the LGE equation can be used for most reservoir engineering purposes. However, for sour gases, the differences between the LGE and CKB correlations can be significant. This leads to the conclusion that the LGE correlation, which was derived for sweet gases only, should not be indiscriminately applied to sour gases.
Gas viscosity is used in several calculations dealing with fluid flow and reservoir behavior. Empirical correlations have been developed to estimate values for viscosity over a range of pressures. Most viscosity correlations are either equation based or in a graphical form derived from laboratory measurements. Typically the correlations are functions of temperature, pressure, gas gravity and composition. Often they are paired with various corrections for non-hydrocarbon components. Applying a correlation without corrections to a sour gas case can lead to large errors in predicting viscosity.
Carr, Kobayashi and Burrows Correlation
The Carr et al correlation (henceforth referred to as CKB) was developed to predict the viscosity of gas hydrocarbon mixtures over a large pressure and temperature range. The limits of the correlation are temperatures between 32 and °400F and reduced pressures up to 20 (which can equate to pressures higher than 12000 psia). It was designed to handle nonhydrocarbon components (CO2, N2, H2S) in concentrations of up to 15 % each.
Calculation of viscosity with the CKB method is a three step process based on graphical methods. The first step is to determine the viscosity at atmospheric pressure based on the gas gravity and temperature using figure 1. Next, corrections are added to the viscosity based on the H2S, CO2 and/or N2 content of the gas. Finally, the viscosity is multiplied by a ratio based on the reduced pressure from figure 2. The only disadvantage of the CKB method is that it is based on several charts, making it difficult to program into a computer. Two methods that have been used to simplify computation have been to generate a series of polynomial curve-fits or to use a table lookup-interpolation scheme(1).