PROGRAM CALCULATES Z-FACTOR FOR NATURAL GAS

    Feb. 16, 1993
    A.K. Coker A.K.C. Technology Sutton Coldfield, U.K. The Fortran program called "Physic" presented in this article (see box) calculates the gas deviation or compressibility factor, Z, of natural gas. The author has used the program for determining discharge-piping pressure drop.1 The calculated Z is within 5% accuracy for natural hydrocarbon gas with a specific gravity between 0.5 and 0.8, and at a pressure below 5,000 psia. Many petroleum engineering and process design calculations require Z.
    A.K. CokerA.K.C. Technology Sutton Coldfield, U.K.

    The Fortran program called "Physic" presented in this article (see box) calculates the gas deviation or compressibility factor, Z, of natural gas. The author has used the program for determining discharge-piping pressure drop.1

    The calculated Z is within 5% accuracy for natural hydrocarbon gas with a specific gravity between 0.5 and 0.8, and at a pressure below 5,000 psia.

    Z-FACTOR

    Many petroleum engineering and process design calculations require Z. But, experimental data from pressure-volume-temperature (PVT) measurements are seldom available.

    Therefore, charts and tables are often used to obtain Z as a function of pseudoreduced temperature, Tr, and pressure, Pr.2 Computer programs are also available to calculate Z solely as a function of temperature and pressure.34

    Numerical methods and mathematical representations of the charts can also be used to estimate Z. However, these charts are often time consuming and involve complex calculations.

    Various methods for estimating Z were reviewed by Takacs.5

    APPLIED EQUATIONS

    The Physic program modifies the method developed by Awoseyin.6

    Z is calculated by Equation 1-6 (see equation box) and the specific gravity, Sg, is determined from either the density or molecular weight of the gas (Equations 7 or 8).

    Table 1 is a typical computer print out for Z as a function of temperature, pressure, and specific gravity. Results for several specific gravities are plotted in Fig. 1.

    REFERENCES

    1. Coker, A. K., "Program sizes compressible flow for discharge piping," OGJ, Dec. 11, 1989, p. 63.

    2. Standing, M.B., and Katz, D.L., "Density of natural gases," Transactions AIME, Vol. 146, 1942, pp. 140-49.

    3. El-Gassier, M.M., "Fortran program computes gas compression," OGJ, July 13, 1987, p. 88.

    4. Gopal, V.N., "Gas Z-factor equations developed for computer," OGJ, Aug. 8, 1977, p. 59.

    5. Takacs, G., "Comparing methods for calculating Z-factor," OGJ, May 15, 1989, pp. 43-46.

    6. Awoseyin, R.S., "Program integrates pressure loss for single and two-phase pipelines," OGJ, June 23, 1986, P. 33.

    Copyright 1993 Oil & Gas Journal. All Rights Reserved.

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