ResvEngine

Gas PVT · ResvEngine standard pack

Gas Z and Bg (DAK)

ResvEngine computes gas compressibility factor Z with Dranchuk–Abou-Kassem (1975) — a numerical fit to the Standing–Katz chart — and pseudocriticals from Sutton (1985), with Wichert–Aziz when CO2 or H2S is present.

Pseudocritical properties (Sutton)

Sweet gas: Ppc and Tpc from gas specific gravity γg (air = 1). Sour components adjust Ppc′ and Tpc′ via Wichert–Aziz before reduced pressure and temperature are formed.

Ppc=756.8−131.07 γg−3.6 γg2[psia]P_{pc} = 756.8 - 131.07\,\gamma_g - 3.6\,\gamma_g^2 \quad [\mathrm{psia}]
Tpc=169.2+349.5 γg−74.0 γg2[∘R]T_{pc} = 169.2 + 349.5\,\gamma_g - 74.0\,\gamma_g^2 \quad [^{\circ}\mathrm{R}]

Z-factor (DAK)

DAK solves Z iteratively from reduced density ρr = 0.27 Pr / (Z Tr). Typical validity: 0.2 ≤ Pr ≤ 30, 1 ≤ Tr ≤ 3. At very low Pr the engine uses an ideal-gas fallback (Z → 1) and flags the row.

Root-finding: Newton–Raphson on ρr with the analytic derivative of the DAK residual, falling back to bisection if Newton doesn't converge — accurate and stable across the full envelope.

Gas FVF Bg

Bg=Z T/pZsc Tsc/pscB_g = \frac{Z\,T/p}{Z_{sc}\,T_{sc}/p_{sc}}

Bg in Rm³/Sm³; psc = 1.01325 bar, tsc = 15.5 °C (METRIC).

Try it

Defaults match a digitized Standing–Katz isotherm (γg = 0.7, T ≈ 52 °C). Edit freely; open the full calculator for tables and export.

Interactive Z vs Standing–Katz

DAK (blue) vs digitized Standing–Katz (orange) at the passport γ_g and T.

Overlay: Standing–Katz (digitized). Standing & Katz chart (docs/Literature JPEG); hand-digitized T_r≈1.55 between printed 1.5 and 1.6; γ_g=0.7 via Sutton P_pc. Uncertainty ~±0.01 in Z.

Auto-updates as you edit
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Hand-digitized from the Standing–Katz chart (not DAK-tuned). Expect ~±0.01 Z digitization error; RMSE vs DAK is incidental.

Open full calculator with these inputs

Full calculator: results table and PVTO/PVTG (.inc) export on the same METRIC basis.

Applicability & limits

  • Dry and sour black-oil gas — not compositional EOS or NGL tables.
  • Extreme high temperature cases may exceed DAK envelope — check flash status in the calculator.
  • Gas viscosity μg is not computed in the current product.
  • Dry-gas tank diagnostics use p/Z (Cole), not the oil Havlena–Odeh F vs E tot plane. The live match is on the material balance calculator.

References

  • Dranchuk, P.M.; Abou-Kassem, J.H. (1975). Calculation of Z Factors for Natural Gases Using Equations of State.
  • Standing, M.B.; Katz, D.L. (1951). Z-factor chart (Standing–Katz).
  • Sutton, R.P. (1985). Pseudocritical properties from γg.
  • Wichert, E.; Aziz, K. Sour-gas correction to pseudocriticals.
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