How to use this calculator
- Choose a unit system, then enter reservoir pressure and temperature.
- Enter gas gravity, oil API, and any CO₂/H₂S/N₂ mol% if known.
- Select pseudo-critical and z-factor correlations suited to your data.
- Provide either Rsb or Pb for oil screening.
- Press Calculate. Results appear above the form.
- Use the download buttons to export CSV or PDF.
Formulas used (engineering correlations)
- Reduced properties:
Pr = P/PpcandTr = T/Tpc. - Impurity adjustment (screening):
Ppc′ = Ppc + 440yCO₂ + 600yH₂S − 170yN₂,Tpc′ = Tpc − 80yCO₂ − 130yH₂S + 250yN₂.
- DAK: solves for reduced density
ρr = 0.27 Pr/(z Tr)iteratively, then evaluates z. - Papay: explicit fit using Pr and Tr.
- Gas density: real-gas equation with molecular weight from gas gravity.
- Bg:
Bg = 0.02827 z T/P(Field units). - Gas viscosity: Lee–Gonzalez–Eakin correlation (uses density and temperature).
- Bubble point: computed from Rsb (or inverted to estimate Rsb from Pb).
- Rs at pressure: for
P ≤ Pb, Standing Rs is used (capped by Rsb). - Bo:
Bo = 0.972 + 0.000147·F^1.175, whereF = Rs·sqrt(gg/γo) + 1.25·T. - Oil viscosity: Beggs–Robinson dead/saturated/undersaturated forms.
Example dataset
| Pressure (psia) | Temp (°F) | Gas gravity | API | CO₂ (mol%) | N₂ (mol%) | Rsb (scf/stb) | z | Bg (rb/scf) | Pb (psia) | Bo (rb/stb) | μo (cP) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 3000 | 180 | 0.75 | 35 | 2 | 1 | 650 | 0.8498 | 0.005123 | 2,634 | 1.3707 | 0.572 |
Correlation coverage and practical limits
This calculator supports reservoir screening when laboratory PVT is unavailable. Gas pseudo-critical properties come from gas gravity using two standard fits. Oil properties use black-oil correlations driven by API gravity, temperature, and solution gas–oil ratio. For consistent behavior, keep gas gravity within 0.55–1.20 and API within 5–60, matching the input checks. The workflow is best for lean to moderate systems; rich condensates should be handled with a compositional EOS. Use sensitivity checks by varying pressure ±10% and temperature ±5°F to understand uncertainty and margins during early planning stages.
Gas z-factor and density workflow
The gas module converts pressure and temperature into reduced properties, Pr = P/Ppc and Tr = T/Tpc, then evaluates compressibility factor z. The iterative option solves for reduced density and is typically stable across a wide Pr range. With z, gas formation volume factor is computed as Bg = 0.02827·z·T/P in Field units. Gas density follows the real-gas equation using molecular weight derived from gas gravity.
Sour-gas adjustment and sensitivity
When CO₂, H₂S, or N₂ are present, corrected pseudo-critical properties can shift z at the same P and T. Increasing CO₂ and H₂S generally raises Ppc and lowers Tpc, increasing Pr and decreasing Tr. That combination often reduces z and increases density. Because responses are nonlinear, export CSV results to compare “correction on” versus “off” cases and document assumptions.
Oil bubble point screening outputs
The oil block estimates bubble point pressure Pb from Rsb, or reverses the relation to estimate Rsb from Pb. At pressures below Pb, Rs is evaluated and capped by Rsb to reflect solution gas depletion. Bo is computed from temperature and Rs, then used with density mixing to estimate live-oil density. Oil viscosity uses dead, saturated, and undersaturated forms; above Pb, viscosity increases with pressure.
Reporting, exports, and engineering QA
Results are displayed above the input form to support fast iteration. The CSV export records inputs, selected methods, and outputs for spreadsheets and audit trails. The PDF report provides a compact snapshot for field notes. For QA, run two passes using different z correlations, confirm trends, and validate key points against separator tests or production data whenever available.
FAQs
Provide pressure, temperature, gas gravity, and oil API. Then enter either Rsb or Pb for the oil block. Optional CO₂, H₂S, and N₂ mol% improve sour-gas screening.
Use the iterative correlation for broader pressure and temperature coverage and smoother behavior near dense-gas conditions. The explicit correlation is fast and useful for quick checks, but may deviate at higher Pr.
Their mol% values adjust pseudo-critical pressure and temperature before computing Pr and Tr. This changes z, Bg, and density. If composition is uncertain, compare corrected versus uncorrected runs and treat the difference as sensitivity.
It compares your pressure to the estimated bubble point. Above Pb it reports undersaturated oil behavior. At or below Pb it flags two-phase likelihood. This is a screening indicator, not a full flash calculation.
Exports follow your selected unit system for pressure, temperature, and density. Some oil and gas ratios are traditionally reported in Field units, so those terms remain in scf/stb style where noted in the CSV.
Correlations are empirical and cannot capture full compositional effects, heavy ends, or tuning. Temperature reference conditions, impurity handling, and region assumptions also matter. Use lab data when available and apply this tool for fast screening and sanity checks.