Black-oil PVT

#Overview

The black-oil workspace on the PVT page computes oil, gas and water properties versus pressure at reservoir temperature from published correlations. You enter four numbers (oil gravity, gas gravity, solution GOR and reservoir temperature), pick a correlation set, then check it against your lab points and tune it if needed. Use it when you have no full composition, or when a correlation fluid is good enough for wells, material balance and decline work.

Every other module reads its fluid from the PVT page. When the Project fluid picker is set to Black-oil correlations, the fluid on this page is the one wells, material balance, network, simulation and the AI Engineer use. See Project fluid.

#Inputs

Pressures are absolute: psia in Field units, bara in Metric.

On a new project, Start with typical black-oil values fills 35 °API, gas gravity 0.75, 650 scf/STB and 200 °F (115 Sm³/Sm³ and 95 °C in Metric). Each of these carries a Typical value, replace it badge until you edit it.

#Methods

Each correlation set supplies bubble point, Rs and Bo, plus matching viscosity and compressibility correlations. The panel subtitle and the note under Correlation set name the parts in use.

The other properties are the same for every set:

• Gas: Sutton (1985) pseudo-critical properties, corrected with Wichert-Aziz when you enter CO₂, H₂S or N₂ (hydrocarbon part mixed by Kay's rule). Z-factor from Dranchuk-Abou-Kassem. Gas viscosity from Lee-Gonzalez-Eakin.
• Water: McCain correlations for Bw, viscosity and density.
• Above the bubble point: Rs stays at the GOR, Bo shrinks with the integrated oil compressibility, and viscosity rises.
• Dead oil: when no bubble point exists above 14.7 psia, pb is set to 14.7 psia and the oil is marked dead. Bo still falls with pressure (compressibility held at its Vasquez-Beggs value at 2,000 psia), and viscosity uses Beal's (1946) undersaturated correction.

Rs is normalised so it is exactly 0 at 14.7 psia and exactly the GOR at pb. A measured bubble point stretches the Rs curve to pass through it.

#Workflow

1. Open PVT and make sure the workspace switch reads Black-oil correlations.
2. On an empty project, choose Enter a fluid, fill Oil gravity, Gas gravity, Solution GOR and Reservoir temperature, then press Create fluid. You can also Load demo fluid, or Import a lab report (AI) to have the AI Engineer read your report.
3. In the Fluid panel, choose a Correlation set and enter any gas impurities.
4. If you have a measured bubble point, tick Honour a measured bubble point and enter Measured pb as an absolute pressure.
5. Read the four tiles (Bubble point, Bo at bubble point, Oil viscosity at pb, Oil density at pb) and the Oil, Gas and Water tabs.
6. Open the Lab match tab. Choose the Lab basis: Differential liberation or Separator (flash).
7. Add your lab points with Paste rows (or Ctrl+V into the table), or Add point. Columns are pressure, Rs, Bo and μo, in the units shown in the headers.
8. For differential-liberation data, enter Separator Bo at pb (Bofb) and Separator GOR (Rsfb) from the separator test.
9. Read Correlation error against lab. If another set fits better, press Use [set] (best fit) under Correlation sets ranked by fit to these points.
10. Press Tune to lab points to fit the correlation to your data. Clear tuning removes it.
11. Check the Project fluid picker in the page header. Every module now reads this fluid.

#Interpreting results

Correlation error against lab reports, per property, the number of points, AARE % (average absolute relative error), Bias % and Max %. Positive bias means the correlation over-predicts. Under 5 % AARE is a usable match. When any property is off by more than 5 %, a warning names it and tells you whether to switch sets or tune.

Correlation sets ranked by fit to these points scores every set, untuned, by the average of its property errors (plus the bubble-point error when you honour a measured pb). Lower is better. The Inputs out of range column counts inputs outside that set's data range.

Tune to lab points fits three things, in order: an Rs shape exponent from the Rs points below pb, then a multiplier and shift on Bo, then a multiplier and shift on oil viscosity. A line under the button shows the fitted Rs exponent, the Bo multiplier and shift, the μo multiplier and shift (cp), and the average error before and after. Tuning cannot move pb; use Measured pb for that. The Rs exponent is bounded to 0.2 to 5 and the multipliers to 0.5 to 2. When a parameter stops at its bound, a warning tells you: the data want more than the bound allows.

The panel subtitle ends in "· tuned" while tuning is active, and the project fluid label becomes, for example, "Black-oil correlations (Standing, tuned)".

#Common pitfalls

#Exports

• Export CSV (page header) downloads pvt-table.csv: pressure, Rs, Bo, μo, ρo, Z, Bg, μg, Bw and μw, in your display units.
• The Table tab shows the same rows on screen.

Simulator keyword tables (PVTO / PVDG and others) come from a compositional model. See Compositional PVT.

#Related

• Project fluid
• Units
• Compositional PVT
• Well performance
• Material balance
• AI Engineer overview