Engineering lesson
Standing versus Vasquez Beggs comparing compatible PVT inputs
Standing and Vasquez–Beggs comparisons require compatible pressure, temperature, gas-gravity and oil-gravity definitions. A difference between two correlations is not an accuracy ranking without suitable independent PVT data.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Match the predicted property
Compare Rs with Rs or Bo with Bo at the same saturated or undersaturated state. A bubble-point prediction and a solution-gas prediction are related but are not interchangeable outputs.
The site's PVT comparison workflow documents its implemented variants and coefficient branches. Its defaults are synthetic, not a laboratory fluid characterization.
Keep gravity and separator corrections explicit
Gas gravity may be defined at a particular reference or corrected for separator conditions. Using a corrected value in one model and an uncorrected value in another introduces an input mismatch.
API gravity is another definition, based on liquid specific gravity at its specified reference. It is not gas gravity and cannot be entered in that field.
Use measurements to assess performance
The explorer reports same-input model outputs. To assess accuracy, compare each with a relevant laboratory dataset and report residuals by pressure and fluid range.
Do not average two correlation predictions merely because they disagree. First verify their domains, coefficient conventions and input definitions.
Explore the example
Original recorded synthetic cases solved with the site's pvt-comparison engine. The full inputs, outputs and diagnostic rows are in the downloadable JSON. These are discrete solved states; the lesson does not interpolate a new solve.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Stock-tank oil gravity (API) = 30. Standing at upper pressure (scf/STB): 547.771053; Vasquez-Beggs at upper pressure (scf/STB): 503.272569.
- Standing at upper pressure (scf/STB)
- Vasquez-Beggs at upper pressure (scf/STB)
Worked example
Standing vs Vasquez-Beggs PVT Comparison
Fixed controls: Lower absolute pressure = 500 psia; Upper absolute pressure = 2500 psia; Measured bubble-point pressure = 3000 psia; Reservoir temperature = 180 °F; Gas gravity on the selected correlation basis = 0.8 dimensionless.
Relation and unit basis
Use absolute pressure and a measured bubble point. The saturated Rs comparison stops at or below that bubble point; it is not an undersaturated-oil extrapolation.
Stock-tank oil gravity is varied within the supported input range. Correlation agreement is not validation. Select or calibrate against measured PVT data; this tool does not rank accuracy or replace a laboratory study.
1. Define the baseline
Stock-tank oil gravity (API) = 30
Fixed controls: Lower absolute pressure = 500 psia; Upper absolute pressure = 2500 psia; Measured bubble-point pressure = 3000 psia; Reservoir temperature = 180 °F; Gas gravity on the selected correlation basis = 0.8 dimensionless.
2. Select the relation
Use absolute pressure and a measured bubble point. The saturated Rs comparison stops at or below that bubble point; it is not an undersaturated-oil extrapolation.
Stock-tank oil gravity is varied within the supported input range. Correlation agreement is not validation. Select or calibrate against measured PVT data; this tool does not rank accuracy or replace a laboratory study.
3. Substitute the values
API = 30 API; with the fixed controls above, the workflow evaluates Standing at upper pressure = 547.771053 scf/STB; Vasquez-Beggs at upper pressure = 503.272569 scf/STB
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Standing at upper pressure (scf/STB) = 547.771053; Vasquez-Beggs at upper pressure (scf/STB) = 503.272569
The input envelope is a conservative overlap of the cited development ranges, not a guarantee of fluid applicability. Standing was developed for California oils. Gas gravity must be supplied on the appropriate separator/correlation basis. No separator correction is calculated; composition effects and volatile oils are outside this comparison.
Result and interpretation
Standing at upper pressure (scf/STB) = 547.771053; Vasquez-Beggs at upper pressure (scf/STB) = 503.272569
The input envelope is a conservative overlap of the cited development ranges, not a guarantee of fluid applicability. Standing was developed for California oils. Gas gravity must be supplied on the appropriate separator/correlation basis. No separator correction is calculated; composition effects and volatile oils are outside this comparison.
The comparison varies one shared input while retaining the same implemented correlation variants.
Exact scenario data
Original recorded synthetic cases solved with the site's pvt-comparison engine. The full inputs, outputs and diagnostic rows are in the downloadable JSON. These are discrete solved states; the lesson does not interpolate a new solve.
| Stock-tank oil gravity (API) | Standing at upper pressure (scf/STB) | Vasquez-Beggs at upper pressure (scf/STB) |
|---|---|---|
| 25 | 460.5724 | 411.6458 |
| 29 | 529.1013 | 483.4453 |
| 30 | 547.7711 | 503.2726 |
| 31 | 567.0996 | 490.0954 |
| 35 | 651.4788 | 569.1641 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. Does the larger Rs prediction necessarily come from the more accurate correlation?
Show the worked answer
No. Accuracy requires independent reference measurements on a compatible fluid and state basis.
2. Reproduce the baseline Standing at upper pressure (scf/STB) without rounding intermediate values.
Show the worked answer
API = 30 API; with the fixed controls above, the workflow evaluates Standing at upper pressure = 547.771053 scf/STB; Vasquez-Beggs at upper pressure = 503.272569 scf/STB. Standing at upper pressure (scf/STB) = 547.771053. The input envelope is a conservative overlap of the cited development ranges, not a guarantee of fluid applicability. Standing was developed for California oils. Gas gravity must be supplied on the appropriate separator/correlation basis. No separator correction is calculated; composition effects and volatile oils are outside this comparison.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Stock-tank oil gravity (API) changes from 25 to 35. Other controls remain as stated in the premise. Standing at upper pressure (scf/STB) changes from 460.572362 to 651.478793.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. Stock-tank oil gravity is varied within the supported input range. Correlation agreement is not validation. Select or calibrate against measured PVT data; this tool does not rank accuracy or replace a laboratory study. The input envelope is a conservative overlap of the cited development ranges, not a guarantee of fluid applicability. Standing was developed for California oils. Gas gravity must be supplied on the appropriate separator/correlation basis. No separator correction is calculated; composition effects and volatile oils are outside this comparison.
Before using the result
- Compare the same property and phase condition.
- Match gas-gravity definitions.
- Use independent PVT data for accuracy claims.
The input envelope is a conservative overlap of the cited development ranges, not a guarantee of fluid applicability. Standing was developed for California oils. Gas gravity must be supplied on the appropriate separator/correlation basis. No separator correction is calculated; composition effects and volatile oils are outside this comparison.
Continue learning
Start with the broader workflow guide · Related method guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.