Undersaturated oil formation volume factor

Al-Marhoun

The Al-Marhoun correlation was specifically developed to calculate the oil formation volume factor above the bubble point pressure. The model is based on data from Middle Eastern oilfields. It accounts for the effects of pressure, temperature, and fluid properties through a power-law relationship. The correlation demonstrates good accuracy over a wide range of conditions but is most accurate for medium and heavy crude oils. 
Recommended applicability range:

• Oil density (API Gravity): 19 – 45 °API
• Gas-oil ratio: 25 – 1600 SCF/STB
• Pressure: < 10,000 psi
• Gas specific gravity (air = 1): 0.75 – 1.35
• Temperature: 75 – 240 °F

De Ghetto et al.

The De Ghetto et al. correlation was developed to calculate the oil formation volume factor above the bubble point pressure. It is based on the analysis of data from heavy (<22.3 °API), medium (22.3–31.1 °API), and light (>31.1 °API) crude oils, making it suitable for a wide range of reservoir types. The model accounts for the effects of pressure, temperature, gas-oil ratio, and fluid properties. It is also suitable for reservoirs with abnormal oil characteristics. 
Recommended applicability range:

• Oil density (API Gravity): 15 – 45 °API
• Gas-oil ratio: 15 – 1500 SCF/STB
• Pressure: < 8000 psi
• Gas specific gravity (air = 1): 0.65 – 1.3
• Temperature: 120 – 320 °F

Glaso

The Glaso correlation is considered one of the most reliable models for calculating the oil formation volume factor in the undersaturated zone. It was developed based on North Sea crude oils. The model incorporates the effects of pressure, temperature, and fluid properties through logarithmic relationships. A key feature of the method is its high accuracy across a wide range of crude oils—from light to heavy. However, it is less accurate for oils with extreme properties (very light or very heavy). 
Recommended applicability range:

• Oil density (API Gravity): 22 – 48 °API
• Gas-oil ratio: 90 – 2600 SCF/STB
• Pressure: < 8000 psi
• Gas specific gravity (air = 1): 0.65 – 1.27
• Temperature: 80 – 280 °F

Petrosky

The Petrosky correlation was specifically developed for crude oils from the Gulf of Mexico. It provides high accuracy in calculating the oil formation volume factor in the undersaturated zone. The model incorporates the effects of pressure, temperature, and fluid properties using power-law relationships. A distinctive feature of the method is its adaptability to different oil types within the defined limits. The average error is ±3–5% within the recommended applicability range. It performs especially well for reservoirs with high gas-oil ratios. 
Recommended applicability range:

• Oil density (API Gravity): 16 – 45 °API
• Gas-oil ratio: 90 – 3000 SCF/STB
• Pressure: < 10,000 psi
• Gas specific gravity (air = 1): 0.6 – 0.9
• Temperature: 120 – 300 °F

Standing

The Standing correlation is a classical method for calculating the oil formation volume factor in the undersaturated zone. It was developed based on data from California oilfields and is widely used in the petroleum industry due to its simplicity and reliability. The model accounts for oil compressibility through an exponential dependence on pressure. The average error is ±5–8% within the recommended applicability range. 
Recommended applicability range:

• Oil density (API Gravity): 22 – 58 °API
• Gas-oil ratio: 20 – 2100 SCF/STB
• Pressure: < 5000 psi
• Gas specific gravity (air = 1): 0.63 – 1.05
• Temperature: 100 – 250 °F

Vasquez & Beggs

The Vasquez & Beggs correlation is one of the most versatile and accurate models for calculating the oil formation volume factor in the undersaturated zone. It was developed using an extensive database of more than 600 crude oil samples. The correlation accounts for separation conditions and categorizes oils into several groups based on density, which enables high accuracy across different reservoir types. 
Recommended applicability range:

• Oil density (API Gravity): 15 – 55 °API
• Gas-oil ratio: 0 – 3000 SCF/STB
• Pressure: < 10,000 psi
• Gas specific gravity (air = 1): 0.56 – 1.18
• Temperature: 70 – 295 °F

Lasater

The Lasater correlation offers an alternative approach for calculating the oil formation volume factor in the undersaturated zone, based on the mole fraction of dissolved gas. It was developed using data from Canadian and U.S. oilfields. The model performs particularly well for oils with high gas-oil ratios. However, the correlation is less accurate for heavy oils (<25 °API). 
Recommended applicability range:

• Oil density (API Gravity): 20 – 48 °API
• Gas-oil ratio: 100 – 3500 SCF/STB
• Pressure: < 5000 psi
• Gas specific gravity (air = 1): 0.65 – 1.2
• Temperature: 100 – 250 °F

Ahmed

The Ahmed correlation is a modern model for calculating the oil formation volume factor in the undersaturated zone. It was developed based on a wide dataset from Middle Eastern and North African oilfields. A distinctive feature of this formula is its non-traditional approach to modeling oil compressibility through a double exponential relationship, which more accurately reflects the behavior of oils with a nonlinear volume factor dependence on pressure. 
Recommended applicability range:

• Oil density (API Gravity): 25 – 40 °API
• Gas-oil ratio: 50 – 2000 SCF/STB
• Pressure: < 10,000 psi
• Temperature: 100 – 250 °F

Arps

The Arps correlation is one of the earliest practical models for estimating oil volume changes in the undersaturated zone. It was developed based on empirical data from U.S. oilfields. The method uses a linear approximation of the relationship between formation volume factor and pressure, making it simple to apply but limited in accuracy under complex reservoir conditions. At elevated pressures (> 2000 psi), the error can reach up to 10%. 
Recommended applicability range:

• Oil density (API Gravity): 20 – 40 °API
• Gas-oil ratio: 100 – 1500 SCF/STB
• Pressure: < 3000 psi
• Temperature: 100 – 220 °F

Kartoatmodjo & Schmidt

The Kartoatmodjo & Schmidt correlation (1994) was developed based on an expanded global database that includes crude oil samples from various regions around the world. The method provides an advanced approach for calculating formation volume factor in the undersaturated zone, accounting for the nonlinear dependence on pressure and fluid properties. A key feature is the use of a power function to more accurately describe oil volume changes. 
Recommended applicability range:

• Oil density (API Gravity): 14 – 45 °API
• Gas-oil ratio: 20 – 2500 SCF/STB
• Pressure: < 8000 psi
• Temperature: 100 – 300 °F