Oil compressibility

A1.4 Compressibility#

Standing

The Standing correlation, developed in 1947, is one of the earliest and most widely used methods for estimating oil compressibility. Standing proposed an empirical relationship that links compressibility with pressure, temperature, gas-oil ratio, and the specific gravities of oil and gas. The method performs particularly well for conventional crude oils with moderate gas content. 
Recommended applicability range:

• Oil density (API Gravity): 20 – 45 °API
• Gas-oil ratio: 50 – 800 SCF/STB
• Temperature: 100 – 220 °F

Vasquez & Beggs

The Vasquez & Beggs correlation is an enhanced method for calculating oil compressibility, developed from an extensive PVT database. Unlike earlier models (such as Standing), this correlation explicitly accounts for the effects of pressure, temperature, gas-oil ratio, and other oil properties through a set of empirical coefficients. The method provides good accuracy across a wide range of crude oils—from light to medium. 
Recommended applicability range:

• Oil density (API Gravity): 15 – 45 °API
• Pressure: < 10,000 psi
• Gas-oil ratio: 50 – 3500 SCF/STB
• Temperature: 100 – 300 °F

Glaso

The Glaso (1980) correlation for oil compressibility is based on generalized data from North Sea oilfields. The method incorporates the effects of pressure, temperature, gas-oil ratio, and oil density, offering more accurate estimates compared to classical approaches (e.g., Vasquez & Beggs). A notable feature of this model is the separation of calculation formulas for different pressure ranges—above and below the bubble point. 
Recommended applicability range:

• Oil density (API Gravity): 18 – 52 °API
• Pressure: 500 – 8000 psi
• Gas-oil ratio: 50 – 3000 SCF/STB
• Temperature: 100 – 300 °F

De Ghetto et al.

This correlation was developed based on a large volume of experimental PVT data for heavy and extra-heavy crude oils, as earlier models (e.g., Standing, Vasquez & Beggs) produced significant errors for oils with API gravity below 25. The key feature of the model is the classification of fluids into multiple categories based on density, with a separate compressibility formula provided for each class. Its main limitation is that it is not universal—it performs best within the scope of the original data set. 
Recommended applicability range:

• Oil density (API Gravity): 10 – 45 °API
• Pressure: < 5000 psi
• Gas-oil ratio: 0 – 2000 SCF/STB
• Temperature: 80 – 260 °F

De Ghetto et al.

This correlation was developed based on a large set of experimental PVT data for heavy and extra-heavy crude oils, as previously existing models (such as Standing and Vasquez & Beggs) produced significant errors when API gravity was below 25. The key feature of the method is the classification of fluids into several categories based on oil density. Each class has its own dedicated oil compressibility correlation. The main limitation is its lack of universality—it performs best within the bounds of the original dataset. 
Recommended applicability range:

• Oil density (API Gravity): 10 – 45 °API
• Pressure: < 5000 psi
• Gas-oil ratio: 0 – 2000 SCF/STB
• Temperature: 80 – 260 °F

Petrosky

The Petrosky correlation was specifically developed for crude oils from the Gulf of Mexico, but it is also applicable to other regions. Key features of the model include consideration of gas-oil ratio, gas specific gravity, oil density, and temperature. It is optimized for light and medium crude oils. Compared to the Standing correlation, it yields more accurate results under high-temperature conditions. 
Recommended applicability range:

• Oil density (API Gravity): 15 – 45 °API
• Pressure: < 8000 psi
• Gas-oil ratio: 100 – 2500 SCF/STB
• Temperature: 100 – 300 °F

Al-Marhoun

The Al-Marhoun (2003) correlation was developed to provide a more accurate estimation of oil compressibility, based on a comprehensive dataset from Middle Eastern reservoirs. The method accounts for the effects of pressure, temperature, gas-oil ratio, and fluid properties, offering improved accuracy compared to classical correlations. A distinctive feature is the use of power-law relationships optimized for various thermobaric (temperature-pressure) conditions. 
Recommended applicability range:

• Oil density (API Gravity): 18 – 44 °API
• Pressure: 500 – 8000 psi
• Gas-oil ratio: 50 – 3000 SCF/STB
• Temperature: 100 – 300 °F

Lasater

The Lasater (1958) correlation was developed based on the analysis of crude oils from Canadian and U.S. fields. The method uses an empirical relationship linking oil compressibility with pressure, gas-oil ratio, and fluid properties. A key feature of this approach is its emphasis on the influence of gas solubility, making it particularly useful for oils with high gas content. At pressures above 3000 psi, the correlation may underestimate compressibility—other models are recommended for such conditions. 
Recommended applicability range:

• Oil density (API Gravity): 20 – 45 °API
• Pressure: 100 – 3000 psi
• Gas-oil ratio: 50 – 1500 SCF/STB
• Temperature: 100 – 250 °F

Ahmed

The empirical correlation proposed by Tarek Ahmed is used to estimate the isothermal compressibility of crude oil. It is based on the analysis of PVT data for light and medium crude oils and is applicable when laboratory measurements are not available. The model accounts for the effects of pressure, dissolved gas, temperature, and oil density. It is known for its simplicity and reliable results under typical reservoir conditions. The correlation is well-suited for early-stage design or reservoir simulators where a quick property estimation is needed. 
Recommended applicability range:

• Oil density (API Gravity): 20 – 45 °API
• Pressure: < 5000 psi
• Gas-oil ratio: 0 – 2000 SCF/STB
• Temperature: 100 – 300 °F

Kartoatmodjo & Schmidt

This correlation is a modern method for calculating oil compressibility, developed using an extensive database of PVT analyses. Unlike classical approaches (e.g., Standing, Vasquez & Beggs), this method is specifically optimized for heavy and highly viscous crude oils, showing particular accuracy under challenging conditions. The formula captures the combined influence of pressure, temperature, gas-oil ratio, and oil density through a system of power-law relationships. 
Recommended applicability range:

• Oil density (API Gravity): 10 – 50 °API
• Pressure: < 8000 psi
• Gas-oil ratio: 20 – 2500 SCF/STB
• Temperature: 80 – 320 °F