The oil formation volume factor (Bo) is calculated as the ratio of the volume of oil (plus dissolved gas) at reservoir conditions to the volume of oil at standard surface conditions (stock tank). In essence, it represents the shrinkage of oil as it travels from the reservoir to the surface.
Here's a breakdown of how to calculate it:
Understanding the Oil Formation Volume Factor
Bo is a crucial parameter in reservoir engineering for estimating oil reserves, predicting production rates, and designing production facilities. It's usually greater than 1 because oil shrinks when pressure decreases and temperature drops as it moves from the reservoir to the surface. The dissolved gas comes out of solution and the liquid oil contracts due to lower temperature.
Calculation Methods
There are several ways to determine the oil formation volume factor:
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Laboratory Measurement (PVT Analysis):
- The most accurate method involves collecting reservoir fluid samples and conducting Pressure-Volume-Temperature (PVT) analysis in a laboratory.
- In a PVT experiment, a known volume of reservoir fluid is subjected to different pressures and temperatures, simulating the conditions encountered during production.
- The volume of oil at reservoir conditions is divided by the volume of stock tank oil (oil at standard conditions, typically 14.7 psia and 60°F) to obtain Bo.
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Empirical Correlations:
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When laboratory data is unavailable, empirical correlations can be used to estimate Bo. These correlations are based on field data and relate Bo to various parameters such as:
- Solution Gas-Oil Ratio (Rs): The amount of gas dissolved in oil at reservoir conditions.
- Oil Gravity (°API): A measure of the oil's density.
- Gas Gravity (γg): The density of the gas relative to air.
- Reservoir Temperature (T): The temperature of the reservoir.
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Common correlations include Standing's correlation, Vasquez-Beggs correlation, and others.
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Standing's Correlation:
Bo = 0.9759 + 0.000120(Rs (γg/γo)^0.5 + 1.25 T)
Where:
- Bo is the oil formation volume factor (bbl/STB)
- Rs is the solution gas-oil ratio (scf/STB)
- γg is the gas specific gravity (air = 1)
- γo is the oil specific gravity (water = 1, can be calculated from API gravity)
- T is the reservoir temperature (°F)
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Vasquez-Beggs Correlation:
Bo = 1.0 + (0.0000467 Rs) + (0.0000175 (T - 60)) + (0.000000011 Rs (T - 60))
Where:
- Bo is the oil formation volume factor (bbl/STB)
- Rs is the solution gas-oil ratio (scf/STB)
- T is the reservoir temperature (°F)
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Important Note: The accuracy of empirical correlations depends on the applicability of the correlation to the specific reservoir fluid. Always check the limitations and assumptions of the correlation before using it.
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Formula Representation
The fundamental formula for Bo is:
Bo = (Volume of oil + dissolved gas at reservoir conditions) / (Volume of oil at standard conditions)
Or, more concisely:
Bo = Vr/Vs
Where:
- Vr = Volume at reservoir conditions
- Vs = Volume at surface (standard) conditions
Example
Suppose a PVT analysis shows that 1 barrel of stock tank oil at surface conditions occupies 1.3 barrels when returned to reservoir conditions, including the dissolved gas. Then:
Bo = 1.3 bbl/STB (barrels per stock tank barrel)
Key Considerations
- Accuracy: PVT analysis provides the most accurate values. Correlations are estimates and should be used with caution.
- Fluid Type: The appropriate correlation depends on the type of reservoir fluid (e.g., black oil, volatile oil, gas condensate).
- Reservoir Conditions: Ensure that the reservoir pressure and temperature are accurately known, as these significantly influence Bo.
By accurately determining the oil formation volume factor, reservoir engineers can more precisely estimate oil reserves, predict production performance, and optimize reservoir management strategies.
