Example Calculation of Gas Formation Volume Factor (Bg)

 To better understand how the Gas Formation Volume Factor (Bg) is calculated, let's go through a simple example.

Problem Statement:

You are given the following reservoir conditions for a gas reservoir:

• Reservoir Pressure (P): 2,500 psia
• Reservoir Temperature (T): 180°F (converted to Rankine: 180°F + 460 = 640°R)
• Gas Compressibility Factor (Z): 0.85

Calculate the Gas Formation Volume Factor (Bg).

Solution:

The formula to calculate Bg is:

$B_g = \frac{0.0283 \times Z \times T}{P}$

Bg​=P0.0283×Z×T​

Substituting the given values:

$B_g = \frac{0.0283 \times 0.85 \times 640}{2500}$

Bg​=25000.0283×0.85×640​

Step-by-Step Calculation:

1. Multiply the constants and variables in the numerator:

$0.0283 \times 0.85 \times 640 = 15.3536$

0.0283×0.85×640=15.3536

2. Divide the result by the reservoir pressure (P):

$\frac{15.3536}{2500} = 0.0061414$

250015.3536​=0.0061414

So, the Gas Formation Volume Factor (Bg) is 0.0061414 rcf/scf.

Interpreting the Result:

This value indicates that at the given reservoir conditions, the volume of gas at reservoir conditions is approximately 0.00614 times the volume of gas at standard conditions. In other words, 1 standard cubic foot (scf) of gas at the surface corresponds to 0.00614 reservoir cubic feet (rcf) of gas at reservoir conditions.

Applications of This Bg Value:

• Reserve Estimation: You can use this Bg value to convert the reservoir gas volume into standard surface conditions for more accurate reserve calculations.
• Production Forecasting: This value aids in predicting how much gas will be produced as the reservoir depletes.

Understanding how Bg is calculated and applied in real-world scenarios is essential for optimizing gas field development and production planning.

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