Gas Formation Volume Factor in Petroleum Engineering Using Excel

Gas formation volume factor (Bg) is a term that describes how much a gas expands or contracts when it is brought from the reservoir to the surface. It is the ratio of the volume of gas at the reservoir conditions (pressure and temperature) to the volume of gas at the standard conditions (usually 14.7 psia and 60°F).

For example, if Bg is 0.005 RB/scf, it means that one standard cubic foot (scf) of gas at the surface occupies 0.005 reservoir barrels (RB) of gas at the reservoir. Conversely, one reservoir barrel of gas at the reservoir expands to 200 scf of gas at the surface.

Bg depends on the pressure, temperature, and composition of the gas. As the pressure decreases or the temperature increases, the gas expands and Bg becomes smaller. As the gas becomes heavier or richer in natural gas liquids, Bg becomes larger.

Bg can be calculated using the real gas equation and the gas deviation factor (z), which accounts for the non-ideal behavior of the gas. However, since you asked for an explanation without formulas, I will not go into the details of the calculation.

Basic Theory

The Gas Formation Volume Factor (Bg) is defined as the ratio of the volume of gas at reservoir conditions (Vg) to the volume of gas at standard conditions (Vg,sc):

$Bg = \frac{Vg}{Vg,sc}$

The standard conditions typically refer to a temperature of 60°F (15.6°C) and a pressure of 14.7 psi (1 atm).

Procedures for Calculation

The Gas FVF can be calculated using the Standing’s correlation, which is a widely accepted method in the industry. The formula is as follows:

$Bg = 0.0283 \times \frac{Z \times T_{sc}}{P_{sc}}$

Where:

$Z$ is the gas compressibility factor,
$T_{sc}$ is the temperature at standard conditions in Kelvin,
$P_{sc}$ is the pressure at standard conditions in psia.

Explanation

Scenario

Let’s consider a scenario with the following parameters:

Reservoir temperature ( $T_r$ ): 150°F
Reservoir pressure ( $P_r$ ): 3000 psia
Gas compressibility factor ( $Z$ ): 0.85

Excel Calculation

Convert Reservoir Temperature to Kelvin:
$T_r(K) = (T_r - 32) \times \frac{5}{9} + 273.15$
Calculate Gas FVF using Standing’s Correlation:
$Bg = 0.0283 \times \frac{Z \times T_{sc}}{P_{sc}}$
Create an Excel Table:

Parameter Value

Reservoir Temperature 150°F

Reservoir Pressure 3000 psia

Gas Compressibility Factor 0.85

Standard Temperature 60°F

Standard Pressure 14.7 psia
Implement Excel Formulas:
- Cell A2: $T_r(K) = (B2 - 32) \times \frac{5}{9} + 273.15$
- Cell B6: $Bg = 0.0283 \times \frac{C4 \times B2}{C5}$

Excel Table

Parameter	Value
Reservoir Temperature	150°F
Reservoir Pressure	3000 psia
Gas Compressibility Factor	0.85
Standard Temperature	60°F
Standard Pressure	14.7 psia

Calculation	Result
Reservoir Temperature	338.15 K
Gas Formation Volume Factor	0.0289

MATLAB Comparison

For MATLAB users, the Gas FVF can be calculated using a similar approach. Here’s a simple MATLAB code snippet:

% Given parameters
Tr = (150 - 32) * 5/9 + 273.15; % Convert to Kelvin
Pr = 3000; % psia
Z = 0.85;

% Standard conditions
Tsc = (60 - 32) * 5/9 + 273.15; % Convert to Kelvin
Psc = 14.7; % psia

% Gas FVF calculation using Standing's correlation
Bg = 0.0283 * (Z * Tsc) / Psc;

% Display the result
fprintf('Gas Formation Volume Factor (Bg): %.4f\n', Bg);