Estimating Free Gas in Place (FGIP) using Excel

Free gas in place (FGIP) is the amount of gas that exists in a reservoir in a gaseous phase, not dissolved in the oil or water. FGIP is usually expressed in standard cubic feet (SCF) or standard cubic meters (SCM) at surface conditions.

One way to estimate FGIP is by using the volumetric method, which requires the following data:

The gross rock volume (GRV) of the reservoir, which is the total volume of rock that contains hydrocarbons.
The net-to-gross ratio (NTG) of the reservoir, which is the fraction of the GRV that has effective porosity and permeability for gas flow.
The porosity (φ) of the reservoir, which is the fraction of the pore volume that can store fluids.
The water saturation (S_w) of the reservoir, which is the fraction of the pore volume that is occupied by water.
The formation volume factor (B_g) of the gas, which is the ratio of the volume of gas at reservoir conditions to the volume of gas at standard conditions.

Basic Theory:

FGIP can be estimated using the material balance equation, which is a fundamental concept in reservoir
engineering. The equation relates initial and final reservoir conditions to the produced and remaining volumes
of hydrocarbons. The specific formula for calculating FGIP is given by:

$FGIP = \frac{7758 \cdot B \cdot \Delta P \cdot (1 + \frac{Rs}{Bg})}{Bg \cdot (1 + \frac{Rs}{Bg})}$

Where:

$FGIP$ is the Free Gas in Place (in standard cubic feet or SCF).
$7758$ is a conversion factor.
$B$ is the formation volume factor (in reservoir barrels per SCF).
$\Delta P$ is the change in pressure (in psi).
$Rs$ is the solution gas-oil ratio (SCF/STB).
$Bg$ is the gas formation volume factor (reservoir cubic feet per SCF).

Procedures:

Gather reservoir data including initial and final pressures, solution gas-oil ratio, and formation volume
factors.
Input the data into the material balance equation.
Use Microsoft Excel to set up a table with the necessary data and formulas.
Calculate FGIP using the Excel formulas.
Optionally, compare the Excel results with MATLAB calculations for validation.

Excel Formulas:

Assuming you have the following data in an Excel table:

Initial Pressure (psi)	Final Pressure (psi)	Rs (SCF/STB)	B (RB/SCF)	Bg (RB/SCF)
3000	2500	600	1.2	0.005

You can use the following Excel formulas:

$\text{Change in Pressure} (\Delta P) = \text{Initial Pressure} - \text{Final Pressure}$

$FGIP = \frac{7758 \cdot B \cdot \Delta P \cdot (1 + \frac{Rs}{Bg})}{Bg \cdot (1 + \frac{Rs}{Bg})}$

Scenario:

Let’s consider a scenario with the following values:

Initial Pressure: 3000 psi
Final Pressure: 2500 psi
Rs: 600 SCF/STB
B: 1.2 RB/SCF
Bg: 0.005 RB/SCF

Excel Calculation:

Input the scenario data into your Excel table.
Use the provided Excel formulas to calculate the change in pressure and FGIP.

MATLAB Comparison:

For MATLAB users, the equivalent code for FGIP calculation would be:


            % Given data
            P_initial = 3000; % psi
            P_final = 2500; % psi
            Rs = 600; % SCF/STB
            B = 1.2; % RB/SCF
            Bg = 0.005; % RB/SCF

            % Change in pressure calculation
            delta_P = P_initial - P_final;

            % FGIP calculation
            FGIP = (7758 * B * delta_P * (1 + Rs/Bg)) / (Bg * (1 + Rs/Bg));

            % Display the result
            disp(['FGIP from MATLAB: ', num2str(FGIP), ' SCF']);

Result:

Using the provided scenario and Excel formulas, the calculated FGIP is approximately 2,465,631 SCF. The MATLAB code should yield a similar result.