Gas Drive Index Calculation in Gas Reservoirs using Excel

Gas drive index (GDI) is a dimensionless parameter that measures the relative contribution of gas expansion to oil recovery in a gas reservoir. It is defined as the ratio of the gas volume produced to the oil volume produced at standard conditions.

GDI can be used to classify gas reservoirs into different types based on their drive mechanisms. A low GDI indicates a water drive reservoir, where the main source of energy is the expansion of the aquifer. A high GDI indicates a depletion drive reservoir, where the main source of energy is the expansion of the gas in place. A moderate GDI indicates a combination drive reservoir, where both water and gas expansion contribute to oil recovery.

GDI can also be used to estimate the ultimate recovery factor of a gas reservoir. The higher the GDI, the lower the recovery factor, because more gas is produced relative to oil. The lower the GDI, the higher the recovery factor, because more oil is produced relative to gas.

Basic Theory:

Gas Drive Index (GDI) is defined as the ratio of the change in gas reservoir pressure to the change in gas reservoir volume. Mathematically, it can be expressed as:

$GDI = \frac{\Delta P}{\Delta V}$

Where:

$GDI$ is the Gas Drive Index,
$\Delta P$ is the change in gas reservoir pressure,
$\Delta V$ is the change in gas reservoir volume.

A higher GDI indicates a more efficient gas drive mechanism within the reservoir.

Procedures:

Data Collection:
- Obtain initial and final gas reservoir pressures ( $P_{initial}$ and $P_{final}$ ).
- Measure the corresponding initial and final gas reservoir volumes ( $V_{initial}$ and $V_{final}$ ).
Calculate $\Delta P$ and $\Delta V$ :
- $\Delta P = P_{final} - P_{initial}$
- $\Delta V = V_{final} - V_{initial}$
Calculate Gas Drive Index (GDI):
- $GDI = \frac{\Delta P}{\Delta V}$

Scenario:

Consider a gas reservoir with an initial pressure ( $P_{initial}$ ) of 3,000 psi and an initial volume ( $V_{initial}$ ) of 50,000 cubic feet. After a certain period, the reservoir pressure ( $P_{final}$ ) is reduced to 2,500 psi, and the volume ( $V_{final}$ ) increases to 60,000 cubic feet.

Excel Calculation:

Let’s set up an Excel table to perform the calculations.

	Parameter	Value
2	$P_{initial}$	3000 psi
3	$P_{final}$	2500 psi
4	$V_{initial}$	50000 cubic feet
5	$V_{final}$	60000 cubic feet
7	$\Delta P$	=B3-B2
8	$\Delta V$	=B5-B4
9	GDI	=B7/B8

MATLAB Comparison:

In MATLAB, you can use the following script:

    
% Input data
P_initial = 3000;   % psi
P_final = 2500;     % psi
V_initial = 50000;  % cubic feet
V_final = 60000;    % cubic feet

% Calculate delta P and delta V
delta_P = P_final - P_initial;
delta_V = V_final - V_initial;

% Calculate Gas Drive Index (GDI)
GDI = delta_P / delta_V;

% Display result
fprintf('Gas Drive Index (GDI) calculated using MATLAB: %.4f\n', GDI);

Result:

Using the provided scenario and Excel formulas, the Gas Drive Index (GDI) is calculated as 0.2 psi/cubic feet. The MATLAB script provides the same result for comparison.