Understanding Gas Cap Ratio in Petroleum Engineering Using Excel and MATLAB

Gas cap ratio is a term used to describe the relative size of the gas cap in a reservoir that contains both oil and gas. The gas cap is the upper portion of the reservoir where gas accumulates due to its lower density than oil. The gas cap ratio is calculated by dividing the volume of the gas cap by the total volume of the reservoir.

The gas cap ratio affects the reservoir pressure and the production of oil and gas. A larger gas cap ratio means that the reservoir has more gas to expand and push the oil towards the wellbore. This results in a higher recovery of oil and a slower decline of pressure. A smaller gas cap ratio means that the reservoir has less gas to provide energy and the pressure drops faster. This results in a lower recovery of oil and a higher production of gas.

The gas cap ratio can be estimated from the reservoir pressure and the producing gas-oil ratio trends. These trends are different for different types of drive mechanisms, which are the natural forces that move oil and gas in the reservoir. The three primary drive mechanisms are solution gas drive, gas cap drive, and water drive. You can learn more about them from this web page.

Basic Theory:

The gas cap ratio (GCR) is defined as the ratio of gas volume (Vg) to total reservoir volume (Vr). Mathematically, it is expressed as:

$GCR = \frac{Vg}{Vr}$

Understanding the gas cap ratio helps reservoir engineers make informed decisions regarding reservoir management, production strategies, and recovery techniques.

Procedures in Microsoft Excel:

Data Collection: Begin by collecting relevant data, including gas volume ( $Vg$ ) and total reservoir volume ( $Vr$ ).
Excel Formula: In Excel, you can use the following formula to calculate gas cap ratio:
$\text{Gas Cap Ratio (GCR)} = \frac{\text{Gas Volume (Vg)}}{\text{Total Reservoir Volume (Vr)}}$

Implement this formula in a designated cell in your Excel spreadsheet.
Excel Table: Create an Excel table to organize your data, including columns for Gas Volume, Total Reservoir Volume, and the Gas Cap Ratio.
Scenario with Real Numbers: Consider a reservoir with a gas volume of 50 million cubic feet ( $Vg = 50 \text{ MMCF}$ ) and a total reservoir volume of 200 million cubic feet ( $Vr = 200 \text{ MMCF}$ ).
Excel Calculation: Input the values into your Excel table and apply the gas cap ratio formula to obtain the result.

Example:

Gas Volume (MMCF)	Total Reservoir Volume (MMCF)	Gas Cap Ratio (GCR)
50	200	=B2/C2

In this scenario, the gas cap ratio would be calculated as $\frac{50}{200} = 0.25$ .

Result:

The Gas Cap Ratio (GCR) for the given scenario is 0.25.

MATLAB Comparison:

   % MATLAB code for Gas Cap Ratio calculation
   Vg = 50; % Gas Volume in MMCF
   Vr = 200; % Total Reservoir Volume in MMCF
   GCR_MATLAB = Vg / Vr; % Gas Cap Ratio calculation

   disp(['Gas Cap Ratio (MATLAB): ' num2str(GCR_MATLAB)]);

Run the MATLAB code to obtain the Gas Cap Ratio using the same data.