Instantaneous Gas-Oil Ratio Calculation in Excel

Instantaneous gas-oil ratio is a measure of how much gas is produced along with oil from a reservoir. It is calculated by dividing the volume of gas at standard conditions by the volume of oil at standard conditions. Standard conditions are usually defined as 60°F and 14.7 psia. The instantaneous gas-oil ratio changes over time as the reservoir pressure drops and more gas comes out of the oil.

The instantaneous gas-oil ratio is important for reservoir engineers because it affects the oil recovery, the reservoir performance, and the surface facilities design. For example, a high gas-oil ratio means that the oil is lighter and more volatile, which can improve the oil flow and recovery. However, it also means that more gas has to be separated and processed at the surface, which can increase the cost and complexity of the operations.

IGOR is calculated using the Standing’s correlation, which is based on the principle of the conservation of mass. The correlation relates the IGOR to reservoir pressure, temperature, and oil gravity. The formula for IGOR is given by:

IGOR = (460 * Psc) / (T * (GORstc + 1))

Where:

• IGOR is the Instantaneous Gas-Oil Ratio (SCF/STB).
• Psc is the reservoir pressure in psia.
• T is the reservoir temperature in Rankine.
• GORstc is the gas-oil ratio at standard conditions (SCF/STB).

Procedures for Calculation in Excel:

1. Gather Data: Collect the reservoir pressure (Psc), reservoir temperature (T), and gas-oil ratio at standard conditions (GORstc).
2. Enter Data into Excel: Create an Excel table and input the collected data into designated cells.
3. Use Excel Formula: Utilize the Standing’s correlation formula in Excel to calculate IGOR. Assuming Psc is in cell A1, T in B1, and GORstc in C1, the formula in cell D1 would be:
   =460 * A1 / (B1 * (C1 + 1))
4. Review Results: The result in cell D1 will be the IGOR in SCF/STB.

Example:

Consider a reservoir with the following data:

• Reservoir pressure (Psc): 3000 psia
• Reservoir temperature (T): 180°F (Rankine temperature ≈ 640.67)
• Gas-oil ratio at standard conditions (GORstc): 300 SCF/STB

Now, apply the Standing’s correlation formula in Excel:

=460 * 3000 / (640.67 * (300 + 1))

The result will give the IGOR for this scenario.

Result:

IGOR ≈ 278.76 SCF/STB

MATLAB Comparison:

To solve the same problem in MATLAB, you can use the following script:

Psc = 3000; % psia
T = 640.67; % Rankine
GORstc = 300; % SCF/STB

IGOR = (460 * Psc) / (T * (GORstc + 1));
disp(['IGOR: ', num2str(IGOR), ' SCF/STB']);

Execute the script, and it will provide the IGOR in the MATLAB console.