Gas Saturation in Water-Drive Gas Reservoirs in Excel

Gas saturation is the fraction of the pore space in a rock that is occupied by gas. In water-drive gas reservoirs, gas is displaced by water that comes from an aquifer, which is a large underground layer of water-bearing rock. As the gas is produced from the reservoir, the pressure drops and the water expands and flows into the reservoir, pushing the gas towards the wells.

However, the water cannot displace all the gas from the rock, because some gas remains trapped in the pores by capillary forces. This is called residual gas saturation, and it depends on the rock properties, the gas and water properties, and the pressure and temperature conditions. Residual gas saturation reduces the amount of gas that can be recovered from the reservoir.

Residual gas saturation is not constant, but changes as the reservoir pressure declines. As the pressure drops, some of the trapped gas becomes mobile and can be produced, while some of the remaining gas becomes trapped again. This means that the residual gas saturation has two values: one for the initial gas entrapment, and one for the gas mobilization under expansion. These values can vary significantly depending on the reservoir characteristics.

Basic Theory:

A water-drive gas reservoir is characterized by the presence of water that displaces gas as the primary driving force. Gas saturation (Sg) represents the fraction of the pore space filled with gas. It is a crucial parameter for reservoir engineers to understand as it directly impacts the reservoir’s productivity and ultimate recovery.

The basic equation for gas saturation is given by:

where:

• Sg is the gas saturation,
• Vg is the volume of gas in the reservoir,
• Vp is the total pore volume of the reservoir.

Procedures:

To calculate gas saturation in a water-drive gas reservoir using Microsoft Excel, follow these steps:

1. Input the necessary data:
   • Reservoir volume (Vp),
   • Volume of gas in the reservoir (Vg).
2. Apply the gas saturation formula in an Excel cell:

      

Scenario:

Let’s consider a reservoir with a total pore volume (Vp) of 500,000 cubic meters and a volume of gas (Vg) equal to 50,000 cubic meters.

This means the gas saturation in the reservoir is 0.1 or 10%.

Excel Calculation:

MATLAB Comparison:

% MATLAB code for gas saturation calculation
Vp = 500000;  % Total Pore Volume in cubic meters
Vg = 50000;   % Volume of Gas in cubic meters

Sg = Vg / Vp; % Gas Saturation calculation

disp(['Gas Saturation (Sg) in MATLAB: ', num2str(Sg)]);

Result: