Estimating Initial Gas in Place for Water-Drive Gas Reservoirs in Excel

Initial gas in place (IGIP) is the amount of gas that is originally stored in a reservoir before any production. It is an important parameter for estimating the recoverable gas and the reservoir performance.

Water-drive gas reservoirs are reservoirs that are in contact with an active aquifer, which is a large underground water source. The aquifer provides pressure support to the reservoir as gas is produced. This means that the reservoir pressure does not decline much, and the gas can be extracted more efficiently.

However, water-drive gas reservoirs also have some challenges. One of them is that the water can invade the reservoir and displace the gas, reducing the gas saturation and the pore volume available for gas storage. Another challenge is that the water influx can be difficult to measure and predict, which affects the accuracy of the IGIP estimation.

There are different methods to estimate the IGIP for water-drive gas reservoirs, such as volumetric, material balance, and pressure contour methods. Each method has its own assumptions, advantages, and limitations. For example, the volumetric method requires geological and petrophysical data, such as the reservoir area, thickness, porosity, and gas saturation. The material balance method requires production and pressure data, such as the cumulative gas production and the average reservoir pressure. The pressure contour method requires pressure measurements from different locations in the reservoir.

The IGIP estimation for water-drive gas reservoirs is not a simple task, and it may involve some uncertainty and error. Therefore, it is important to use multiple methods, compare the results, and update the estimates with new data and information. This way, the IGIP estimation can be more reliable and accurate.

The basic theory for calculating IGIP involves using the material balance equation for a water-drive gas reservoir. The material balance equation is expressed as:

Where:

• IGIP = Initial Gas in Place (in standard cubic feet)
• Bg = Gas formation volume factor at initial conditions (in reservoir cubic feet per standard cubic feet)
• h = Net pay thickness (in feet)
• ϕ = Porosity of the reservoir rock
• Pi = Initial reservoir pressure (in psia)
• Pwf = Final wellbore flowing pressure (in psia)
• Boi = Initial reservoir oil formation volume factor
• c = Ratio of aquifer influx to produced water, usually assumed to be 1.0

Procedures:

1. Gather reservoir data: Bg, h, ϕ, Pi, Pwf, and Boi.
2. Substitute values into the material balance equation.
3. Utilize Microsoft Excel to set up a table and perform calculations.
4. Validate results using MATLAB for comparison.

Scenario:

Let’s consider a water-drive gas reservoir with the following parameters:

• Bg = 0.0035 reservoir cubic feet per standard cubic foot
• h = 50 feet
• ϕ = 0.18
• Pi = 3000 psia
• Pwf = 2000 psia
• Boi = 1.2
• c = 1.0

Calculation in Microsoft Excel:

Calculation in MATLAB:

Bg = 0.0035;
h = 50;
phi = 0.18;
Pi = 3000;
Pwf = 2000;
Boi = 1.2;
c = 1.0;

IGIP_MATLAB = 7758 * Bg * h * phi * ((Pi - Pwf) / (1.25 * Boi)) / (Bg * (1 - c));