## Material Balance for a Water Drive Mechanism

This article will demonstrate a material balance equation in a natural water drive mechanism.  A full material balance equation is shown below:

Figure 1 – Full Material Balance Equation

Assumptions

• Reservoir pressure above the bubble point (Pb). Above a bubble point, Rs = Rsi.
• No water injection
• No gas injection
• No gas cap
• Neglect formation and connate water compressibility (Cf and Cw have little effect for a gas cap drive mechanism.)

Figure 2 – Material Balance Equation with Assumption for a Natural Water Drive Mechanism

## Material Balance for Gas Cap Drive Mechanism

Figure 1 – Full Material Balance Equation

Assumptions

• No water production
• No water injection
• No gas injection
• No water influx
• Neglect formation and connate water compressibility (Cf and Cw have little effect for a gas cap drive mechanism.)

## Solution Gas Drive Mechanism Explained in Material Balance Equation

Material balance equation can be applied for any drive mechanism and this article demonstrates how to apply the material balance equation in a solution drive mechanism. For a solution drive mechanism, there are 2 cases. The first case is when reservoir pressure is above a bubble point and the second case is when reservoir pressure is below a bubble point.

Solution Gas above Bubble Point

Figure 1 – Full Material Balance Equation

## Material Balance Equation in Reservoir Engineering

Material balance is a mathematic way to express mass conservation in a reservoir and a simple key principle is “what reservoir is produced must be replaced by other mass.

Volume Produced = Volume Replaced

Volume Produced comes from Gas Production, Oil Production, and Water Production.

Volume Replaced comes from volume expansion, water in flux and water/gas injection.

Figure 1 shows the relationship of the material balance.

Figure 1 – Concept of Material Balance

Let’s take a look at each component of equation. Continue reading