Oil Material Balance Equation

 Figure 5.6 shows an initial condition of a reservoir with original gas cap and the

setting when the reservoir pressure as dropped due to fluid expansion. The material

balance equation uses the principle of conservation of mass. It states that the total

amount of hydrocarbon withdrawn is equal to the sum of the expansion of the oil

plus the original dissolved gas plus the primary gas plus the expansion of the connate

water & decrease in pore volume plus the amount of water the encroached into the

reservoir.

From the diagram, we have that

The derivation of the general material balance is presented below

Quantity of Oil Initially in the Reservoir

NBoi

5.6.2.2 Quantity of Oil Remaining in the Reservoir

 The Free/Liberated Gas in the Reservoir

Expansion of Oil Zone

In the oil zone, will have the original volume of oil plus the original dissolved gas in

the oil

Expansion of Connate Water and Decrease in Pore Volume

The rock compressibility is expressed as

Total Underground Withdrawal

The total underground withdrawal (TUW) due to the pressure drop is the sum of the

oil + gas + water production. Mathematically, it is

Quantity of Injection Gas and Water