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amount of hydrocarbon withdrawn is equal to the sum of the expansion of the oil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