# Material Balance Equation

## Definition - What does Material Balance Equation mean?

Material Balance Equation is the equation used to express the conservation of mass in a reservoir. This equation is applicable for initially saturated volatile and black oil reserves. It is a written as:

Free-gas expansion + Free oil expansion + Free water expansion + pore volume contraction + water influx = net gas withdrawal + oil withdrawal + net water withdrawal.

Its simplified general equation can be written as** ****F = NE**_{t}** ****+ W**_{e}**B**_{w }

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## Petropedia explains Material Balance Equation

During hot conditions the evaporation of connate water takes place and hence the hydrocarbon content decreases. Material balance methods are used, with the help of static pressure and production data so as to determine the original fluid in place. This material balance is used to calculate the hydrocarbon volume in a reservoir. In simple words, the sum remains constant and is equal to the pore volume of the interconnected reservoir. The simple material balance equation can be slightly misleading as the vapor pore volume is decreased by the condensation below the dew point of the hydrocarbons. Hence simple material balance equations have to be changed slightly for getting the right balance.