The formation volume factor of a gas is defined as the ratio of the gas volume in the reservoir at reservoir pressure and temperature to the standard produced volume of gas at the surface. In fact, in reservoir and gas engineering, one of the main usages of the real gas equation of state is to relate the surface volume of a gas to the reservoir volume. In order to obtain this correlation, gas formation volume factor (Bg) and gas expansion factor (E) are used. The gas formation volume factor is calculated according to Eq. (1.37) as follows: