Gas deviation factor

The above equation is valid at low pressures where the assumptions hold. However, at typical reservoir temperatures and pressures, the assumptions are no longer valid, and the behaviour of hydrocarbon reservoir gases deviate from the ideal gas law. In practice, it is convenient to represent the behaviour of these real gases by introducing a correction factor known as the gas deviation factor, (also called the dimensionless compressibility factor, or z-factor) into the ideal gas law ... 

Compressibility factor is also known as gas deviation factor, supercompressibility, or z-factor. Sometimes the reciprocal of compressibility factor is called supercompressibility. 

The fluid displacement experiments were carried out at constant temperature and the downstream pressure was kept atmospheric. The pressure drops across the porous medium were always less than 50 psi and thus, the gas deviation factor can be neglected (43). Cumulative volume (q) of the fluid collected at gas breakthrough time (t) was recorded at room temperature and atmospheric pressure. Therefore, the flow rate term Qg can be replaced by the term q/t, which represents an average flow rate during an experiment until gas breakthrough. Equation (1) can be rewritten as follows. 

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