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U.S. Geological Survey
Open-File Report 2005-1451

Methane Gas Volume Expansion Ratios and Ideal Gas Deviation Factors for the Deep-Water Bering Sea Basins

Ideal Gas Deviation or Z-Factors

Z-factor, or ideal gas deviation factor, describes the departure of the gas behavior from that of the Ideal Gas Law (PV = nRT), which does not account for variation in forces between molecules as a function of pressure. For molar volume (n=1), the Real Gas Law is

PVm = zRT

where P is pressure, Vm is molar volume, R is the ideal gas constant, T is absolute temperature and z is the ideal gas deviation factor.

Note that z-factors greater than 1 mean the real gas volume is greater than the Ideal Gas Law would predict for the given pressure and temperature conditions, while z-factors less than 1 indicate that the real gas is more compressed than the ideal gas prediction. For the Bering Basin conditions described below, z-factor increases with depth.

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For more information, contact: Ginger Barth
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last modified January 6, 2006 (mfd)