This article is to introduce about properties of natural gas.

### Specific Gravity of Gas

It is defined as the ratio of the apparent molecular weight of the gas to that of air. The molecular weight of air is usually taken as equal to 28.97 ( ~79% nitrogen and 21% oxygen). Therefore, the gas specific gravity is

Gas composition is usually determined in a laboratory and reported in mole fractions of components in the gas. Gas specific gravity varies between 0.55 and 0.9.

### Gas Pseudo-Critical Pressure and Temperature

Similar to gas apparent molecular weight, the critical properties of a gas can be determined on the basis of critical properties of compounds in the gas using the mixing rule. The gas critical properties determined in such a way are called "pseudo-critical properties". Gas pseudo-critical pressure and pseudo-ctirical temperature are, respectively, expressed as

If the gas composition is not know but gas-specific gravity is given, the pseudo-critical pressure and temperature can be determined from various charts or correlations developed based on the charts. Applications of the pseudo-critical pressure and temperature are normally found in petroleum engineering through pseudo-reduced pressure and temperature.

### Viscosity of Gas

Dynamic viscosity in centipoise is usually used in petroleum engineering. Kinematic viscosity is related to the dynamic viscosity through density

Kinematic viscosity is not typically used in natural gas engineering. Direct measurement of gas viscosity are preferred for a new gas. If gas composition and viscosities of gas components are known, the mixing rule can be used to determine the viscosity of the gas mixture. Viscosity of gas is very often estimated with charts or correlations developed based on the carts.

### Gas Compressibility Factor

It is also called "deviation factor" or "z-factor". Its value reflects how much the real gas deviates from the ideal gas at a given pressure and temperature.

Introducing the z-factor to the gas law for ideal gas results in the gas law for real gas as below.

Gas compressibility factor can be determined on the basis of measurements in PVT laboratories. For a given amount of gas, if temperature is kept constant and volume is measured at 14.7 psia and an elevated pressure, z-factor can then be determined with following formula:

Very often the z-factor is estimated with the chart developed by Standing and Katz (1954). This chart has been set up for computer solution by a number of individuals.

### Density of Gas

Because of gas is compressible, its density depends on pressure and temperature. Gas density can be calculated from gas law for real gas with good accuracy:

### Formation Volume Factor of Gas

Gas formation volume factor is defined as the ratio of gas volume at reservoir condition to the gas volume at standard condition.

Gas formation volume factor is frequently used in mathematical modeling of gas well-inflow performance relationship (IPR).

### Gas Compressibility

Gas compressibility factor is defined as

Combining with gas law for real gas, equation becomes as below.

Since the second term in the right-hand side is usually small, gas compressibility is approximately equal to the reciprocal of pressure.

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