Gas Properties Reference Material
Subtopics:
Critical Pressure (pc)
Critical pressure represents the pressure above which distinct liquid and gas phases do not exist. As the critical pressure is approached, the properties of the gas and liquid phases become the same, resulting in one phase known as supercritical fluid. Its value is used in the definition of reduced pressure (pr = p / pc) which in turn is used directly in correlations or Equations Of State (EOS) to determine various Pressure, Volume, Temperature (PVT) properties of natural gases (e.g., viscosity, compressibility, z-factor, etc.).
Critical Temperature (Tc)
Critical temperature represents the temperature above which distinct liquid and gas phases do not exist. As the critical temperature is approached, the properties of the gas and liquid phases become the same, resulting in one phase known as supercritical fluid. Its value is used in the definition of reduced temperature (Tr = T / Tc) which in turn is used directly in correlations or equations of state (EOS) to determine various PVT properties of natural gases (e.g., viscosity, compressibility, z-factor, etc.).
Gas Compressibility (cg)
The compressibility of a substance is the change in volume per unit volume per unit change in pressure. Gas compressibility should not be confused with the compressibility factor (z), which is the deviation factor from ideal gas behavior. Gas compressibility is calculated from the BWR equation of state as published in the ERCB manual "Gas Well Testing - Theory and Practice".
The gas compressibility is a very strong function of pressure, and increases as the pressure decreases. Mathematically, it can be expressed as:
Where p is the specified pressure and z is the gas compressibility factor (z) at that pressure. Thus, the magnitude of gas compressibility (cg) is of the order of 1 / p. Moreover, when the pressure drawdown at the wellbore is large, the difference in compressibility values at the initial pressure (pi) and at the flowing pressure is significant and must be accounted for by using the pseudo-time (ta) variable, instead of regular time.
Gas Compressibility Factor (z)
The compressibility factor of a natural gas is a measure of its deviation from ideal gas behavior. Its value is usually between 0.8 and 1.2, but it can be as low as 0.3, and as high as 2.0. It is used in the calculation of gas pseudo-pressures (y), and in converting gas volumes and rates from standard conditions to reservoir conditions (and vice versa). It is sometimes called the super-compressibility factor, and is often confused with the term "compressibility" (which is the change in volume per unit volume per unit change in pressure). The gas compressibility factor directly affects the gas compressibility (cg).
Gas Formation Volume Factor (Bg)
The gas formation volume factor, defined below, is a function of the fluid composition and the pressure / temperature ratio between reservoir (in-situ) and standard
| Note: | It is a very strong function of pressure, and a weak function of temperature and gas composition. |
Gas Gravity (g or gg)
Gas gravity is the molar mass (molecular weight) of the natural gas divided by the molar mass of air (28.94). It ranges from 0.55 for dry sweet gas to approximately 1.5 for wet, sour gas. The default value is 0.65.
The gas gravity affects the calculations of gas viscosity (mg), compressibility (cg), compressibility factor (z), and solution gas oil ratio (Rs). The effect of gas gravity on these properties is not significant, so it is not detrimental if the gas gravity is not precisely known, using the default or an average value is sufficient in most cases.
Gas Viscosity (mg)
Gas viscosity is a measure of the amount of resistance to flow the gas has. Higher values indicate more resistance to flow. For gas, the viscosity increases with increasing temperature and pressure. Usually it is not measured, but is obtained from the Carr, Kobayashi, and Burrows correlations, which include corrections for H2S, CO2, and N2.
For sour gases, this correlation is preferred to the Lee, Gonzalez, and Eakin formulation which does not account for H2S, CO2, and N2. Gas viscosity is used in numerous equations, most notably in the definitions of pseudo-pressure (y) and pseudo-time (ta). Typically, gas viscosity is in the range of 0.015 to 0.03 cp or 15 to 30 micro-Pa.s.