Introduction
Properties of natural gas include gas-specific gravity, pseudocritical pressure and temperature, viscosity, compressibility factor, gas density, and gas compressibility. Knowledge of these property values is essential for designing and analyzing natural gas production and processing systems. Because natural gas is a complex mixture of light hydrocarbons with a minor amount of inorganic compounds, it is always desirable to find the composition of the gas through measurements. Once the gas composition is known, gas properties can usually be estimated using established correlations with confidence.
Specific Gravity
Gas-specific gravity (yg) is defined as the ratio of the apparent molecular weight of a natural gas to that of air, itself a mixture of gases. The molecular weight of air is usually taken as equal to 28.97 (approximately 79% nitrogen and 21% oxygen). Therefore the gas gravity is
where the apparent molecular weight of gas can be calculated on the basis of gas composition. Gas composition is usually determined in a laboratory and reported in mole fractions of components in the gas. Let yt be the mole fraction of component i, the apparent molecular weight of the gas can be formulated using mixing rule as.
where MWt is the molecular weight of component i, and Nc is the number of components. The molecular weights of compounds (MWi) can be found in textbooks on organic chemistry or petroleum fluids such as that by McCain (1973). A light gas reservoir is one that contains primarily methane with some ethane. Pure methane would have a gravity equal to (16.04/28.97) = 0.55. A rich or heavy gas reservoir may have a gravity equal to 0.75 or, in some rare cases, higher than 0.9.
Pseudocritical Properties
Similar to gas apparent molecular weight, the critical properties of a gas can be determined on the basis of the critical properties of compounds in the gas using the mixing rule. The gas critical properties determined in such a way are called pseudocritical properties. Gas pseudocritical pressure (ppc) and pseudocritical temperature (Tpc) are, respectively
expressed as
For the gas composition given in the following text, determine apparent molecular weight, pseudocritical pressure, and pseudocritical temperature of the gas.
Viscosity
Gas viscosity is a measure of the resistance to flow exerted by the gas. Dynamic viscosity (jug) in centipoises (cp) is usually used in the natural engineering:
Gas viscosity is very often estimated with charts or correlations developed based on the charts. The gas viscosity correlation of Carr, Kobayashi, and Burrows (1954) involves a two-step procedure: the gas viscosity at temperature and atmospheric pressure is estimated first from gas-specific gravity and inorganic compound content. The atmospheric value is then adjusted to pressure conditions by means of a correction factor on the basis of reduced temperature and pressure state of the gas.
Compressibility Factor
Gas compressibility factor is also called deviation factor, or z-factor. Its value reflects how much the real gas deviates from the ideal gas at given pressure and temperature. Definition of the compressibility factor is expressed as:
where n is the number of moles of gas. When pressure p is entered in psia, volume V in ft 3 , and temperature in 0 R, the gas constant R is equal to
The 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 P1, z-factor can then be determined with the following formula:
Gas Density
Because natural gas is compressible, its density depends upon pressure and temperature. Gas density can be calculated from gas law for real gas with good accuracy:
Formation Volume Factor and Expansion Factor
Formation volume factor is defined as the ratio of gas volume at reservoir condition to the gas volume at standard condition, that is
Compressibility of Natural Gas
Real Gas Pseudopressure
Real gas pseudopressure m(p) is defined as
(2.52) where pb is the base pressure (14.7 psia in most states in the U.S.). The
pseudopressure is considered to be a "pseudoproperty" of gas because it depends on gas viscosity and compressibility factor, which are properties of the gas. The pseudopressure is widely used for mathematical modeling of IPR of gas wells. Determination of the pseudopressure at a given pressure requires knowledge of gas viscosity and z-factor as functions of pressure and temperature. As these functions are complicated and not explicit,a numerical integration technique is frequently used.
Real Gas Normalized Pressure
Real gas normalized gas pressure n(p) is defined as
(2.53) where pr is the pseudoreduced pressure. For the convenience of engineering applications, the normalized gas pressures of sweet natural gases at various pressures and temperatures have been generated with the spreadsheet program NormP.xls. The results are presented in Appendix B.
Source :
Properties of natural gas - Specific Gravity , Vicosity and more
Properties of natural gas include gas-specific gravity, pseudocritical pressure and temperature, viscosity, compressibility factor, gas density, and gas compressibility. Knowledge of these property values is essential for designing and analyzing natural gas production and processing systems. Because natural gas is a complex mixture of light hydrocarbons with a minor amount of inorganic compounds, it is always desirable to find the composition of the gas through measurements. Once the gas composition is known, gas properties can usually be estimated using established correlations with confidence.
Specific Gravity
Gas-specific gravity (yg) is defined as the ratio of the apparent molecular weight of a natural gas to that of air, itself a mixture of gases. The molecular weight of air is usually taken as equal to 28.97 (approximately 79% nitrogen and 21% oxygen). Therefore the gas gravity is
where the apparent molecular weight of gas can be calculated on the basis of gas composition. Gas composition is usually determined in a laboratory and reported in mole fractions of components in the gas. Let yt be the mole fraction of component i, the apparent molecular weight of the gas can be formulated using mixing rule as.
where MWt is the molecular weight of component i, and Nc is the number of components. The molecular weights of compounds (MWi) can be found in textbooks on organic chemistry or petroleum fluids such as that by McCain (1973). A light gas reservoir is one that contains primarily methane with some ethane. Pure methane would have a gravity equal to (16.04/28.97) = 0.55. A rich or heavy gas reservoir may have a gravity equal to 0.75 or, in some rare cases, higher than 0.9.
Pseudocritical Properties
Similar to gas apparent molecular weight, the critical properties of a gas can be determined on the basis of the critical properties of compounds in the gas using the mixing rule. The gas critical properties determined in such a way are called pseudocritical properties. Gas pseudocritical pressure (ppc) and pseudocritical temperature (Tpc) are, respectively
expressed as
For the gas composition given in the following text, determine apparent molecular weight, pseudocritical pressure, and pseudocritical temperature of the gas.
Viscosity
Gas viscosity is a measure of the resistance to flow exerted by the gas. Dynamic viscosity (jug) in centipoises (cp) is usually used in the natural engineering:
Gas viscosity is very often estimated with charts or correlations developed based on the charts. The gas viscosity correlation of Carr, Kobayashi, and Burrows (1954) involves a two-step procedure: the gas viscosity at temperature and atmospheric pressure is estimated first from gas-specific gravity and inorganic compound content. The atmospheric value is then adjusted to pressure conditions by means of a correction factor on the basis of reduced temperature and pressure state of the gas.
Compressibility Factor
Gas compressibility factor is also called deviation factor, or z-factor. Its value reflects how much the real gas deviates from the ideal gas at given pressure and temperature. Definition of the compressibility factor is expressed as:
where n is the number of moles of gas. When pressure p is entered in psia, volume V in ft 3 , and temperature in 0 R, the gas constant R is equal to
The 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 P1, z-factor can then be determined with the following formula:
Gas Density
Because natural gas is compressible, its density depends upon pressure and temperature. Gas density can be calculated from gas law for real gas with good accuracy:
Formation Volume Factor and Expansion Factor
Formation volume factor is defined as the ratio of gas volume at reservoir condition to the gas volume at standard condition, that is
Compressibility of Natural Gas
Real Gas Pseudopressure
Real gas pseudopressure m(p) is defined as
(2.52) where pb is the base pressure (14.7 psia in most states in the U.S.). The
pseudopressure is considered to be a "pseudoproperty" of gas because it depends on gas viscosity and compressibility factor, which are properties of the gas. The pseudopressure is widely used for mathematical modeling of IPR of gas wells. Determination of the pseudopressure at a given pressure requires knowledge of gas viscosity and z-factor as functions of pressure and temperature. As these functions are complicated and not explicit,a numerical integration technique is frequently used.
Real Gas Normalized Pressure
Real gas normalized gas pressure n(p) is defined as
(2.53) where pr is the pseudoreduced pressure. For the convenience of engineering applications, the normalized gas pressures of sweet natural gases at various pressures and temperatures have been generated with the spreadsheet program NormP.xls. The results are presented in Appendix B.
Source :
Properties of natural gas - Specific Gravity , Vicosity and more
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