# 18: Properties of Natural Gas and Condensates I

Learning Objectives

• Module Goal: To highlight the important properties used to characterize natural gas and condensate systems.
• Module Objective: To present the most popular models for estimating properties of natural gas and condensate systems.

Fluid properties are used to characterize the condition of a fluid at a given state. A reliable estimation and description of the properties of hydrocarbon mixtures is fundamental in petroleum and natural engineering analysis and design. Fluid properties are not independent, just as pressure, temperature, and volume are not independent of each other. Equations of State provide the means for the estimation of the P-V-T relationship, and from them many other thermodynamic properties can be derived. Compositions are usually required for the calculation of the properties of each phase. For a VLE system, using the tools we have discussed in the previous lectures, we are ready to predict some important properties of both the liquid (condensate) and vapor (natural gas) phases — by means of the known values of composition of both phases. Some of the most relevant are discussed next.

• 18.1: Molecular Weight
The molecular weight (MW) of each of the phases in a VLE system is calculated as a function of the molecular weight of the individual components (MWi), provided that both the composition of the gas (yi) and the liquid (xi) are known.
• 18.2: Density
Density is customarily defined as the amount of mass contained in a unit volume of fluid. Density is the single-most important property of a fluid, once we realize that most other properties can be obtained or related to density. Both specific volume and density — which are inversely proportionally related to each other — tell us the story of how far apart the molecules in a fluid are from each other.
• 18.3: Specific Gravity
Specific gravity is defined as the ratio of fluid density to the density of a reference substance, both defined at the same pressure and temperature. These densities are usually defined at standard conditions (14.7 psia and 60°F). For a condensate, oil or a liquid, the reference substance is water and for a natural gas, or any other gas for this matter, the reference substance is air.
• 18.4: API Gravity
Petroleum and Natural Gas Engineers also use another gravity term which is called API gravity.
• 18.5: Volumetric Factors (Bo and Bg)
Due to the dramatically different conditions prevailing at the reservoir compared to the conditions at the surface, we do not expect that 1 barrel of fluid at reservoir conditions could contain the same amount of matter as 1 barrel of fluid at surface conditions. Volumetric factors were introduced in petroleum and natural gas calculations to readily relate the volume of fluids that are obtained at the surface (stock tank) to the volume that the fluid actually occupied in the reservoir.
• 18.6: Isothermal Compressibilities
For liquids, the value of isothermal compressibility is very small because a unitary change in pressure causes a very small change in volume for a liquid. For natural gases, isothermal compressibility varies significantly with pressure.
• 18.7: Surface Tension
Surface tension is a measure of the surface free energy of liquids, i.e., the extent of energy stored at the surface of liquids. Although it is also known as interfacial force or interfacial tension, the name surface tension is usually used in systems where the liquid is in contact with gas. Qualitatively, it is described as the force acting on the surface of a liquid that tends to minimize the area of its surface, resulting in liquids forming droplets with spherical shape, for instance.
• 18.8: Action Item