C.1 Introduction

This Appendix catalogs some commofnly used gas fundamental expressions that are useful when operating gas wells.

C.2 Phase Diagram

A hydrocarbon gas is a mixture of different hydrocarbon molecules in varying composition. The type and amount ofeach molecular species in the gas determines the mixture properties at a given pressure and temperature.

As shown in Figure C-1, Critical temperature (T _C) is the temperature of a gas above which it cannot be liquefied by increasing pressure.

Figure C-1: Typical gas well reservoir phase diagram

Critical pressure (P _C) is the pressure a gas exerts when in equilibrium with the liquid phase at the critical temperature.

Critical volume (V _C) is the volume of one pound of gas at the critical temperature and pressure.

Cricondenbar is the highest pressure at which a gas can exist. Cricondenterm is the highest temperature at which a liquid can exist. Bubble point is the pressure at a given temperature above which the mixture is 100% liquid. Dew point is the pressure, at a given temperature, above which the mixture is 100% gas.

C.3 Gas Apparent Molecular Weight and Specific Gravity

Molecular weight is defined for a specific molecule but not for a mixture of different molecular species. For gas mixtures, the apparent gas molecular weight M is defined to represent the average molecular weight of all the molecules in the gas. Thus, M

where y _j = mole fraction of molecule j

M _j = molecular weight...