Reservoir fluid properties are normally measured in the laboratory. *Pressure-Volume-Temperature* (*PVT*) properties relate these properties to each other at equilibrium conditions. These variables, typically used for volumetric related reservoir behavior, are measured in a laboratory *PVT Cell*. A PVT Cell is a high-pressure vessel (container) that allows for the control and measurement of pressure, volume, and temperature.

In addition to laboratory measurements, reservoir PVT properties can be determined from *Equations-of-State* (*EOS*). Equations-of-state are theoretically derived equations that relate the *State Variables*: pressure, volume, and temperature (state variables are variables that define the thermodynamic state of a system). Three common examples of equations-of-state include:

Isothermal compressibility:

Real gas law:

Van der Waal’s Cubic EOS:

In **Equation 3.26**, the negative sign is required because the volume of a fluid decreases as pressure increase (i.e., the derivative is negative). Note that all of these relationships allow for the determination of one of the state variables, p, V, or T, if the two other variables are known (two degrees of freedom). For the isothermal compressibility EOS, **Equation 3.26**, temperature can be considered a variable if we have tables or equations where the value of cf can be defined for a specific temperature.

In addition to the equations-of-state, fluid PVT correlations are also used in the oil and gas industry. These correlations can be either graphical or mathematical in nature. Fluid property correlations are simply plots, curve fits, or regressions of many laboratory measurements covering a wide range of data. In general, these correlations may not be as accurate as laboratory measurements or equations-of-state, but they have their uses in reservoir engineering.