A large series of experiments was performed to measure the downhole rheology of various quality foams of Carbon Dioxide, Nitrogen and mixtures of the two with aqueous phases consisting of non-crosslinked and crosslinked polymer solutions. Traditional methodology for rheological data analysis, used in small scale horizontal pipe experiments, could not be applied to these large scale vertical experiments because of the interrelation between friction and hydrostatic pressure drops. Therefore, a two-step methodology was developed which consisted of obtaining friction factors from primary data by solving a two-phase vertical flow equation. Then a rheological model was applied to describe the friction factor behavior. A new variable, the specific volume expansion ratio, was found the most useful to correlate friction factor data. The obtained correlation, named Volume Equalized Power Law model, is suitable to represent foam rheology. Model parameters are given for various liquid formulations. A calculational procedure is presented, applicable in field conditions, where a varying quality is always evident.