Compressibility Factor for Gas Condensates

The compressibility factor of natural gases is necessary in many petroleum engineering calculations. Some of these calculations are the following: evaluation of a newly discovered formation, pressure drop from flow of gas through a pipe, pressure gradient in gas wells, gas metering, gas compression, and processing. Typically, the gas compressibility factor is measured by laboratory experiments. These experiments are expensive and time-consuming. Occasionally, experimental data became unavailable and the gas compressibility factor is estimated from correlations using gas composition or gas gravity. This paper presents new methods for calculating the gas compressibility factors for gas condensates at any temperature and pressure. The method is based on compositional analysis of 1200 compositions of gas condensates collected worldwide. When the gas composition is known, this study presents a simple mixing rule to calculate the pseudo-critical properties of the gas condensate. The new mixing rule accounts for the presence of the heptane plus fraction and none hydrocarbons. In case the gas composition is unavailable, the study presents a new gas gravity correlation to estimate pseudo-critical properties of the gas condensate. This study also presents an evaluation of eight methods to characterize the plus fraction, three widely used mixing rules, and six methods to calculate the gas compressibility factor. Thus, this study presents an evaluation of one hundred forty-four possible methods of calculating the gas compressibility factor for gas condensates. The accuracy of the new mixing rule and the gas gravity correlation has been compared to other published methods. The comparison indicates that the proposed methods are consistent and provide accurate results.