Summary
The focus of this work is on the behavior of hydrocarbon-gas viscosity and gas density. The viscosity of hydrocarbon gases is a function of pressure, temperature, density, and molecular weight, while the gas density is a function of pressure, temperature, and molecular weight. This work presents new approaches for the prediction of gas viscosity and gas density for hydrocarbon gases over practical ranges of pressure, temperature, and composition. These correlations can be used for any hydrocarbon-gas production or transportation operations.
In this work, we created a large database of measured gas viscosity and gas density. This database was used to evaluate existing models for gas viscosity and gas density. We also provide new models for gas density and gas viscosity, as well as optimization of existing models, using our new database.
The objectives of this research are as follows:• To create a large-scale database of measured gas-viscosity and gas-density data. This database will contain all the information necessary to establish the applicability of various models for gas density and gas viscosity over a widerange of pressures and temperatures.• To evaluate a number of existing models for gas viscosity and gas density.• To develop new models for gas viscosity and gas density using our research database; these models are proposed and validated.
For this study, we created a large database from existing sources available in the literature. The properties in our database include composition, viscosity, density, temperature, pressure, pseudo reduced properties, and the gas compressibility factor. We use this database to evaluate the applicability of existing models used to determine hydrocarbon-gas viscosity and hydrocarbon-gas density (or, more specifically, the gas z-factor). Finally, we developed new models and calculation approaches to estimate the hydrocarbon-gas viscosity, and we also provide an optimization of the existing equations of state (EOS) typically used for for the calculation of the gas z-factor.
Introduction
Hydrocarbon-Gas Viscosity. NIST—SUPERTRAP Algorithm. The state-of-the-art mechanism for the estimation of gas viscosity is most likely the computer program SUPERTRAP, developed at the U.S. Natl. Inst. of Standard sand Technology (NIST). SUPERTRAP was developed from pure-component and mixture data and is stated to provide estimates within engineering accuracy from the triple point of a given substance to temperatures of 1,340.33°F and pressures of 44,100 psia. Because the SUPERTRAP algorithm requires the composition for a particular sample, it generally would not be suitable for applications in which only the mixture gas gravity and compositions of any contaminants are known.
Carr et al. Correlation. Carr et al. developed a two-step procedure to estimate hydrocarbon-gas viscosity. The first step is to determine the gas viscosity at atmospheric conditions (i.e., a reference condition). Once estimated, the viscosity at atmospheric pressure is then adjusted to conditions at temperature and pressure using a second correlation. The gas viscosity can be estimated with graphical correlations or using equations derived from these figures.
Jossi et al. Correlation. Jossi et al. developed a relationship for the viscosity of pure gases and gas mixtures; this correlation includes pure components such as argon, nitrogen, oxygen, carbon dioxide, sulfur dioxide, methane, ethane, propane, butane, and pentane. This "residualviscosity" relationship can be used to predict gas viscosity with the "reduced"density at a specific temperature and pressure, as well as the molecular weight. The critical properties of the gas (i.e., the critical temperature and critical pressure) are also required.
Our presumption is that the Jossi et al. correlation (or at least a similar type of formulation) can be used for the prediction of viscosity for pure hydrocarbon gases and hydrocarbon-gas mixtures. We will note that this correlation is rarely used for hydrocarbon gases (other correlations are preferred); however, we will consider the formulation given by Jossi etal. as a potential model for the correlation of hydrocarbon-gas-viscosity behavior.
