Equation of State Comparisons and Evaluations for Applications Through Gas Property Testing and Derivations

Accurate gas property prediction is a necessary component throughout the oil and gas industry for end users, operators and equipment manufacturers for proper sizing and
selection of equipment, improving overall efficiency, and reducing operating costs. Various Equation of State (EOS) models are utilized to predict thermophysical gas properties needed for such calculations. These are semi-empirical models that allow the calculation of thermodynamic and dynamic properties such as density, enthalpy, and entropy of gas mixtures for known pressures and temperature (and vice versa). While there is a large body of work available comparing the results of various EOS models, there is currently limited or no data publicly available to verify the results of these EOS calculations for the range of pressures, temperatures and gas compositions relevant to compression and pipeline operations[1-5]. This is specifically true for natural gas compositions containing heavier hydrocarbons, sour or acid gas components or high CO2 content as well as operating points near the critical phase, high pressures, or dense phase (supercritical) operation. Thus, the users of EOS (operators and manufacturers) often have no precise data to determine whether a particular EOS will give sufficiently accurate results or which EOS will provide calculations closest to measured values.

In order to have an improved understanding of the applicability of standard EOS in pipeline applications, a set of gas physical property tests were undertaken with sweet and
sour natural gas and CO2 mixtures at typical pipeline compositions and conditions, including new high pressure dense phase applications. Specific gas properties tested include gas density (ρ), specific heat at constant volume (cv), and speed of sound (c or SOS). These results were compared to several of the most commonly used EOS, including NIST, GERG, AGA8, PR, and SRK, which were also compared to each other.This project was a joint effort with funding and direction from the Gas Machinery Research Council (GMRC) and the Pipeline Research Council International (PRCI).. A committee formed by representatives from GMRC, PRCI and the Southwest Research Institute® (SwRI®) defined the gas mixtures and test points presented in this paper. Table 1 and Table 2 provide the gas mixture component concentrations (in % mol) for the GMRC and PRCI portions of the project, respectively.The purpose of this project is not to provide a comprehensive database, but rather provide data sets for multi-component gas mixtures, at pressures and temperatures relevant to gas compression and pipeline metering applications. While a large body of test data exists for pure substances and binary mixtures, data on gas compositions typical for many upstream and midstream oil and gas projects is very rare. Typical gas mixtures contain methane and heavier hydrocarbons, but may also include CO2, nitrogen and water.

The data sets obtained in this project can be used to test EOS model predictions against. Also included in the scope of work for this project is the methodology for calculating enthalpy and entropy from experimental data. These properties cannot be measured and must be derived by their relationship to measured properties. Analyses performed for compressor performance, station operation, pipeline simulation all depend on these properties. Therefore, deriving equations that allow enthalpy and entropy 2 to be calculated from the tested properties is vital. The methodology for deriving these equations, as well as, the equations themselves are also presented.