HPHT Reservoir Acid Gas: Experimental Determination of Z-Factor and Equation of State Modeling
Abstract
Abstract The paper presents experimental compressibility factor (Z-factor) determination and equation of state (EOS) modeling for natural gas under high pressure and high temperature (HPHT) reservoir conditions in a field of the Uzbekistan Republic. The gas contains up to 20 mol.% of non-hydrocarbon components, including acid gases. The discrepancy between various EOS and empirical correlation predictions of the gas compressibility factor increases at higher pressures and temperatures, leading to uncertainties in reservoir performance and production modeling. This work includes experimental determination of Z-factors across pressure and temperature ranges, selection of the most suitable method for modeling gas behavior, and adjustment of common equations of state for production modeling. Gas chromatographic analysis was conducted on reservoir gas samples to determine gas composition for EOS modeling. The experiments with the PVT cell were conducted on the same samples to determine the Z-factor. The experiments involve measuring the volumes of the gas stabilized at specific pressure and temperature values. A special study was conducted to account for the thermal expansion and compressibility of mercury used for gas compression and the cell walls within the same pressure-temperature range. Experimental Z-factor values were compared with predictions from several empirical correlations and equations of state to assess their suitability for field conditions. During the experiments, several parameters were measured: pressure, temperature, volume – each with particular accuracy. The relative uncertainty in experimental Z-factor determination was assessed by combining all measurement errors. The calculated uncertainty was found to be significantly lower than the discrepancy between EOS predictions. Therefore, these experimental values provide a reliable reference for evaluating EOS models and empirical correlations. Six widely used models were evaluated in the study: equations of state – Redlich-Kwong (RK), Peng-Robinson (PR), and Brusilovsky (BR) –and empirical correlations – Dranchuk-Purvis- Robinson (DPR), Dranchuk-Abou-Kassem (DAK), and Hall-Yarborough (HY). The predictions were compared with experimental results across pressure ranges from 150 bar to 700 bar and temperatures from 30°C to 150°C. The comparison revealed that the four- parameter Brusilovsky equation provided the best fit to the experimental data, with a maximum error of 3% across the entire range of pressures and temperatures. Two equations commonly used in commercial software – PR and RK – were adjusted to fit experimental data by introducing correction multipliers for equation parameters. Physical experiments with acid-rich natural gas under HPHT conditions present technical challenges. The presented results appear to be the first experimental dataset obtained for the region of interest. The results validate the application of adjusted equations of state for modeling reservoir and surface facilities performance, reserves estimation, and production forecasting.