Evaluation of the Best Equation of State for Enhanced Oil Recovery Use in Ultra-high Pressure Hydraulic Calculation

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Enhanced oil recovery (EOR), is an essential part of oil and gas production nowadays. Gases used include carbon dioxide (CO2), natural gas, or nitrogen (N2). The discharge pressure of the platform injection compressor could be as high as 12,000 psi. The proper selection of the size of the gas injection system and platform becomes critically important and is found to be heavily affected by the simulated results from process involving the equation of state (EOS). The EOS of a system has been proven to be very reliable in predicting the properties of most hydrocarbon based fluids. An engineering design starts with EOS selection, process simulation, heat and material balance calculation, equipment sizing and finally detailed engineering. This study focuses on the investigation of the most probable and applicable equations of state (EsOS) such as GERG-EOS, BWRS-EOS, LKP-EOS and PR-EOS in the high pressure compression simulation industry.
Aspen HYSYS is a commercial process modeling tool for conceptual design, optimization, and performance monitoring for oil & gas production, gas processing, petroleum refining, and air separation industries. Because the critical thermodynamic properties including enthalpy, entropy, vapor pressure and density are shown to be related to the compressibility factor, this study adopted Aspen HYSYS as the simulation tool to evaluate all four EsOS. The predicted compressibility factors (Z) from the different EsOS were compared to experimental data obtained from a wide variety of sources. The results suggest that for the case of pure CO2 and pure N2, all EsOS tested within the low pressure range up to 1000 psia can produce accurate results. For high pressure conditions up to 12,000 psia, the GERG can provide the most accurate predictions. Considering the hydrocarbon/CO2 mixture and hydrocarbon/N2 mixture, for low pressure system it is found the results from GERG, LKP and PR EsOS fit better with the experimental data than those from BWRS. However for high pressure system, it appears that GERG, BWRS and LKP can provide good prediction. Furthermore, for high temperature case, the LKP proves to give the most accurate results. It is recommended to use LKP for offshore EOR gas injection operations.

EOS comparison, Equation of state for EOR, Deep-water EOR, Gas injection, Enhanced oil recovery (EOR), High pressure compression
Portions of this document appear in: Elliot, D., Kuo, J.C., and Nasir, P. (2008). Plant Processing of Natural Gas, University of Texas Continuing Education Petroleum Extension Service, Austin, Texas, pp. 146 -152.