An Improved Compressibility Model for Gas Service with High Concentrations of CO2
In the oil and gas industry, CO2 compression occurs primarily in Enhanced Oil Recovery (EOR), gathering, and sequestration applications. CO2 has a critical point (88°F/ 31°C and 1070 psia/ 74 bar) that lies in the operating range of compressors typically used in these applications. Near this point, common equation-of-state (EOS) models do not adequately account for CO2 compressibility effects, making compressor performance predictions inaccurate. Research organizations such as the National Institute of Standards and Technology (NIST) have created EOS models that accurately predict CO2 compressibility. However, these models require complex algorithms that are difficult to implement in real-time field devices such as analyzers or programmable logic controllers (PLC)s. This paper presents a simplified EOS model for CO2 compressibility that is functionally equivalent to the more complex models available today. This model is ideal for analyzer and PLC use, and is valid for vaporous and supercritical CO2. This paper uses analyzer results from compressors in 98% CO2 service to demonstrate the new models increased accuracy over common EOS models. The new correlations show improvement in temperature and clearance calculations compared to similar calculations done with the Redlich-Kwong EOS and with the routines built into an RTwin analyzer (owned by Windrock). The new equations also improve the flow balance between cylinder ends, though the flow balance between stages was not improved.