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    Tech Talk

#14 Modeling of CO2 Properties

Just 10 years ago, the oil industry primarily saw CO2 as a gas that could potentially be used to enhance the oil recovery through injection into oil reservoirs. CO2 is relatively miscible with heavy hydrocarbons and a miscible drive can develop at moderate reservoir pressures. 

Today, much focus on CO2 has shifted to Carbon Capture Storage (CCS). It is the process of capturing waste carbon dioxide (CO2), transporting it to a storage site, and depositing it where it will not enter the atmosphere. The logistics around the transport and storage of CO2 requires several engineering simulations calling for accurate simulation models for the properties of not only pure CO2, but also CO2 with impurities. 

Calsep has just completed a validation project comparing the performance of four industry standard equations of state for simulation of the below properties of CO2 and CO2 with impurities. 

  • VLE
  • Density
  • Heat capacity
  • Joule-Thomson coefficient
  • Velocity of sound

The equation of state tested were 

  • SRK-Peneloux with constant and T-dependent volume correction
  • PR-Peneloux
  • GERG-2008
  • PC-SAFT 

The results are documented in two validation reports, one for pure CO2 and one for CO2 with impurities. The reports are available to all PVTsim users with a maintenance agreement for PVTsim. 

The GERG-2008 equation turned out to give accurate predictions of all the tested properties. For pure CO2 it is almost as accurate as the Span-Wagner equation, which is a kind of standard for pure CO2. Waste CO2 will often be polluted by other gas components as for example N2 and C1. Unlike Span-Wagner GERG-2008 handles mixtures of CO2 and other gas components, making it a perfect choice as fluid model in CCS engineering calculations. 

Below are selected plots from the validation reports.

PVTsim users with a maintenance agreement for PVTsim can sign in/up and download the the PVTsim Nova validation reports here: Link

References

Kunz, O. and Wagner, W.”The GERG-2008 Wide-Range Equation of State for Natural Gases and Other Mixtures: An Expansion of GERG-2004”, J. Chem. Eng. Data. 57, 2012, 3032-3091.

Span, R. and Wagner, W., “A New Equation of State for Carbon Dioxide Covering the Fluid Region from the Triple-Point Temperature to 1100 K at Pressures up to 800 MPa”, Journal of Physical and Chemical Reference Data 25, 1996, 1509-1596.