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.