[1] measured MMP of carbon dioxide for twelve different oils using the slim tube and RBA methods to compare these two methods reliability.

They showed that depending on the criteria chosen for MMP in the slim tube test, MMP varies. During their test they used three different usual criteria for MMP: Pressure at 90% oil recovery, pressure at 95% recovery and the break over pressure in the oil recovery plot (the pressure for which oil recovery did not increase more than 1% per 100 psi pressure increase). Their results showed that a slim-tube oil recovery criterion of 90% will usually yield MMPs lower than those from the criteria for the slim tube. They mentioned that the oil recovery break over criterion produced an MMP from the slim tube that was in excellent agreement with MMP from the RBA.

The pressure drop across the slim tube complicates interpretation of slim tube results. As mentioned before it is assumed that the pressure different between input and output of the slim tube is ignorable compare to the MMP.  Because the final report of a slim tube experiment is the oil recovery at a given pressure, the pressure drop becomes an uncertainty in pressure at which the oil recovery is measured, so improper design and operation of a slim tube can give excessive pressure drop.

Asphaltene deposition is a phenomenon that occurred during the solvent injection. Elshalkaway et al. defined the blind spot of the slim tube method as the measuring of the MMP for systems with asphaltene deposition problem during the dilution process. Precipitation of the asphaltene could block the slim tube more or less and increase required displacement pressure. Sometimes deposited asphaltenes completely plug the tube and the test fails. In RBA it does not appear to be a problem. Specks of asphalt can be seen precipitating out on the walls of tube; however bubble behavior can still be observed.

Sometimes there is not enough test fluids (oil and solvent) to run slim tube test several times to find the MMP or MME. RBA could work at this time because it requires considerably less fluid to determine the MMP or MME.

RBA could be a good chose when the time factor is important. The RBA takes 1-2 hours per MMP determination (excluding preparation time), while the slim tube takes one to two weeks per MMP determination.

References

[1] “Measurement and Modeling of Fluid-Fluid Miscibility in Multicomponent Hydrocarbon Systems”, Ayirrala S. C., 2005

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