Thermodynamic stability and phase behavior of asphaltenes in oil and of other highly asymmetric mixtures
Ting, Pei-Lun David
Chapman, Walter G.; Hirasaki, George J.
Doctor of Philosophy thesis
Asphaltenes are the polydisperse fraction of heavy organics from petroleum whose phase behavior is important in petroleum production and processing because of its potential to precipitate and plug tubulars. The molecular framework used in this work is that van der Waals (dispersion) interactions dominate asphaltene phase behavior in oil. Using a proposed reservoir fluid fractionation method and an equation of state (EOS) asphaltene characterization method that requires only ambient condition titration data, the Statistical Associating Fluid Theory (SAFT) EOS was extended to model/predict asphaltene phase behavior in oil. Studies on model asphaltene systems (polystyrene-hexane, polystyrene-toluene-ethane, long-chain and short-chain n-alkanes, and phenanthrene-decane-methane mixtures) show that SAFT can describe the phase behavior of fluids dominated by molecular size and shape interactions. Comparison between predicted and experimental asphaltene stability and oil bubble point curves of a recombined oil and a model live oil measured in this work show good agreement. The asphaltene stability and the bubble point measurements for the two oils were made under reservoir conditions as functions of pressure, temperature, and dissolved gas concentration. Both theory and experiment show significant temperature effects on asphaltene stability and the asphaltene instability onset pressures are nearly linear functions of dissolved gas concentration at each temperature. Furthermore, both SAFT-calculated and experiment derived mixture solubility parameters/refractive indices along the asphaltene instability curves are nearly constant at each temperature. A SAFT investigation into the effects of asphaltene polydispersity shows that the lower molecular weight (MW) asphaltenes (including resins) play a significant role in stabilizing higher MW asphaltenes in oil, despite the inclusion of only dispersion interactions in the model. Resin's stabilizing effects on (polydisperse) asphaltene is greatest in the region of incipient asphaltene instability. In n-alkane titrations, SAFT shows that the heaviest asphaltenes will precipitate first, followed by the precipitation of smaller asphaltenes on further oil dilution. The ability to calculate changes in asphaltene MW distribution may be useful in deposition models.
Engineering, Chemical; Engineering, Civil; Engineering, Petroleum