Page 147 - Handbook Of Multiphase Flow Assurance

Asphaltenes 143

The plot of the square root of molar volume of n-alkane precipitants (such as n-C5, n-C7 up
to n-C15) versus the precipitation onset parameter has a linear shape and is the asphaltene
instability trend. The linear trend is used to predict asphaltene precipitation at high pressure
conditions. The precipitation onset parameter is calculated from the measurements by cor-
relations derived from the Flory-Huggins polymer theory. The method has been gradually
gaining acceptance as the predictive technique for oilfield projects.
PC-SAFT or the perturbed chain statistical associating fluids theory is another method
used to estimate phase equilibria and asphaltene precipitation conditions. The method was
developed by Gross and Sadowski (2001) for chain molecules based on SAFT (Chapman
et al., 1988). The liquid phase is modeled as pseudocomponents, and asphaltenes as pseudo-
components with same size or a range of sizes. Despite its promising nature as an equation
of state, the relative difficulty in implementing this method and the requirement to regress
the equation of state parameters for each oil based on laboratory measurements of asphaltene
titration makes this a research method.
Of the above methods the asphaltene onset pressure method has seen the most use in proj-
ect design work as it is a direct laboratory measurement. Nonetheless, laboratory methods
may have results of varying precision which may be affected by drilling mud contamination
and should be complemented with predictive methods such as ASIST or PC-SAFT for asphal-
tene risk analysis.

Light oil and EOR

Enhanced oil recovery can be accomplished by light oil miscible flooding, CO 2 flooding or
water flooding.
Light oil solubilizes resins which causes asphaltene flocculation. Injection of light oil
may destabilize asphaltenes, similar to titration by n-paraffins. Laboratory tests should be
able to tell whether miscible flood would not cause reduced permeability due to solids
precipitation.
CO 2 flooding increases the acidity which leads to asphaltene flocculation (Srivastava et al.,
1999).
Another study (Srivastava et al., 1995) indicated that asphaltene began to flocculate at
about 42 mol% CO 2 concentration in oil.
Water flooding, if applied after the reservoir pressure dropped below the bubble point, at
some later time after the start of production may cause asphaltene deposition in the reservoir
as it is re-pressurized. As the gas cap light ends get redissolved in oil due to pressure mainte-
nance, the light ends solvate resins which destabilizes asphaltenes.

Gas condensate

Gas condensate and light oils can cause asphaltene deposition from heavy oils when com-
ingled from several production streams. Caution should be used in combining production
from several wells into the same flowline, or several flowlines into the same separator.
In some hydrocarbon fluids condensate may be carried by gas as droplets rather than by
being dissolved in gas. Such condensate can contain asphaltenes which can precipitate in
high shear locations of the production system such as the choke.