Page 151 - Handbook Of Multiphase Flow Assurance

Asphaltenes 147

• Irreversible thermodynamics
In cases when pressure excursion into the asphaltene instability envelope is significant, res-
ins may flow away from flocculating asphaltene particles and be unavailable to re- solubilize
asphaltenes if pressure were to increase again. In this case the flocculation process becomes
irreversible, and flocculated asphaltenes are more likely to deposit on production system sur-
face such as on well tubing walls or in a separator. A model which accounts for the stabilizing
effect of resins was developed by Pan and Firoozabadi (1998). Once asphaltenes deposit on
a surface of a rock in the reservoir or on steel or polymeric production system surface, the
solubility loss becomes irreversible and an increase in pressure above the upper asphaltene
instability envelope or a decrease in pressure below the lower asphaltene instability bound-
ary would not lead to redissolution of the precipitated asphaltene. Asphaltene deposition tool
ADEPT was developed to forecast the rate of asphaltene deposition.
Prediction of asphaltene solids precipitation may be done with the ASIST method based on
laboratory measurements. Precipitation may be reversible, but not always is.
Prediction of asphaltene deposition may be done with the ADEPT method. Deposition is
usually irreversible. Unlike wax deposits which can be redissolved by increasing temperature
(e.g. by hot oiling) or hydrate deposits which can be dissociated by either pressure reduction
or by temperature increase (slowly), the asphaltene deposit does not respond to a change in
either pressure or temperature. Asphaltene pyrolizes at very high temperatures of over 500 °C
which cannot be practically achieved in production systems. Solvent, mechanical removal or
flow path replacement (either by pipe section replacement or reservoir re-fracturing) may be
the only methods to restore production after irreversible asphaltene deposition.

Prevention of asphaltenes

Production operation should avoid or control asphaltene formation and accumulation
which is usually irreversible by normal operating procedure. Asphaltene flocculation may be
reversible with pressure but should not be relied upon in production.
Pressure threshold for the start of chemical treatment with asphaltene inhibitor, dispersant
or solvent may be used if supported by live fluid laboratory data. Chemical injection for as-
phaltene treatment should be set shallower than the scale treatment injection depth because
scale deposit may be more difficult to remedy. While asphaltene may be dissolved by tolu-
ene, barite scale cannot be easily dissolved and requires either mechanical milling or costly
chelant treatment. Note that acid workover for scale formation damage tends to destabilize
asphaltene which can form a deposit or a stable emulsion.
Asphaltene inhibitor effectiveness is measured in a high pressure laboratory cell
equipped with infrared light and meter. A reduction in the amount of asphaltene deposited
on the cell walls after depressurization, determined by light absorbance, and the amount
precipitated in oil, determined by filtration of live oil, indicate chemical effectiveness (Yin
et al., 2000).
Technologies which could be considered for management of asphaltene in produced fluids
include:
• asphaltene dispersant/inhibitor chemical from topside, wet tree or downhole
• avoid commingling of dissimilar produced hydrocarbon fluids
• pressure maintenance above asphaltene onset pressure AOP