Page 150 - Handbook Of Multiphase Flow Assurance

146 5. Flow restrictions and blockages in operations

to distinguish between types of restrictions based on pressure, temperature, flow rates of oil,
gas and water, and on the deviation trend signature indicative of the solid type.

Remediation of asphaltene plugging
Deposited asphaltenes may be removed from the flow system by several methods:
• Chemical techniques
Toluene soak can dissolve asphaltene deposits in risers, flowlines, wells and reservoirs
pores, provided that the solvent chemical can reach the asphaltene buildup.
• Mechanical techniques
Topsides asphaltene removal from separators, heat exchangers and other process equip-
ment may be safer, cheaper and faster than the use of flammable toxic solvent.
Scraping is also a mechanical method of removing asphaltenes from production flowlines.
• Novel materials
Non-stick coatings have been shown to reduce asphaltene adhesion. Effectiveness of dif-
ferent coatings should be verified in laboratory for a specific oil.
• Emerging techniques
Pressure pulsation is an effective novel method either for a complete removal of pliable
asphaltene deposit from a flowline or for opening a communication channel through an as-
phaltene blockage so that a solvent could be circulated through the blockage. However, com-
posite blockages for example of a less pliable hydrate and asphaltene may be more difficult
to remove using this method.

Environmental impact of various techniques
Each method carries its own set of risks for an operator. Such risks and hazards have to be
identified in a review. Mitigation plan should be developed for the credible risks to minimize
potential environmental impact should a treated production system lose continuity.

Modeling of asphaltenes

• Reversible thermodynamics
Asphaltene precipitation (flocculation) is usually reversible within small excursion into
the asphaltene instability phase envelope. If asphaltenes precipitate as pressure is decreased,
they get re-solubilized when pressure increases again.
The Flory-Huggins solubility model of asphaltenes is
(
(
2
−
Volfraction_of_asphaltene_in_oil = exp −(1 V asp /V oil) −V asp /RTTd _ asp − d _ oil) )
.
3
with units T [K], V [m /mol], where V asp is molar volume of asphaltene, V oil is molar vol-
ume of oil, d_asp is solubility parameter of asphaltene, d_oil is solubility parameter of oil, R
is universal gas constant, and T is system temperature.
At the asphaltene solubility limit (flocculation), the solubility of asphaltene in oil equals
the volume fraction of asphaltene in live oil. The Hirschberg model is an example of using an
equation of state to predict reversible flocculation of asphaltene in oil.