Asphaltene
- Also known as
- asphaltene deposition
Asphaltenes are the heaviest, most polar fraction of crude oil, defined by solubility: they dissolve in aromatic solvents such as toluene but not in light alkanes such as heptane. When they come out of solution they form sticky deposits that can plug rock, wells and equipment.
What are asphaltenes?
Asphaltenes are a class of large, complex molecules in crude oil, made of fused aromatic ring systems with side chains and small amounts of nitrogen, sulfur, oxygen and metals. They are not defined by one chemical structure but by how they behave: asphaltenes are the fraction of crude oil that is soluble in toluene and insoluble in a light alkane such as n-heptane or n-pentane.
In a standard SARA analysis, crude oil is split into saturates, aromatics, resins and asphaltenes. In the reservoir, asphaltenes are kept dispersed in the oil, partly stabilized by the resins.
Why asphaltenes precipitate
Asphaltenes stay in solution as long as the oil’s composition, pressure and temperature favor it. Changes can push them out:
- Pressure drop. As oil is produced and pressure falls toward the point where gas comes out of solution, many oils become unstable. Asphaltene problems often appear near the wellbore and in tubing for this reason.
- Gas injection. Injecting carbon dioxide or hydrocarbon gas for enhanced oil recovery can destabilize asphaltenes.
- Mixing oils. Blending crudes from different sources can cause precipitation.
- Acid treatments. Contact with acid during acidizing can produce asphaltic sludge; see acid sludge.
Once precipitated, asphaltene particles can clump together and stick to surfaces.
Where asphaltenes cause problems
- In the reservoir near the well, where deposits reduce permeability, a form of formation damage.
- In tubing and chokes, restricting flow.
- In flowlines, separators and tanks, where they add to sludge.
- At oil-water interfaces, where asphaltenes stabilize tight emulsions that are hard to separate.
Asphaltene deposition is one of the main concerns of flow assurance.
How asphaltenes are managed
- Inhibitors and dispersants, injected continuously or by squeeze treatment, which keep asphaltene particles small and suspended so they do not deposit.
- Aromatic solvents, such as xylene, which dissolve deposits. They are effective but raise safety and environmental concerns; see BTEX.
- Mechanical removal, such as scraping tubing or cleaning equipment.
- Operating practices that avoid the pressure and mixing conditions at which a given oil becomes unstable, informed by laboratory stability testing.
Asphaltenes and paraffins
The two deposits are often confused and sometimes occur together, but they differ:
| Asphaltene | Paraffin | |
|---|---|---|
| What it is | Heavy, polar, aromatic molecules | Long-chain waxy hydrocarbons |
| Main trigger | Pressure drop and changes in oil composition | Cooling below the wax appearance temperature |
| Appearance | Dark, brittle to tar-like | Waxy, often lighter in color |
| Solvent response | Dissolves in aromatic solvents | Melts with heat; dissolves in many solvents |
Asphaltenes at Locus
Locus’s Avert is a solvent-free biosurfactant that Locus describes as remediating and inhibiting paraffin, asphaltene and mixed solids to restore flow in producing and injection wells.
Frequently asked questions
Mainly changes that make the oil a poorer solvent for asphaltenes: pressure falling during production, injection of carbon dioxide or light gas, mixing incompatible oils, or contact with acid.
They are related. Asphalt, or bitumen, is a heavy petroleum material that is rich in asphaltenes, but asphaltenes are a specific fraction defined by solubility, present in varying amounts in most crude oils.
With aromatic solvents, chemical dispersants or mechanical cleaning. Because deposits tend to return, operators often combine removal with continuous inhibitor treatment.
Sources
- asphaltene, SLB Energy Glossary
- Asphaltene deposition, SPE PetroWiki