Challenges and solutions to asphaltene and wax deposition

Major economic problem for production operations

Kosta J. Leontaritis, President
AsphWax, Inc.
Kosta Oil Field Technologies, Inc.

PART I: The following is part I of a three-part series on asphaltene and wax deposition. Parts II and III will appear in following issues.

The seriousness of asphaltene and wax deposition and the economic impact it has in oil recovery cannot be overstated. Many investigators have reported such field problems and case histories. This paper presents a coalesced description of the occurrence of asphaltene and wax problems offshore that could assist in oil recovery operations around the globe.

The accompanying asphaltene and wax world map (A&W Map) summarizes the occurrence of asphaltene and wax problems, and has been a very useful tool for the author in his worldwide consulting operations. It is intended to be a qualitative correlation tool for geologists, geophysicists, geochemists, and other professionals. By providing insight into potential asphaltene and wax problems, oil companies can use it to reduce the cost of exploration and production.

There are two contrasting, striking, and salient features in the A&W Map:

Oil reservoirs in the Far East contain paraffinic oils.
Oil reservoirs in and around the Mediterranean Sea contain asphaltenic oils.

The map colors indicate the relative occurrence of waxy or asphaltenic oils. For example, in Indonesia, both oil types are found, but the occurrence of waxy oils is much higher. In contrast, waxy oils (Bombay crude) dominate India.

Regional differences

Other regions have their own pecularities:

West Africa & South America: Waxy oils are found in west central Africa. Angola, Gabon, and Nigeria are examples of areas producing primarily waxy oils. For the time being, offshore Nigeria is indicated as a mixture of oil types. Some observers, including the author, anticipate that offshore Nigeria may contain asphaltenic oils similar to those found in Brazil's Campos and Santos Basins. Offshore Brazil is indicated on the A&W Map to contain both asphaltenic and waxy oils.

Colombia seems to contain both oil types, although asphaltene problems are more dominant. Venezuela contains oils that have asphaltene behavior similar to those in the Mediterranean. In Venezuela, considerable asphaltene-induced organic deposition problems occur. Venezuelan reservoirs are generally prolific, so the asphaltene problems are somewhat balanced economically by the high production rates.

Waxy oils dominate onshore and offshore Argentina. Hence, transportation issues are important in this area, especially considering the cold ambient conditions.

North Sea: Both asphaltene and wax problems have been reported in the North Sea. It appears that wax deposition is more common.
North America and Russia: The United States (including Alaska), Canada, and Russia seem to have a little of both type of oils, although Russia is somewhat more asphaltenic. Russian researchers have published much asphaltene-related research, as have Canadian researchers. US scientists have developed many fundamental concepts and theories regarding asphaltene and paraffin problems.

The Gulf of Mexico, US side, is another wax-dominated area. The author is aware of two unconfirmed asphaltenic cases.

The term unconfirmed is used here to leave open the possibility of misdiagnosed problems. Misdiagnosing a field problem is one reason for the A&W Map being in error. The author has found several misdiagnoses around the world. A frequent reason for misdiagnosis is the use of asphaltene and wax analysis of field deposits to determine the type of problem.

This type of analysis is insufficient to provide diagnosis of the problem because asphaltenes are found in deposits of both oil types. Only a specialized expert has any chance of determining the truth with these types of data alone. The asphaltene deposition envelope (ADE) and wax deposition envelope (WDE) are the most important and simplest tools for diagnosing asphaltene and wax problems.

Australia: Waxy oils dominate Australia. Unconfirmed asphaltene concerns are being raised in the coastal Western Australia region. This area is currently in a development boom.
Central Asia: The Balkan area is considered primarily asphaltenic. Romanian engineers report asphaltene problems although the literature has been sparse. The author does not have first-hand experience in the Caspian Sea but there are several indications that both problems are present with asphaltenes being predominant. The author is aware of two major reservoirs in Kazakhstan with paraffin problems.
Middle East: Finally, the Middle East seems to have a little bit of both with the asphaltenes being more dominant. Egypt and the Suez have reported both asphaltene and paraffin problems. In Kuwait, the Marrat reservoir suffers from asphaltene problems. In Abu Dhabi, the Zacum reservoir has asphaltene problems. In Bahrain, wax deposition problems in pipelines have been indicated. Aramco has also reported some asphaltene problems.

Iran is another area where asphaltene problems seem to dominate oil production. Several papers have been written regarding asphaltene problems in Iran. Overall, the Middle East is more of an asphaltenic than a paraffinic region.

Depositional differences

In general, asphaltene problems are more difficult to solve than paraffin problems. Asphaltene problems occur mostly (but not exclusively) inside the rock formation and downhole. Paraffin problems usually occur in the upper part of production tubing and in surface production facilities. In subsea satellite production systems with long offsets, paraffin deposition can be devastating. On balance asphaltene problems cost more to solve than paraffin problems.

Paraffins can be easily upgraded to gasoline and fuel in the refinery, the two most valuable components of crude, whereas asphaltenes are generally disposed of as low-valued petroleum coke. Paraffinic oils usually have a higher dollar value than asphaltenic oils (sometimes as much as $5-10/bbl). The Far East oils generally cause serious transportation problems due to wax deposition and oil congealing. These problems can be solved with reasonable cost.

Colloidal oil model

Petroleum is a colloidal sol, a colloidal solution or suspension of asphaltene micelles or particles in liquid hydrocarbons (Figure 2). Although the petroleum model depicted is widely accepted, not all investigators believe in the colloidal nature of petroleum. The power of the natural dispersants (resins) is easily demonstrated by the ease with which asphaltenes are dispersed in normal alkanes such as n-pentane, n-hexane, and n-heptane via addition of small amounts of resins or similar dispersants.

The paraffins are primarily in molecular state although a small amount, probably with high carbon numbers (C45-C50), may be associated with the micelles through van der Waals or London dispersion interactions with the resin paraffinic ends. The resin-asphaltene interactions are most likely polar, hydrogen bond, or associative type. The resin-oil interactions are also van der Waals or London dispersion types.

The presence and abundance of asphaltenes and high carbon number paraffins usually, but not always, earmarks the potential for asphaltene and/or paraffin deposition during oil recovery. The relative abundance of resins and aromatics lessens the propensity of asphaltenes and paraffins to drop out of solution during petroleum production.

Hence, one has to take into account the entire petroleum composition, not only the presence of asphaltenes and paraffins, but also the relative abundance of resins and aromatics.

If there are no asphaltenes and high carbon number paraffins present in a crude oil, no asphaltene and/or paraffin deposition should be expected. Their presence, however, does not necessarily imply problematic organic deposition. The detailed analysis of petroleum will be addressed detail in later articles.

Tracking origins

Asphaltenes and high carbon number paraffins usually hold the secret of the oil's origin. IFP researchers have shown that asphalt enes contain very similar macromolecular moieties with kerogen. Since asphaltenes migrate with the rest of the oil from source-rocks to reservoirs, they carry with them the biomarkers that identify the origin of the oil in a given reservoir.

Asphaltene and paraffin phase behavior is also indicative of the oil's origin. Asphaltene flocculation points, cloud points, and pour points are usually very indicative of the oil's origin. Often, oils sampled from different zones of the same well during a drill stem test (DST) have widely differing phase behavior characteristics thus indicating either different origin or different in-situ maturation.

Reservoir tar-mats

Reservoir tar-mats can also have a significant impact on oil recovery. A tar-mat is a heavy oil zone or carpet enriched in asphaltenes and resins situated at the reservoir water-oil interface. The asphaltene content and API gravity vertical distribution in a reservoir containing a tar-mat follows the trend shown in Figure 3. Just above the water-oil interface, the asphaltene-resin content increases exponentially, revealing the presence of the tar-mat. The oil API gravity shows an equivalent trend. The presence and extent of a tar-mat in a given reservoir is very important in the production facility design and oil recovery mechanism.

The location of producing well perforations and water injection are especially affected by the tar-mat. Water injection below a tar-mat is almost never a good idea because of the high probability of channeling. Hence, engineers designing a water injection project in a reservoir containing a tar-mat must decide the best depth to inject water. If the water injection zones are perforated too high, some good oil will be bypassed or lost.

The evolution of oil from kerogen, following the kerogen-asphaltene-resin-oil sequence, is significant in explaining tar-mats in certain reservoirs. If heavy asphaltene-containing oil migrated into a reservoir containing light paraffinic oil, the result would be the formation of a tar-mat because of asphaltene flocculation at the water-oil interface where the two oils mix.

The above evolution theory appears to be in contrast with the view held by many refining researchers that resins are oxidation products of oil and asphaltenes are oxidation products of resins. The author believes that the "asphaltenic oil migration theory" explains the formation of tar-mats much more soundly than the "asphaltene in-situ generation theory."

References

Pelet, R., Behar, F., and Monin, J. C.: "Resins and asphaltenes in the generation and migration of petroleum," Advances in Organic Geochemistry 1985, Organic Geochemistry vol. 10.

Author's Note: The information contained in the Asphaltene and Wax World Map reflects more than 20 years of effort by the author. It is probably neither complete nor error-free but is the best that could be done with the available data. The map should be viewed, as a tool owned jointly by all in the oil industry. The author and others, including published work in professional journals and magazines, supplied information in the map. Future authors are welcome to add to, correct, and distribute the map freely. The author welcomes comments, additions or corrections from readers.