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Remediating wax deposition
Wax precipitation may result in wax deposition in tubing, pipelines and other production equipment. This deposition can pose significant flow problems requiring remediation. This page discusses the problems created by wax deposition and available methods for remediating those issues.
Effect of wax deposition in pipes
Crystallization of waxes in crude oils leads to non-Newtonian flow characteristics, including very high yield stresses that are dependent on time and the shear and temperature histories of the fluid. This crystallization may cause three problems:
- High viscosity, which leads to pressure losses
- High-yield stress for restarting flow
- Deposition of wax crystals on surfaces
Wax precipitation-induced viscosity increases and wax deposition on pipes are the primary causes of high flowline pressure drops. In turn, these pressure losses lead to low flow rates that make conditions for wax deposition more favorable. In extreme cases, pumping pressure can exceed the limits of the system and stop flow entirely. A related problem is the high-yield stress for restarting flow. When oil is allowed to stand in a pipeline at temperatures below its pour point, a certain pressure is required to break the gel and resume flow. Again, this pressure may be higher than the pressure limits of the pumps and pipelines.
Methods of wax prevention and removal
Wax can deposit on surfaces in the production system and in the formation. Wax deposition can be prevented or removed by a number of different methods. These methods fall into three main categories:
Because wax precipitation is highly temperature dependent, thermal methods can be highly effective both for preventing and removing wax precipitation problems. Prevention methods include steam- and electrical-heat tracing of flowlines, in conjunction with thermal insulation. Thermal methods for removing wax deposition include:
- Hot oiling
- Hot watering
Hot water treatments cannot provide the solvency effects that hot oiling can, so surfactants are often added to aid in dispersion of wax in the water phase. Surfactants are discussed under chemical methods.
Hot oiling is one of the most popular methods of deposited wax removal. Wax is melted and dissolved by hot oil, which allows it to be circulated from the well and the surface producing system. Hot oil is normally pumped down the casing and up the tubing; however, in flowing wells, the oil may be circulated down the tubing and up the casing. There is evidence that hot oiling can cause permeability damage if melted wax enters the formation.
Higher molecular-weight waxes tend to deposit at the high-temperature bottom end of the well. Lower molecular-weight fractions deposit as the temperature decreases up the wellbore. The upper parts of the well receive the most heat during hot oiling. As the oil proceeds down the well, its temperature decreases and the carrying capacity for wax is diminished. Thus, sufficient oil must be used to dissolve and melt the wax at the necessary depths.
The types of chemicals available for paraffin treatment include:
- Wax crystal modifiers
Solvents can be used to treat deposition in production strings and also may be applied to remediate formation damage. Although chlorinated hydrocarbons are excellent solvents for waxes, they generally are not used because of safety and processing difficulties they create in the produced fluid. Hydrocarbon fluids consisting primarily of normal alkanes such as condensate and diesel oil can be used, provided the deposits have low asphaltene content. Aromatic solvents such as toluene and xylene are good solvents for both waxes and asphaltenes. Solvents are mostly used in large batch treatments.
Wax crystal modifiers act at the molecular level to reduce the tendency of wax molecules to network and form lattice structures within the oil. Wax crystal modifiers which are used to prevent wax deposition, reduce oil viscosity and lower the wax gel strength are only effective when used continuously. Since they work at the molecular level they are effective in concentrations of parts per million, as opposed to hot oil or solvents, which must be applied in large volumes. Wax crystal modifiers have a high-molecular-weight and as a result they have high pour points, so their use can be limited in cold climates.
Dispersants are a type of surfactants that helps disperse the wax crystals into the produced oil or water. This dispersing of the wax crystals into the produce oil or water helps prevents deposition of the wax and also have a positive effect on the viscosity and gel strength. Dispersants can help break up deposited wax into particles small enough to be carried in the oil stream. To prevent wax deposition dispersants must be used continuously. To remediate deposited wax, dispersants can be used continuously or in batch treatments. Dispersants generally have a very low pour point making their use suitable for cold climates. These chemicals are used in low concentrations and can be formulated in both aqueous and hydrocarbon solutions, making them relatively safe and inexpensive.
Surfactants are a general class of chemicals that are most often used to clean vessels, tanks, pipes, machinery or any place where wax may deposit. Surfactants or dispersants can also be used in combination with hot oil and water treatments.
Scrapers and cutters are used extensively to remove wax deposits from tubing because they can be economical and result in minimal formation damage. Scrapers may be attached to wireline units, or they may be attached to sucker rods to remove wax as the well is pumped. Deposits in surface pipelines can be removed by forcing soluble or insoluble pigs through the lines. Soluble pigs may be composed of naphthalene or microcrystalline wax. Insoluble pigs are made of plastic or hard rubber.
Another method of mechanical intervention to prevent deposition is the use of plastic or coated pipe. Low-friction surfaces make it more difficult for wax crystals to adhere to the pipe walls. Deposition will still occur if conditions are highly favorable for wax precipitation, and deposits will grow at the same rate as for other pipes once an initial layer of material has been laid down; therefore, the pipe and coating system must be capable of withstanding one of the other methods of wax removal.
- Misra, S., Baruah, S., and Singh, K. 1995. Paraffin Problems in Crude Oil Production And Transportation: A Review. SPE Prod & Oper 10 (1): 50-54. SPE-28181-PA. http://dx.doi.org/10.2118/28181-PA
- Allen, T.O. and Roberts, A.P. 1982. Production Operations, second edition, Vol. 2. Tulsa, Oklahoma: Oil and Gas Consultants International.
- Becker, J.R. 2000. Oilfield Paraffin Treatments: Hot Oil and Hot Water Compared to Crystal Modifiers. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, 1-4 October. SPE-63123-MS. http://dx.doi.org/10.2118/63123-MS
- Fan, Y. and Llave, F.M. 1996. Chemical Removal of Formation Damage From Paraffin Deposition Part I - Solubility and Dissolution Rate. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, USA, 14-15 February. SPE-31128-MS. http://dx.doi.org/10.2118/31128-MS
Noteworthy papers in OnePetro
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read
1. Paraffin Cleanout in a Single Subsea Flowline Using Xylene Bailey, J. Claire, Anadarko Petroleum Corp, Allenson, Stephan J., Nalco Co. 125131-PA SPE Journal Paper - 2009
2. Paraffin Cleanout in a Single Subsea Flowline Environment: Glycol to Blame? Bailey, J. Claire, Anadarko Petroleum Corp. Allenson, Stephan John, Nalco Company 19566-MS OTC Conference Paper - 2008
Jamaluddin, Abul. 2013. Flow Assurance – Managing Flow Dynamics and Production Chemistry. https://webevents.spe.org/products/flow-assurance-managing-flow-dynamics-and-production-chemistry-2
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