What Is Paraffin?
Crude oils contain three main groups of compounds: saturated hydrocarbons or paraffins, aromatic hydrocarbons, and resins and asphaltenes. Paraffins are normal (straight chain) or branched alkanes of relatively high molecular weight and are represented by the general formula CnH2n+2. Paraffin refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range (may have more than 80 carbon atoms); they are found in the solid state at room temperature and begin to enter the liquid phase past approximately 37 °C (99 °F). And a typical melting point is between about 115°F and 154°F (46 and 68°C) having a density of around 0.9 g/cm3. The melting point of the paraffin increases as the size of the molecule increase. Methane (CH4) is the simplest paraffin and the most common compound in petroleum reservoir fluids. Heavier members of the series, such as octane, C8H18, and mineral oil appear as liquids at room temperature. The solid forms of paraffin, called paraffin wax, are from the heaviest molecules from C20H42 to C40H82. It is insoluble in water, but soluble in ether, benzene, and certain esters. This class of hydrocarbons is essentially inert to chemical reactions and is, therefore, resistant to attack by bases and acids.
What Is the Problem?
Paraffin can be deposited in the well bore flow line, pore walls, other surface equipment such as separators. For paraffin deposition to be a significant problem, the paraffin must deposit on the pore walls or the tubing surface. If the paraffin remains entrained in the crude oil, it usually offers few production problems. The most serious damage occurs on the face of sand and a short distance back in the pore. Many wells have been severely damaged or totally plugged by the deposit. Paraffin buildup in the tubing can lead to overload of rod-pumps and cause rod breaks. Finally, production decreases in wells producing paraffinic oil may be caused by paraffin deposition. Typical problems caused by paraffin deposition include:
- Increase the viscosity and gelling of the oil
- Reduction or plugging of pipework, blocking flow
- Increased surface roughness on the pipe wall which can increase back pressure
- Interference with valve and instrumentation operation
- Interference with valve and instrumentation operation
- Costly and technically challenging removal especially in deep water
- Disposal problem due to deposited paraffin.
Why Paraffin Is Deposited?
The primary cause of wax or paraffin deposition is simply a loss in solubility in the crude oil. This loss of solubility is usually a result of changes in temperature, pressure, or composition of the crude oil as a result of loss of dissolved gases. Under the condition of pressure, temperature and oil composition occurring in the reservoir, the paraffin remain in the solution. But crude rises to the surface, the solubility of paraffin may be exceeded. Since paraffin characteristics content vary from reservoir to reservoir, production problems and their solutions also vary.
Paraffins that have the highest melting point and molecular weight are usually the first to separate from solution, with lower molecular- weight paraffins separating as the temperature decreases further. For example, a C60 alkane with a melting point of about 215°F will deposit at a much higher temperature than a C20 alkane with a melting point of 98̊ F. Deposits vary in consistency from soft accumulations to hard, brittle deposits. Usually the deposits are firmer and harder as the molecular weight of the paraffin deposits increases.
Paraffin can precipitate from crudes when equilibrium conditions change s lightly, causing a loss of solubility of the wax in the crude. The point of deposition in a well's producing system is normally determined by how close the crude is to its solubility saturation point and the amount of wax in the crude. Loss of wax solubility, however, is not the only factor in deposition. Wax crystals normally have a needle-like shape, and if they remain as single crystals, they tend to disperse in the crude instead of depositing on a surface. A nucleating material is usually present that gathers wax crystals into a bushy particle that is much larger than single crystals; these agglomerates may then separate from the crude and form deposits in the well’s producing system. Asphaltenes are frequently the nucleating material that causes paraffin crystals to agglomerate. Other nucleating materials are formation fines and corrosion products.
The ability of the crude oil to hold the paraffin in solution is generally quantified with two indicators: a pour point and a cloud point. The procedure for measuring the pour point and cloud point may be found in ASTM manuals (D2500-66 for cloud points and D97-66 for pour points). The cloud point is defined as the temperature at which paraffins begin to come out of solution and a clear solution of hydrocarbons turns cloudy. Obviously, it is difficult to measure the cloud point for dark crude oil because cloudiness is not visible. In such cases, the presence of paraffin crystals may have to be detected with a polarizing light microscope. The pour point is defined as the temperature at which the crude oil no longer flows from its container. As the temperature is lowered, wax crystals form an interlocking network that supports the hydrocarbon liquid within it. This network of paraffin crystals is quite shear sensitive and loose when first formed but can harden and become extremely rigid as fluid is lost from it. Pour points are relatively easy to measure in the field and provide a good indication of conditions under which large quantities of paraffin will fall out of solution in crude oils.
The most common cause of loss of solubility of the paraffin in the crude oil is a decrease in temperature, which may occur for a variety of reasons: cooling produced by the crude oil and associated gas expanding through the perforations, gas expansion while lifting fluids to the surface, radiation of heat from the tubing to the surrounding formation induced by intrusion of water into or around the wellbore, and loss of lighter constituents in the crude oil because of vaporization. Several other possible reasons for a decrease in temperature can be envisioned. In offshore installations, for example, paraffin problems are usually associated with the rapid change in temperature as the crude oil from the wellbore enters subsea pipelines that are immersed in seawater at 4°C. Large volumes of paraffins can be deposited on the surfaces of the pipelines, which require periodic pigging. At some point in the system, the temperature drops below the wax melting point, and wax may start to accumulate on production equipment.
Pressure itself has little or no influence on the solubility of paraffin in crude oil. However, it does have a significant impact on the composition of the crude oil. Reductions in pressure usually lead to loss of volatiles from the crude oil and can induce the precipitation of paraffins. This is the primary reason why paraffin problems are more common in the more mature regions of the world. As the reservoir pressure is depleted and the lighter components of the crude oil are produced in preference to the heavier fractions, the likelihood of paraffin precipitation is significantly increased.
The injection of fluids such as stimulation fluids or injection water into the wellbore can often induce paraffin deposition problems. This is particularly true if the surface temperature is significantly colder than the reservoir temperature. Also the injection of cold fracturing or acidizing fluids into the oil reservoir can cause significant cooling of the crude and formation. If the crude is cooled below its cloud point, paraffin can precipitate in flow channel. Heating the fluids above the formation temperature will prevent the paraffin from deposition. Additional paraffin deposits occur in the downhole due to hot oiling for paraffin. Additional of hot oil to bottomhole areas can leave paraffin deposits when hot oil cools and it can no longer hold paraffin in solution at bottomhole conditions.
Certain signs can indicate the start of paraffin deposition. Congealed paraffin can be separated from oil by centrifuging. A change in crude appearance, such as cloudiness, indicates that paraffin is coming out of solution. Accumulation of paraffin in stock tanks usually indicates that paraffin deposition may be expected soon in the flowline, tubing, and possibly later in the wellbore.
Paraffins that have the highest melting point and molecular weight are usually the first to separate from solution, with lower molecular- weight paraffins separating as the temperature decreases further. For example, a C60 alkane with a melting point of about 215°F will deposit at a much higher temperature than a C20 alkane with a melting point of 98̊ F. Deposits vary in consistency from soft accumulations to hard, brittle deposits. Usually the deposits are firmer and harder as the molecular weight of the paraffin deposits increases.
Paraffin can precipitate from crudes when equilibrium conditions change s lightly, causing a loss of solubility of the wax in the crude. The point of deposition in a well's producing system is normally determined by how close the crude is to its solubility saturation point and the amount of wax in the crude. Loss of wax solubility, however, is not the only factor in deposition. Wax crystals normally have a needle-like shape, and if they remain as single crystals, they tend to disperse in the crude instead of depositing on a surface. A nucleating material is usually present that gathers wax crystals into a bushy particle that is much larger than single crystals; these agglomerates may then separate from the crude and form deposits in the well’s producing system. Asphaltenes are frequently the nucleating material that causes paraffin crystals to agglomerate. Other nucleating materials are formation fines and corrosion products.
The ability of the crude oil to hold the paraffin in solution is generally quantified with two indicators: a pour point and a cloud point. The procedure for measuring the pour point and cloud point may be found in ASTM manuals (D2500-66 for cloud points and D97-66 for pour points). The cloud point is defined as the temperature at which paraffins begin to come out of solution and a clear solution of hydrocarbons turns cloudy. Obviously, it is difficult to measure the cloud point for dark crude oil because cloudiness is not visible. In such cases, the presence of paraffin crystals may have to be detected with a polarizing light microscope. The pour point is defined as the temperature at which the crude oil no longer flows from its container. As the temperature is lowered, wax crystals form an interlocking network that supports the hydrocarbon liquid within it. This network of paraffin crystals is quite shear sensitive and loose when first formed but can harden and become extremely rigid as fluid is lost from it. Pour points are relatively easy to measure in the field and provide a good indication of conditions under which large quantities of paraffin will fall out of solution in crude oils.
The most common cause of loss of solubility of the paraffin in the crude oil is a decrease in temperature, which may occur for a variety of reasons: cooling produced by the crude oil and associated gas expanding through the perforations, gas expansion while lifting fluids to the surface, radiation of heat from the tubing to the surrounding formation induced by intrusion of water into or around the wellbore, and loss of lighter constituents in the crude oil because of vaporization. Several other possible reasons for a decrease in temperature can be envisioned. In offshore installations, for example, paraffin problems are usually associated with the rapid change in temperature as the crude oil from the wellbore enters subsea pipelines that are immersed in seawater at 4°C. Large volumes of paraffins can be deposited on the surfaces of the pipelines, which require periodic pigging. At some point in the system, the temperature drops below the wax melting point, and wax may start to accumulate on production equipment.
Pressure itself has little or no influence on the solubility of paraffin in crude oil. However, it does have a significant impact on the composition of the crude oil. Reductions in pressure usually lead to loss of volatiles from the crude oil and can induce the precipitation of paraffins. This is the primary reason why paraffin problems are more common in the more mature regions of the world. As the reservoir pressure is depleted and the lighter components of the crude oil are produced in preference to the heavier fractions, the likelihood of paraffin precipitation is significantly increased.
The injection of fluids such as stimulation fluids or injection water into the wellbore can often induce paraffin deposition problems. This is particularly true if the surface temperature is significantly colder than the reservoir temperature. Also the injection of cold fracturing or acidizing fluids into the oil reservoir can cause significant cooling of the crude and formation. If the crude is cooled below its cloud point, paraffin can precipitate in flow channel. Heating the fluids above the formation temperature will prevent the paraffin from deposition. Additional paraffin deposits occur in the downhole due to hot oiling for paraffin. Additional of hot oil to bottomhole areas can leave paraffin deposits when hot oil cools and it can no longer hold paraffin in solution at bottomhole conditions.
Certain signs can indicate the start of paraffin deposition. Congealed paraffin can be separated from oil by centrifuging. A change in crude appearance, such as cloudiness, indicates that paraffin is coming out of solution. Accumulation of paraffin in stock tanks usually indicates that paraffin deposition may be expected soon in the flowline, tubing, and possibly later in the wellbore.
How To Remove Paraffin Deposit?
Paraffin accumulations are removed by methods that can be broadly placed into three categories: (1) mechanical removal of paraffin deposits, (2) the use of solvents to remove paraffin deposits (chemical treatment), and (3) the use of heat to melt and remove the wax (Thermal treatment). However, the use of only one method to remove paraffin may not be efficient and combination of two or all three methods is recommended for effective removal.
What Is Mechanical Removal?
There are several mechanical methods for removing deposited paraffin from tubing, flow lines and pipelines. These include scraper, cutter and also the pipeline pigging method.
However, scraping can cause perforation plugging if it is necessary to circulate scraped paraffin down the tubing and out of the casing. If frequent cleanout is required, mechanical cleaning becomes more costly, especially when the value of lost production is added to cleanout costs.
The major advantage of mechanically removing paraffin is that positive cleaning is assured. However, some disadvantages are:
However, scraping can cause perforation plugging if it is necessary to circulate scraped paraffin down the tubing and out of the casing. If frequent cleanout is required, mechanical cleaning becomes more costly, especially when the value of lost production is added to cleanout costs.
The major advantage of mechanically removing paraffin is that positive cleaning is assured. However, some disadvantages are:
- Application is limited due to time and equipment involved
- Treatment may be more expensive than other methods due to personal, time and special equipment required and
- Danger of fishing for tools that may be lost in the hole during the cleaning operation.
What Is Chemical Treatment?
There are three types of chemical that are used to treat paraffin problem;
Solvents are added to restore the solvent properties to the crude oil which has lost those properties due to escape of dissolved gases or reduction of temperature. The use of solvents is relatively common, but care must be observed in solvent selection.
Selection of a solvent for any application should be based on its cost effectiveness in dissolving a specific organic deposit. Solvent application must be adapted to fit well conditions. One procedure is to circulate solvent down the annulus and back through the tubing.
Soaking or surging of the solvent over a period of time will usually dissolve the maximum amount of paraffin per gallon of solvent. If the formation is partially plugged with wax, squeezing solvent and surfactant into the formation and soaking for 24 to 72 hours is very effective. Severe paraffin buildup in the tubing of rod-pumping wells often makes rod removal very difficult. In these instances, pumping a solvent down the tubing softens paraffin and facilitates rod pulling.
Dispersant and Pour-point dispersant
Dispersants are compound which break the paraffin up into finer particles and dispersant it more thoroughly into the crude. These chemicals will cause the molecule of the paraffin deposit to repel each other as well as metal surface. Dispersants are generally used in elimination tank-bottom build up and fallout in surface. Dispersants can be used for both inhibitory effect and remedial benefit. If it used for inhibitory effect, the dispersants must be added into the system before the deposition is occurred.
Pour-point dispersant is for lowering the pour point temperature of the crude oil and preventing paraffin crystallization. This chemical should be added to the system before the paraffin deposition is occured.
- Solvents
- Dispersant and Pour-point dispersant
- Paraffin Inhibitor
Solvents are added to restore the solvent properties to the crude oil which has lost those properties due to escape of dissolved gases or reduction of temperature. The use of solvents is relatively common, but care must be observed in solvent selection.
Selection of a solvent for any application should be based on its cost effectiveness in dissolving a specific organic deposit. Solvent application must be adapted to fit well conditions. One procedure is to circulate solvent down the annulus and back through the tubing.
Soaking or surging of the solvent over a period of time will usually dissolve the maximum amount of paraffin per gallon of solvent. If the formation is partially plugged with wax, squeezing solvent and surfactant into the formation and soaking for 24 to 72 hours is very effective. Severe paraffin buildup in the tubing of rod-pumping wells often makes rod removal very difficult. In these instances, pumping a solvent down the tubing softens paraffin and facilitates rod pulling.
Dispersant and Pour-point dispersant
Dispersants are compound which break the paraffin up into finer particles and dispersant it more thoroughly into the crude. These chemicals will cause the molecule of the paraffin deposit to repel each other as well as metal surface. Dispersants are generally used in elimination tank-bottom build up and fallout in surface. Dispersants can be used for both inhibitory effect and remedial benefit. If it used for inhibitory effect, the dispersants must be added into the system before the deposition is occurred.
Pour-point dispersant is for lowering the pour point temperature of the crude oil and preventing paraffin crystallization. This chemical should be added to the system before the paraffin deposition is occured.
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Paraffin Inhibitor
Inhibitors (Paraffin Crystal Modifiers) are usually polymeric materials which prevent paraffin deposition by disrupting nucleation, crystallizing or modifying the crystals. Paraffin comes out of solution as single crystals, which tend to agglomerate around a nucleus to form relatively large particles. Removal of the nucleating agent will prevent agglomeration of paraffin crystals and prevent deposition on metal surfaces. Polymers have been used successfully as crystal modifiers in some areas; their use should expand as more effective polymers are developed. The key properties which determine how effective these treatment chemicals will be, depending on the solubility of chemical in the oil or condensate and structure of chemical. Inhibitors also affect the crude oil viscosity, pour point and pumpability. For optimum effectiveness, inhibitors must be introduced into the oil before the crude oil has reached its cloud point. To properly inhibit paraffin deposition, the chemicals should be used on continuous injection basic or squeezed into the formation resulting in the continuous feedback of chemicals. |
Schematic presentation of the influence of wax inhibitor on the wax deposition (Source: Al-Yaari, 2011) Copyright 2011 by the Society of Petroleum Engineers.
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What Is Thermal Treatment?
This method normally consists of addition of external heat to the system. Procedures such as steaming the flowlines, installing bottomhole heaters, and circulating of hot oil or hot water are examples of the application of heat in an effort to melt or increase the solubility of the deposit. Any application of heat to remove paraffin should be carried out before large deposits have accumulated. Effective removal of paraffin from tubing that is almost plugged is very difficult since loosened or partially-melted paraffin may bridge in the tubing. If large deposits have accumulated, mechanical removal of some of the paraffin may be advisable prior to heat application. The use of hot oil at regular intervals has proved to be effective in wells in which paraffin buildup rates are known.
References
- Larry W. Lake, ‘Petroleum Engineering Handbook’ Joe Dunn Clegg, Editor, Volume IV “Production Operation Engineering”, by SPE, 2007.
- Malcolm A. Kelland, “Production Chemical for the Oil and Gas Industry”, by CRC Press, 2014.
- Thomas O. Allen and Alan P. Roberts, “Production Operation” Volume 2, Published by OGCI (Oil & Gas Consultants International, Inc.), 1989.
- https://www.slb.com/products-and-services/innovating-in-oil-and-gas/well-production/production-chemicals-and-services/paraffin-management#:~:text=paraffin%20deposition%20inhibitors%E2%80%94high%2Dmolecular,restore%20production%20to%20optimal%20levels.
- The SLB energy glossary.
