Paraffins and Asphaltenes

Basic Properties Paraffins are organic deposits caused by changing wellbore conditions that upset the chemical equilibrium so that various materials in the crude oil precipitate out of solution. These materials separate and deposit a waxy material called paraffin (in some cases asphaltene). Figure 1 illustrates the development of paraffin in a flowing well. FIGURE 1 Paraffin, usually deposited in the wellbore, extends to the top of the well and into flowlines from a depth where the formation temperature is approximately the same as that at which the oil became saturated with the separated wax. Asphaltene deposits, usually hard and brittle, occur at the bottom of the wellbore adjacent to the producing formation wall. The buildup of paraffin or asphaltene can completely plug up the oil flow in a producing well if the problem is ignored or left untreated. Properties Paraffins are either normal, branched, or cyclic alkanes with the chemical formula CnH2n+2. Paraffin deposition is difficult to inhibit because these hydrocarbons are generally very inert and therefore resistant to attack by acids, bases, and oxidizing agents. Crude paraffin deposits are made up of individual alkanes ranging from a carbon chain length of 20 with a melting point of 98° F to a chain length of 60 with a melting point of about 215° F. Crude wax or paraffin deposits consist of very small crystals that tend to agglomerate and form granular wax particles about the size of ordinary table salt grains. Deposited paraffin may also contain gums, resins, asphaltic material, crude oil, sand and silt, corrosion products, scale, and often water. The primary cause of wax separating from crude oil is loss of solubility, often resulting from changing environmental conditions that disturb solution equilibrium. Factors affecting this equilibrium include temperature and pressure changes, evaporation, and loss of dissolved gases. Those paraffins having the highest melting point and molecular weight are the first to separate from solution. The quantity of lower melting point waxes with smaller molecular weights to separate from solution depends on how much the oil's equilibrium changes prior to deposition - that is, the solubility of different paraffin waxes in specific crude oil at a given temperature decreases with an increase in melting point and molecular weight. Asphaltenes are black carbonaceous petroleum components characterized by relatively high molecular weights and insolubility in light paraffinic hydrocarbon solvents, such as pentane and petroleum ether. The asphaltene fraction of petroleum is made up of polycyclic, condensed aromatic rings with few side chains. These compounds are considered polar materials due to the presence of sulfur, nitrogen, oxygen, and complex metals. Asphaltenes occur in many crude oils in the form of colloidally suspended solid particles. Asphaltene precipitation takes place when crude oil loses its ability to disperse the particles. Asphaltene stability is dependent on crude oil composition and the nature of reservoir rock surface. The balance that holds them in a stable suspension is dependent on the same conditions that cause paraffin to separate from crude. Pour point and cloud point measurements refer to crude oil's ability to hold paraffin in solution. Cloud point is the temperature at which paraffin begins to come out of solution. This point is visible in clear solutions as a slight cloudiness. Since the cloud point of a dark crude oil is not visible, instrumental methods may be used to obtain crude oil cloud points. If crude oil is slowly cooled below the cloud point without agitation, small wax crystals gradually form an interlocking network which inhibits flow. The pour point is reached when oil ceases to flow. Factors Affecting Deposition of Paraffins Temperature: When crude oil cools, it loses its solubility, and can no longer keep the particles in suspension. Paraffin solidifies, its particles settle and accumulate into waxy deposits. Pressure: Pressure holds dissolved gases and volatile constituents in solution and helps maintain the fluid at formation temperature. Obviously, it is impossible to produce oil without having a considerable pressure drop; two factors, then, work simultaneously. As pressure drops, the temperature drops, because gases evolve and expand, and heavier components settle out, forming paraffin. Loss of Volatile Constituents: Loss of the lighter constituents of the crude definitely reduces the quantity of paraffin the oil can hold in solution. The reduction in the volume of oil results in less solvent available to dissolve the same amount of wax. Suspended Particulate Material: Evidence shows that as paraffin begins to separate when temperature drops, formation fines such as sand and silt speed this process. Co-produced Water: Although water has no direct effect on wax solubility in oil, it may reduce acute paraffin problems in wells that began producing it in appreciable amounts. One possible explanation is that steel tubing will water-wet, thus reducing the tendency for paraffin to deposit. Conditions Favoring Paraffin Deposition: Even though wax may separate from crude oil, the produced paraffin does not necessarily deposit on the tubular goods and other equipment. It seems logical, in fact, for the wax to remain suspended in the crude oil itself. This ideal situation often exists, and in wells producing a very waxy crude, few paraffin problems occur. Some factors that cause paraffin accumulation in a well are alternate coating of the pipe and draining of the oil. When a surface is intermittently coated with oil, the thin film left on the pipe surface is too thin and its movement too slow to carry away wax particles.presence of only an oil film contact with the pipe while the well is flowing.contact of the oil with an unusually cold surface, such as in oil production through aquifers; this can cause paraffin crystals to grow directly on the pipe wall. (It is estimated that heat loss by conduction from a pipe in contact with water is approximately eight times greater than that which would occur in contact with air or dry earth.)rough pipe surfaces, which often provide excellent sites for paraffin deposition.electrical charges on various materials in the crude oil, which tend to promote migration of the separated waxes to the pipe walls. Paraffins Removal and Control Techniques There are basically three methods for removing and/or controlling paraffin deposition: mechanical, thermal, and chemical. Mechanical Methods: Several mechanical techniques remove paraffin deposition in tubing, flowlines, and pipelines. These include rod scraperswireline scrapersflowline scrapersballs, pigs, and/or soluble plugsfree-floating pistonshollow rods for circulating hot fluids The major advantage of using mechanical methods is that positive cleaning is assured. There are some disadvantages, however. Application is rather limited because of the time, expense, personnel, and special equipment involved. If tools are lost in the hole, a fishing job may be necessary. One preventive mechanical method is the use of smooth-surface pipe. The internal surface of pipe can be made smooth by plastic coating or removal of mill scale. Smooth pipe surfaces do not prevent paraffin deposition, but in most cases they slow the process. There are three widely used plastic coatings for pipe: phenol-formaldehyde, epoxy phenolic, and polyurethane. Phenol-formaldehyde has been in use longest, and is the most widely used coating in oilfield tubulars. It is a highly crosslinked polymer, with excellent resistance to temperature, chemicals, and infusion by small molecules (H2O, CH4, H2S). Its surface is extremely smooth and has a high gloss. It is a brittle plastic which becomes extremely rough when abraded by sand.Epoxy phenolic is the second most widely used material in oilfield tubulars. Less resistant to temperature, chemicals, and infusion than phenol-formaldehyde, its surface is almost as smooth, but it is less brittle and does not deform as badly when abraded by sand.Polyurethane is a relative newcomer, and the least crosslinked of the three. It has lower temperature, chemical, and infusion resistance than the other two, and its surface is not quite as smooth as phenol-formaldehyde. Polyurethanes are quite flexible, and although (like the other two materials) its film thickness is reduced by sand abrasion, the plastic is only slightly deformed and maintains most of its smoothness. Plastic-coated pipe either works extremely well or is a dismal failure. Numerous examples of both cases can be found in oil-field operations. Given the descriptions above, it is apparent why particular plastics may perform well or not, depending on their application and the experience of the particular oil operator. In areas where the oil stream contains no sand or other abrasive solids, plastics effectively prevent paraffin buildup. When brittle plastics (such as phenol-formaldehyde) are used in wells containing abrasive materials, the plastic deforms badly, leaving a rough, abraded surface which only invites severe paraffin deposits. There are three important factors regarding the use of plastic coatings in the prevention/reduction of paraffin deposition: Paraffinic plastics such as polytetrafluoroethylene, polyethylene, and polypropylene, no matter how smooth, do not reduce paraffin buildup. In fact, because they are themselves, they cause paraffin buildup by either hydrogen bonding or a phenomenon closely resembling cocrystallization.Smooth, nonparaffinic plastics reduce paraffin deposits only as long as they remain smooth. Wirelines, fishing tools, piano wire equipment, and other workover or measuring devices destroy the effectiveness of the plastic coating by damaging the smooth surface.All metal surfaces(nipples, valves, etc.) must be coated with plastic. One uncoated nipple can effectively choke the well. Thermal Methods: All thermal techniques use heat to remove the paraffin. Methods in current use include bottomhole heatershot oil, a common method which melts the waxy accumulation; lease crude oil is run through heat exchangers and pumped into the well at temperatures greater than 300 °Fsteam, which melts the paraffin so that it can be flowed with the oil from the wellheat-liberating chemicals(such as magnesium bars) followed by HCl; these produce chemical reactions which release heat, thereby melting the paraffin These approaches ensure paraffin removal. In order to prevent further deposition, the working temperature must be maintained above the melting point of the wax. Thermal techniques have some disadvantages. Bottomhole heaters require special equipment and consume additional power. Hot fluids pose a danger to personnel, usually require the use of pump trucks, and sometimes dissolve the paraffin only to redeposit it deeper than before. Chemical Methods: Chemical methods for paraffin control are separated into two distinct categories: solvents and inhibitors. Solvents: The best solvents for paraffin removal are too toxic, hazardous to handle, or damaging to refinery catalysts. Two such chemicals are carbon disulfide (CS2) and carbon tetrachloride (CCl4). Aromatic hydrocarbons, such as xylene and toluene, are acceptable solvents and dissolve both wax and asphaltenes; they are suitable for crude paraffin deposits. Solubility of paraffin in xylene is shown in Figure 1. FIGURE 1 Inhibitors: A large number of paraffin inhibitors are also available. Explanations of their effectiveness vary widely. Some are said to be dispersants; others, wax crystal modifiers; and still others claim to alter the surface conditions of equipment. Most paraffin inhibitors also contain paraffin solvent as the carrier. Surfactants: Of the vast number of paraffin chemicals available, wetting agents and dispersants constitute the highest percentage. Wetting agents are designed to work much like plastic pipe: the chemical reacts with produced water or by itself and forms an aqueous film on the pipe surface. This surface film retards paraffin deposition by covering the sites where paraffin can adhere. However, it is very difficult to get the pipe surfaces water-wet and keep them that way. The theory behind the dispersant approach is that certain chemicals tend to cause the crude paraffin molecules to repel each other as well as the metal surface.Both wetting agents and dispersants have seen some success. The most serious drawback to their use is that no supplier can be absolutely certain of their effectiveness. The operator has to decide whether to continue a particular paraffin removal program or to run the trial and error method with all available chemicals and hope to find one that is satisfactory.Crystal Modifiers: Generally, crystal modifiers have many of the same drawbacks as surfactants. Their effectiveness appears to be somewhat unpredictable. With the proper placement technique, however, these chemicals seem to work fairly well and to show the most promise among chemical inhibitors. When viewed under a microscope, they appear to modify the wax crystals precipitating from a wax solution (Figure 2, paraffin crystal clustering and Figure 3, effect of inhibitor on paraffin crystals). FIGURE 2 FIGURE 3 Crystal modification occurs for one of two reasons: (1) the chemical comes out of solution at a temperature slightly higher than that required for paraffin precipitation, causing nucleation, or (2) the modifier comes out of solution and co-crystallizes with the paraffin crystals. In either case, the normal crystal habit of paraffin is deformed sufficiently to inhibit further growth. This causes the crude oil to become saturated with much smaller paraffin crystals than normal with less tendency to adhere to pipe surfaces. Although they do not completely cure the paraffin problem, crystal modifiers offer the best chemical approach for reducing its severity. Assessment Paraffins and Asphaltenes 1. Which of the following affects deposition of paraffin? A .Pressure and temperatureB .Volatile constituentsC .Suspended particulatesD .Water contentE .A, B and C onlyF .A, B C and D 2. Cloud point for an oil is at a lower temperature than the temperature of the pour point. A .TrueB .False 3. Which is the most effective paraffin solvent used in the field? A .Carbon disulfideB .Carbon tetrachlorideC .DieselD .Xylene Recommended for You Introduction to Acidizing Additives in Acidizing Fluids Matrix Acidizing Fracture Acidizing Diverting Materials in Acidizing Hydraulic Fracturing Fundamentals Production Logging of Multiphase Flow in Horizontal Wells