What is a wax inhibitor?

A wax inhibitor is a special chemical agent used in oil production to prevent paraffin from precipitating, crystallizing, and depositing on the inner walls of oil wells, pipelines, and equipment. It works by changing the structure of wax crystals, improving the low-temperature fluidity of crude oil, or modifying the surface properties of metal pipes, so that wax cannot stick and accumulate, thereby keeping oil wells and pipelines unobstructed and ensuring stable production.

Causes and Hazards of Oil Well Wax Deposition

Oilfield wax is mainly composed of n-alkanes, which are solid at room temperature and contain not only pure alkane substances but also other components such as oil, resin and asphaltene, with its appearance ranging from pasty liquid to hard solid. The core cause of oil well wax deposition is the high wax content in crude oil. Under the high temperature and pressure conditions of the formation, wax can be fully dissolved in crude oil. As crude oil flows upward from the bottom of the well to the wellhead, the external pressure and temperature decrease continuously, leading to a sharp drop in the solubility of wax in crude oil, and wax will gradually precipitate from crude oil. The entire wax deposition process can be divided into three stages: wax precipitation, wax crystal growth and deposition. If wax precipitates from the active sites on the surface of solid steel such as sucker rods, oil pipes and pipelines, it will continue to grow and accumulate on these solid surfaces, eventually causing blockage of sucker rods, oil wells and various pipeline equipment, seriously interfering with the normal production operations of oilfields and becoming an urgent technical problem to be solved in the process of petroleum exploitation and gathering and transportation. Although resin and asphaltene naturally contained in crude oil can play a certain basic wax inhibition role through their own characteristics, they cannot fundamentally solve the problem of wax deposition. Therefore, artificially developed petroleum wax inhibitors have become the core means to inhibit oil well wax deposition.

Main Types and Action Mechanisms of Petroleum Wax Inhibitors

At present, the mature petroleum wax inhibitors used in industry are mainly divided into three types: polycyclic aromatic hydrocarbon type, surfactant type and polymer type. Different types of wax inhibitors have their own unique component composition and action mechanisms, and all can cooperate with the natural resin and asphaltene in crude oil to improve the overall wax inhibition effect. The core effective components of polycyclic aromatic hydrocarbon type wax inhibitors are polycyclic aromatic hydrocarbons and their derivatives, among which polycyclic aromatic hydrocarbons refer to aromatic hydrocarbons formed by two or more benzene rings sharing two adjacent carbon atoms, with naphthalene, anthracene and phenanthrene as typical representatives, and derivatives such as methylnaphthalene and naphthol also having the same wax inhibition effect. The polycyclic aromatic hydrocarbons used in practical applications are all mixed polycyclic aromatic hydrocarbons, mainly extracted from coal tar. For example, the product cut from the 232-388℃ fraction of coal tar has an aromatic hydrocarbon mass fraction of at least 0.90, among which the mass fraction of mixed hydrocarbyl C1-C2 naphthalene is at least 0.50, which is a high-efficiency polycyclic aromatic hydrocarbon type wax inhibitor. The core action mechanism of this type of wax inhibitor is to participate in the formation of wax crystal nuclei and twist the crystal nucleus structure, which fundamentally hinders the continuous growth of wax crystals. At the same time, the resin in crude oil is soluble in oil and can participate in the formation of crystal nuclei in oil, playing the same wax inhibition role as polycyclic aromatic hydrocarbons. Asphaltene is insoluble in oil and exists as solid particles dispersed in oil to become the crystal nuclei of wax. A large number of dispersed crystal nuclei will hinder the growth and enlargement of wax crystals, making tiny wax crystals be carried away with the oil flow in a suspended state. Any type of crude oil contains a certain amount of resin and asphaltene, which are the basic wax inhibitors of crude oil itself, and other artificial wax inhibitors can give full play to their wax inhibition effect only with the cooperation of the two. Surfactant type wax inhibitors achieve wax inhibition by adsorbing on the surface of wax crystals or wax-deposited solid surfaces, and are mainly divided into oil-soluble and water-soluble types. Among them, the HLB value of oil-soluble surfactants needs to be less than 7, with petroleum sulfonates and polyoxyethylene alkyl alcohol ethers as commonly used types. Their action mechanism is to adsorb on the surface of wax crystals, turning the originally non-polar wax crystal surface into a polar surface, reducing the adsorption force between wax molecules and hindering the further deposition of wax molecules. Water-soluble surfactants are represented by polyoxyethylene sorbitan fatty acid esters, sodium alkylbenzene sulfonates and polyoxyethylene alkyl alcohol ethers, etc. They adsorb on the wax-deposited surfaces such as oil pipes, sucker rods and pipelines, making these solid surfaces polar and adsorbing a water film. Utilizing the characteristic that wax is insoluble in water, they prevent wax from depositing on solid surfaces. Moreover, water-soluble surfactants do not need to be added additionally and can be generated by the reaction of waxy oil with corresponding chemical reactants. For example, treating the surface of oil pipes with SO3-containing liquid and NaOH solution can make the sulfonatable substances in wax undergo sulfonation reaction first, and then generate surfactants through alkali neutralization. These surfactants will adsorb on the surface of oil pipes to form a water-wet surface, achieving the purpose of wax inhibition. Polymer type wax inhibitors are a class of polymers with excellent wax inhibition effect. The non-polar segments on their chain links and the non-polar parts on the polar chains can form eutectics with wax, while the polar segments will twist the crystal form of wax crystals, hindering the continuous growth of wax crystals to form a network structure, thus realizing wax inhibition. Polyacrylates are commonly used polymer type wax inhibitors, and the carbon number of their branched chain R is usually in the range of C14-C26. The specific carbon number depends on the peak alkane carbon number of wax in crude oil. When the average carbon number of the paraffin-like branched chain of the polymer is close to the peak alkane carbon number of wax in crude oil, it is most conducive to the precipitation of wax on it and can produce the best wax inhibition effect, which also makes the polymer type wax inhibitor have the characteristic of adaptive adjustment according to the characteristics of crude oil.

Application Methods of Petroleum Wax Inhibitors

The application of petroleum wax inhibitors needs to combine the actual situation of oil well wax deposition and select appropriate methods to ensure the wax inhibition effect. At present, there are three mainstream application methods, all of which exert their effects by making wax inhibitors fully contact with crude oil and dissolve slowly. The first one is to prepare an oil solution for use. During operation, the wax inhibitor is prepared into an oil solution and directly injected into the position below the wax deposition section of the oil well to make the oil solution fully mix with the rising crude oil, so that the wax inhibitor can exert its wax inhibition effect throughout the flow of crude oil. This method is simple to operate and suitable for the wax inhibition needs of most conventional oil wells. The second one is to make hollow wax inhibitor blocks for use. After the wax inhibitor is made into hollow wax inhibitor blocks, it is installed in a special wax inhibition pipe and run into the position below the wax deposition section of the oil well together with the oil pipe. Utilizing the immersion effect of crude oil, the wax inhibitor is slowly dissolved into the crude oil to realize long-term and continuous wax inhibition and reduce the workload of frequent wax inhibitor injection. The third one is to deposit it in the near-well zone for use. This method also relies on the slow dissolution of the wax inhibitor by crude oil to exert its effect. During specific operation, equal volumes of methanol and 1~3% wax inhibitor oil solution are alternately injected into the near-well zone of the oil well, and the well is shut in for 24 hours after injection to make the wax inhibitor stably deposit in the near-well zone. When crude oil flows from the formation to the wellbore, it will continuously dissolve the wax inhibitor in the near-well zone, making the wax inhibitor enter the wellbore and subsequent pipelines with the crude oil to achieve the wax inhibition effect.

Application Scope and Development Trend of Petroleum Wax Inhibitors

The application scope of petroleum wax inhibitors is not limited to the oil well exploitation link, but covers many fields such as petroleum exploitation, gathering and transportation, and refined oil processing. In different links, it can solve the problem of wax precipitation and deposition by improving the low-temperature fluidity of wax, and at the same time achieve the goal of cost reduction and efficiency increase. In terms of oil well wax inhibition, wax inhibitors can effectively inhibit the precipitation and deposition of wax crystals in the wellbore, maintain the smoothness of sucker rods and oil pipes, ensure the normal progress of oil recovery in oilfields, and reduce equipment maintenance and wax removal operations caused by wax deposition. In terms of crude oil gathering and transportation, wax inhibitors can play the role of pour point depression and viscosity reduction, effectively reduce the pour point of crude oil, improve the low-temperature fluidity of crude oil, prevent or reduce the precipitation and deposition of wax in the process of crude oil gathering and transportation, avoid the blockage of oil transmission pipelines, and at the same time reduce the number of heating stations built and their operation time in the gathering and transportation process, cutting down the energy consumption of crude oil heating. In terms of refined oil processing, wax inhibitors can be used as diesel pour point depressants to solve the problems of poor fluidity, wax deposition, blockage of engine filters and the lower flow port of oil storage pipes of diesel oil at low temperatures, improve the low-temperature fluidity of diesel oil, reduce the pour point of diesel oil, and ensure the normal storage and use of diesel oil in low-temperature areas. With the continuous development of petroleum exploitation technology, oilfield development is gradually advancing to complex working conditions such as deep wells and high-wax oil wells, and petroleum wax inhibitors also show a clear development trend. First, among the three types of wax inhibitors, polymer type wax inhibitors have become the fastest-growing category due to their good wax inhibition effect, various varieties and strong adaptability, and will continue to be the research focus in the future, with their molecular structure and performance continuously optimized. Second, the compound use of various wax inhibitors has become an important direction. By utilizing the synergistic effect of different types of wax inhibitors, the limitations of a single wax inhibitor are made up for, and the overall wax inhibition effect is greatly improved. Third, the long-term application of wax inhibitors has become a development trend. Wax inhibitors are placed in key positions such as formations to be slowly dissolved by crude oil, exerting a continuous wax inhibition effect for a long time, reducing repeated operations and improving the efficiency and economy of wax inhibition operations.

Supporting Wax Removal Agent Technology for Petroleum Wax Inhibition

In the process of petroleum production, if wax deposits have formed in oil wells and pipelines, wax inhibitors alone cannot remove the wax layer, and wax removal agents need to be used in conjunction with operations at this time. Wax removal agents are chemicals that can effectively remove wax deposits. For wax-deposited pipelines and equipment, physical methods such as hot oil circulation and downhole heater heating are preferred for wax removal, and wax removal agents can be used as an effective supplement. At present, wax removal agents are mainly divided into two types: oil-based wax removal agents and water-based wax removal agents. Oil-based wax removal agents are solvent-based wax removal agents with high wax dissolving capacity, with aromatic hydrocarbons as the core components. Pure aromatic hydrocarbons such as benzene, xylene, toluene, ethylbenzene and isopropylbenzene, as well as mixed aromatic hydrocarbons such as petroleum hydrocarbon reforming fractions and coal tar extract aromatics are all commonly used raw materials, and petroleum fractions such as gasoline, diesel oil and kerosene can also be used as solvents for oil-based wax removal agents. Benzene and toluene are usually selected in practical applications. It should be noted that although substances such as CS2, CCl4 and CHCl3 have excellent wax solubility, they will cause serious equipment corrosion and catalyst poisoning in the post-processing of crude oil, so they are strictly prohibited from use. Since oilfield wax contains polar substances, the dissolving capacity of a single solvent is limited. Therefore, polar structure cosolvents need to be added to oil-based wax removal agents to improve the dissolving capacity. Commonly used cosolvents include alcohols such as n-propanol, isopropanol and ethylene glycol, ethers with carbon number less than 12 such as butyl ether, amyl ether and hexyl ether, and alcohol ethers such as butanediol ethyl ether and ethylene glycol butyl ether. A typical formula of oil-based wax removal agent is 45-85% kerosene, 5-45% benzene, 0.5-6% ethylene glycol butyl ether and 1-15% isopropanol. Water-based wax removal agents take water as the dispersion medium and are mainly composed of surfactants, cosolvents and alkaline substances. Among them, water-soluble surfactants such as sulfonate type, quaternary ammonium salt type, polyether type and Tween type can be selected as surfactants, whose function is to reverse the wettability of the wax-deposited surface to a hydrophilic surface, facilitating the detachment of wax from the surface and preventing wax from re-depositing on the surface. Cosolvents select alcohols or alcohol ethers such as methanol, ethanol and ethylene glycol butyl ether to increase the mutual solubility of oil and water and improve the contact efficiency between wax removal agents and wax. Alkaline substances include alkalis such as sodium hydroxide and potassium hydroxide, and salts such as sodium silicate, sodium phosphate and sodium pyrophosphate which are alkaline when dissolved in water, which can react with polar substances such as asphaltene in wax to make the reaction products easily dispersed in water, thus removing wax from the solid surface. The development of wax removal agents is synergistic with that of wax inhibitors. At present, the main development directions are, on the one hand, developing water-in-oil wax removal agents combining oil-based and water-based wax removal agents, whose oil phase selects sulfur-free and chlorine-free wax solvents, and water phase takes water-soluble surfactants as emulsifiers, which have both the good wax dissolving effect of oil-based wax removal agents and the auxiliary wax inhibition effect of water-based wax removal agents, and can reduce subsequent wax deposition while removing wax; on the other hand, combining wax removal agents with chemical heating agents for use, increasing the temperature of the wax removal environment through chemical heating agents, and cooperating with the dissolving and peeling effects of wax removal agents to further improve the wax removal efficiency, suitable for complex scenarios with hard wax layers and serious wax deposition.