Causes of wax formation in casing and tubing

Date:2024-05-06Tags:Casing and Tubing, Casing, well casing, API 5CT

Due to long-term uninterrupted work, casing and tubing will accumulate a lot of impurities in their internal management, hindering the transportation of oil and gas, and in severe cases may even lead to corrosion or blockage. Wax formation in casing and tubing is one of the more serious hazards. As oil flows in the casing, a mixture of paraffin, colloid, condensate, sand and other impurities of a certain thickness gradually deposits on the inner wall of the pipeline, collectively called wax formation. The phenomenon of reduced transportation capacity due to wax accumulation in pipelines is a common problem that must be solved during the operation of heated and transported waxy crude casing and tubing.

casing and tubing


Definition and causes of wax accumulation in casing and tubing

Casing and tubing Waxing is a mixture of paraffin, colloid, asphaltene, condensate, sand and other mechanical impurities that gradually deposits a certain thickness on the inner wall of the tube. The internal cause of wax formation on the inner wall of the pipe is the high wax content of the crude oil. The external cause is mainly the drop in the transportation temperature, and is also related to factors such as crude oil flow rate, temperature, the difference between the oil temperature and the pipe wall temperature, and the roughness of the inner wall of the pipe. As the outside temperature decreases, the temperature of the crude oil also decreases. When the temperature drops below the cloud point temperature of crude oil (also called wax precipitation temperature), the paraffin in the crude oil precipitates from the crude oil and adheres to the pipe wall.The mechanism of wax formation is relatively clear: when the temperature drops below the cloud point temperature. Paraffin wax crystallizes and separates from the crude oil. It forms a network cavity structure with some crude oil molecules. Since the temperature of the pipe wall is lower than the temperature of the crude oil fluid, the resulting temperature gradient becomes the driving force for the diffusion of paraffin crystals to the molecules of the pipe wall. . The aging phenomenon of paraffin makes the wax removal operation more difficult.


Casing and tubing are steel pipes used to support the well walls of oil and gas wells to ensure the normal operation of the entire oil well during the drilling process and after completion. Each well uses several layers of casing based on different drilling depths and geological conditions. Cement must be used to cement the casing after it is lowered into the well. Unlike oil pipes and drill pipes, it cannot be reused and is a one-time consumable material.In order to keep the casing and tubing smooth and clean, a series of measures are required to prevent the deposition of paraffin in the pipeline system through the cross-section, and to clean up the paraffin deposits that have formed in the pipeline. All application of measures should be based on studies of the conditions and properties of paraffin deposition in the specific oil field. The method of wax removal depends on the physical and chemical conditions of the underlying fluid. First of all, methods to prevent paraffin deposition should be studied. Prevention is more important than cleaning.


At present, there are several methods to prevent paraffin deposition during oil extraction, storage and transportation:

1. Take casing and tubing insulation measures.

2. It is forbidden for paraffin in the storage tank to enter oil casing and tubing.

3. Clean the residual oil in the storage tank regularly.

4. Mix crude oil with different paraffin content.

5. Use various methods to heat the transported oil.


Several methods of wax removal using casing and tubing

1. Chemical additives for wax prevention and wax removal

In recent years, China's State Oilfield has conducted experiments on several oil wells using surfactant aqueous solutions to prevent wax formation in oil pipes, and has achieved certain results. This anti-wax agent can prevent wax molecules from continuing to precipitate on the waxed surface, and has a hydrophilic and oil-draining effect on the surface of the steel pipe. In foreign countries, polyethylene, vinyl acetate and other high molecular polymers or methylnaphthalene and other thickeners are also used. Ring compounds are injected into the bottom of wells or gathering pipelines to inhibit paraffin deposition, thus receiving certain economic benefits. The function of these additives is to disperse the paraffin wax products in the oil flow and keep them suspended, preventing the agglomeration or deposition of wax crystals.

2. Use plastic pipes or paint coatings on the inner walls of steel pipes to reduce wax buildup.

Using fiberglass pipes, plastic pipes, etc. to transport oil, or coating the inner wall of steel pipes with certain coatings or special paints can reduce or even prevent wax formation. Based on the comprehensive performance of oil resistance, wax resistance, strength and ease of spraying, modified epoxy silicone resin and ethoxy-free methyl silicone resin are more effective as anti-wax coatings. However, when the above measures are applied to long-distance pipelines, a series of issues such as construction technology and material strength still need to be further solved.

3. Use a pipe cleaner to remove wax

The principle of pig wax removal is as follows: after the pig is sent out by the sending device of the oil transmission station, it moves with the oil flow. When the pig is in a free state, its diameter is slightly larger than the inner diameter of the pipeline, and the pig itself has many steel brushes and scrapers. When the pipe pig moves with the oil flow, the steel brush and scraper form a large friction force on the inner wall of the pipe, so that the pipe pig produces a good wax removal effect.

 

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