In recent years, the application of slotted casing in sand control technology for oil and gas wells has achieved remarkable success and has become the preferred solution for many oil fields. The excellent sand control effect and low cost make slotted casing widely used in sand control operations in oil wells. However, after long-term use, grooved casing will face complex corrosion problems in the underground environment. These problems seriously affect its service life and sand control effect. To this end, the industry continues to explore and improve the anti-corrosion technology of slotted casing to improve its corrosion resistance and ensure long-term stable production of oil and gas wells.
Basic structure and technical characteristics of slotted casing
Slotted casing is made by processing slits of a certain width on the body of the casing and tubing. These slits block larger sand particles carried in the formation fluid, thereby achieving the function of blocking sand and oil seepage. Its main technical features are as follows:
Pipe and seam in one body: The slotted casing adopts a single-layer overall structure and has a large inner diameter, which can ensure a high flow capacity.
Simple structure: low processing difficulty, convenient for mass production and application.
High precision: Using advanced laser cutting technology, the seam width is uniform and the precision is extremely high.
Low cost: It has wide applicability and can be used in different types of oil and gas wells, including vertical wells, inclined wells, sidetracking wells and horizontal wells, etc.
Corrosion challenges of slotted casing
Oilfield water has long-term contact with oil and gas, and its composition is much more complex than ordinary natural water, containing a variety of anions and cations. As a result, casing and slotted casing mainly face electrochemical corrosion problems in the downhole environment. Electrochemical corrosion mainly occurs in parts where the absolute value of the electrode potential is high, which poses a serious threat to the long-term use of grooved bushings.
In order to solve this problem, the new grooved sleeve introduces plasma processing technology in the manufacturing process. While cutting the seams, the surface of the seam cavity is subjected to high-frequency quenching and electrochemical treatment to form a 0.1mm to 0.2mm thick hard anti-corrosion layer on the surface of the screen tube. Its hardness is twice the hardness of the pipe body, and it is formed from the outside to the inside. Gradually decrease, making the combination between the hardened layer and the pipe body more reliable. The cutting process is completed in a special composite water-based working fluid. The surface of the cutting seam is smooth, free of slag and micro-cracks, which greatly improves the wear resistance and corrosion resistance of the grooved casing.
Application of anti-corrosion technology for slotted casing
The anti-corrosion technology of new slotted casing has achieved remarkable results in practical applications. The following is a detailed introduction to several key technologies:
High-frequency quenching technology: Through high-frequency quenching, a uniform hardened layer is formed on the surface of the grooved casing to enhance its corrosion resistance. High-frequency quenching can quickly heat the slot surface and then cool it quickly to form a high-hardness quenching layer, which can effectively prevent erosion by corrosive media.
Electrochemical treatment: On the basis of quenching, electrochemical treatment is performed to form a dense oxide film on the surface of the grooved casing. This oxide film has good corrosion resistance, can effectively isolate the contact between the corrosive medium and the metal matrix, and prolong the service life of the grooved casing.
Composite water-based working fluid: The use of special composite water-based working fluid during the cutting process can effectively reduce oxidation and cracks in the cutting heat-affected zone, improve the smoothness and integrity of the cutting surface, and further improve the anti-corrosion performance.
Erosion study of slotted casing
Indoor tests show that the erosion and wear of slotted casing is mainly affected by fluid flow rate, sand diameter, sand concentration and erosion angle:
Effect of flow rate: As the crude oil flow rate increases, the erosion and wear rate of the grooved casing increases exponentially. Therefore, when designing slotted casing, it is necessary to reasonably control the flow rate to avoid excessive erosion and wear caused by excessive flow rate.
Sand diameter: The erosion wear rate of slotted casing increases with the increase of the sand diameter in crude oil. However, when the sand diameter reaches a certain value, as the diameter continues to increase, the wear rate decreases instead. This shows that reasonable design of slit width can effectively reduce erosion and wear and extend the life of the screen tube.
Sand concentration: The erosion wear rate of slotted casing shows a similar change pattern to the sand concentration, that is, as the sand concentration increases, the erosion wear first increases and then decreases. By controlling the sand concentration, erosion wear of grooved casing can be reduced.
Erosion angle: The erosion angle has a significant impact on the wear of the grooved casing. When the erosion angle reaches 90°, the wear rate is the highest. This phenomenon is different from the erosion and wear mechanism of plastic materials, indicating that when designing slotted casing, it is necessary to consider the influence of erosion angle and optimize the slot arrangement.
The future development trend of slotted casing
With the continuous development of sand control technology for oil and gas wells, the future development trend of slotted casing is mainly reflected in the following aspects:
Material upgrade: Develop new materials with high strength and corrosion resistance to further improve the performance of slotted casing and adapt to harsher underground environments.
Process optimization: Continuously improve laser cutting and plasma processing technology to improve the accuracy and consistency of cutting processing and enhance anti-corrosion and sand prevention effects.
Intelligent monitoring: Introduce advanced sensors and monitoring systems to realize real-time monitoring of the working status of slotted casing downhole, timely discover and deal with potential problems, and improve the safety and efficiency of sand control operations.
Comprehensive protection: Combined with other sand control measures, such as chemical sand control and mechanical sand control, a comprehensive protection system is formed to improve the overall sand control effect of oil and gas wells.
In short, slotted casing, as an important sand control technology, has broad application prospects in oil and gas wells. Through continuous technological innovation and improvement, the anti-corrosion performance of slotted casing will be further improved, providing a more reliable guarantee for the stable production of oil and gas wells.
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