With the development of oil and gas exploration and development, oil and gas mining faces increasingly harsh environments. These environments are places with high temperature and pressure, high chloride ion content, high salinity and high water content, high carbon dioxide content, and high hydrogen sulfide content. Affected by all these factors and their interactions, corrosion of
OCTG pipes is becoming increasingly serious. For a long time, production accidents caused by corrosion of H2S, CO2, and CL- have occurred from time to time. The development and production of pipes suitable for use in such harsh environments have attracted attention. Corrosion caused by H2S is mainly divided into hydrogen-induced cracking (HIC) and sulfide stress corrosion (SSC). When pipeline steel is corroded in an oil and gas rich in H2S, the hydrogen produced by entering the steel and causing cracks is called hydrogen-induced cracking. When delayed brittle fracture of metal is caused by the combined action of corrosion and tensile stress in the sulfide solution, and the failure occurs when the stress is much less than the yield strength, this phenomenon is called SSC. It can be seen that there is a need to research and develop seamless OCTG pipes that are resistant to corrosion caused by H2S.
At present, Anshan Iron and Steel has successfully developed high-performance OCTG pipes for sour oil and gas fields, which have good strength, thermal matching and good resistance to sulfide stress corrosion. In the production process, the inclusion content in the steel is strictly controlled, and using the special heat treatment well meets the needs of oilfield users.
2.
Ingredients Control
2.1
Alloy element control
Anshan Iron and Steel's corrosion-resistant OCTG pipes are mainly made from chromium-molybdenum steel. The addition of chromium makes the steel have good resistance to hydrogen corrosion and high-temperature resistance; the stability of carbides is high, which reduces the chance of carbides and precipitated carbon reacting with hydrogen to generate methane, and increases the stability of hydrogen under high temperature and pressure. Cr-containing steel can form a corrosion product film against CO2 corrosion, which can prevent further corrosion. Molybdenum in steel can improve hardenability and thermal strength, prevent temper brittleness, increase remanence and coercive force, and improve corrosion resistance in certain media. In quenched and tempered steel, molybdenum can improve the quenching depth and quenching penetration of larger sections, improve the tempering resistance or tempering stability of the steel, and can be tempered at higher temperatures, thereby more effectively eliminating or reducing residual stress, improve plasticity and corrosion resistance. In corrosion-resistant steel, molybdenum can further improve acid corrosion, especially by preventing pitting corrosion tendencies caused by the presence of chloride ions.
2.2
Inclusion control
There are strict requirements for the control of non-metallic inclusions, especially inclusions extending along the length direction. At present, the control technology for type A inclusions is very mature, but there are still many problems in the control of type B inclusions. The processes involved in non-metallic inclusions include raw material preparation, blowing, tapping, ladle bottom argon blowing, slag making, deoxidation, vacuum treatment, wire feeding, soft blowing, bale protection, speed control, liquid level control, electromagnetic stirring, exception handling and other processes. In H2S, cracks often originate from Mn S inclusions, so reducing the sulfur content can reduce the total amount of Mn S inclusions. In order to minimize the value of spherical inclusions, the long Mn S inclusions can be transformed into spherical Ca S through calcium treatment. Anshan Iron and Steel adopts pure steel smelting technology to effectively reduce sulfur and phosphorus; harmful elements containing low sulfur and phosphorus and low stress concentration coefficient of spherical Ca S are factors that enable H2S corrosion-resistant OCTG pipes to have better stress and lower SSC sensitivity. In addition, by increasing the tap temperature of the converter, optimizing the LF operation, shortening the cycle, controlling the LF furnace top slag system and weak blowing after wire feeding to ensure the floating of inclusions, VD weak blowing in the later stage of pressure holding, maintaining the constant speed of the casting machine can ensure Anshan Iron and Steel's daily accumulation of production experience such as stable parameter control in the production process and also effectively reduced the content of inclusions. The inspection results of the actual inclusion content are shown in Table 1.
Table 1 Non-metallic inclusion levels
3.
Main equipment
3.1
PQF five-stand three-roll rolling mills
The PQF five-stand three-rolling continuous rolling mill is the most advanced continuous rolling mill in the world manufactured by SMS Mill of Germany. At the same time, it is equipped with the GT3000 series frequency conversion controller from Italy's Ansaldo Company, and the roll gap is automatically adjusted by the hydraulic servo HCCS system of Mill Company. It has advantages of having high rolling precision, excellent deformation, complete process control systems, computer-aided manufacturing systems, equipping with a quality assurance system that can perform online length measurement, thickness measurement, diameter measurement, and temperature measurement of the full length of the steel pipe. It has head and tail sharpening during rolling to ensure the head and tail walls. The outer circle tolerance accuracy reaches ±0.5%D (standard ±0.79%D) and the wall thickness accuracy reaches ±8%S (standard ±12.5%S).
3.2
Ultrasonic flaw detectors
The computerized ultrasonic thickness measurement and flaw detection system produced by the American Tuboscope Company has reached the international leading level. Its advantage is that it can detect longitudinal defects, transverse defects, oblique defects and wall thickness in all directions at the same time, and the ultrasonic coverage rate is greater than and equal to 110%. The system tracks the inspected steel pipe in three dimensions to maintain the consistency of flaw detection conditions.
4.
Production quality control
4.1
Intensity control
According to the results of laboratory tests, samples with lower strength have better corrosion resistance. Therefore, production is controlled according to the lower limit on the premise of meeting the standard requirements. Figure 1 is the statistical curve of the yield strength of AG95S H2S corrosion resistant OCTG pipes in a certain month. Figure 2 is the statistical curve of the yield strength of AG80S-3Cr CO2 corrosion resistant OCTG pipes in a certain month.
Figure 1 Statistics of the yield strength of AG95S H2S corrosion resistant OCTG pipes in a certain month
Figure 2 Statistics of the yield strength of AG80S-3Cr anti-CO2 corrosion resistant OCTG pipes in a certain month
4.2
Heating systems
Strict requirements are put forward on the heating system to ensure stable performance, especially the tempering temperature. After quenching at 900°C plus tempering at a higher tempering temperature above 700°C, good, homogeneous tempered sorbite is obtained (Figure 3). The structure grains are fine and uniform, meeting the requirement for the quality of each steel grade. These steel grades of OCTG pipes have excellent resistance to sulfur stress for their strength grade.
4.3
Straightening
The temperature of conventional products is required to be below 60°C before straightening and below 20°C after straightening. To develop corrosion resistant oil casing, thermal straightening is required to prevent pipe defects such as dislocations from occurring in the straightening process after heat treatment, which will affect the anti-corrosion performance. The straightening temperature is required to be 480°C or above. Recovery often occurs after straightening due to the high temperature, resulting in bending and secondary re-straightening, which seriously affects production speed and production efficiency. Therefore, measures are formulated to achieve uniform heating and cooling based on the actual equipment conditions; measures such as redesigning the straightening curve are also developed. After hard work, we overcame equipment defects and lack of experience, and initially mastered the straightening production process, especially the thermal straightening production process, which improved the straightening effect and produced products that meet standard requirements. At present, a stable production process has been formed, and users have given good feedback, and there has not been a single objection to straightening quality.
Figure 3 Microstructure of H2S corrosion resistant OCTG pipes
(a) AG80S petroleum casings (b) AG80S coupling blanks (c) AG95S casings (d) AG95S coupling blanks (e) AG80S-1Cr petroleum casings (f) AG80S-3Cr petroleum casings
