(1. China Institute of Petrochemical Engineering and Technology, Beijing10010/; 2. Youshi University, China, Beijing 102249)
The purpose of this paper is to study how to improve the collapse strength of expansion pipe, and analyze the external factors affecting the material and strength respectively. Firstly, the main basis of material selection for high performance expansion pipe is put forward. Based on this, the external factors affecting the collapse strength of expanded tube are studied, and their influence and sensitivity are illustrated by experimental analysis, micro-theoretical analysis and simulation calculation. Secondly, the influence of different tube expanding processes on tube expanding performance is studied, and the basic basis for selecting the process is given. In addition, on the basis of fully absorbing the previous research in this field at home and abroad, this paper tentatively puts forward a new type of expansion pipe material, and explores the future development direction of expansion pipe technology, in order to obtain expansion pipes with good comprehensive performance and make forward-looking preparations for expanding its application fields.
Key words: Influencing factors of mechanical properties and collapse strength of expanded tube materials
Research progress of expansion pipe materials and improvement of collapse strength
Liu Xiaodan 1, 2, Tao Xinghua 1, Niu Xinming 1
(1. China Petrochemical Petroleum Engineering Research Institute, Beijing10010/; 2. China Shiyou University, Beijing 1 02249, China)
The purpose of this paper is to improve the collapse strength of expansion pipe. The analysis begins with how to choose the expansion pipe material and how external factors affect the collapse strength. First of all, it gives the principle of material selection. On this basis, the main related factors of collapse strength are studied. In the research process, experiments, micro-theoretical analysis, simulation and other methods are adopted. The influence of various factors on the anti-extrusion strength and its sensitivity are analyzed. The influence of expansion technology on collapse strength was studied. How to choose the appropriate method in some cases is given. On the other hand, based on the previous research results, a new type of expansion tube material is proposed and the future development of expansion tube technology is explored. The main purpose is to obtain an expansion tube with good comprehensive performance, so as to expand the operation field.
Keywords expansion tube; Materials; Mechanical properties; Collapse intensity; Effective factors
Expansion pipe technology is to run a special pipe smaller than the inner diameter of the upper casing into the well during drilling construction, and push the expansion cone head into the well by hydraulic or mechanical means, so that the pipe string is permanently deformed radially and the inner diameter is increased, thus achieving the purpose of plugging complex strata and repairing damaged casing. The research on expansion tube technology abroad began in 1970s and 1980s. Then it developed rapidly, and as early as 1993, the concept test of expansion tube technology was carried out. 1998, Shell conducted a casing prototype test with model J55 and size 133/8 in Gasmer test well, and the expansion seal was successful [1]. It has reached the commercialization level of 1999. In June, 2004, Enventure Company has completed 247 technical services for 58 users in 20 countries around the world, and the accumulated tube expanding length has reached 258755ft? The reliability is above 95%. Domestic research started late and the foundation is weak. In 2000, the concept of expansion tube was introduced, and it was found that the domestic demand market was large. Therefore, scientific research institutes, Dagang and other institutions in southwest China have carried out relevant research and experiments. In 2004, the Drilling and Production Technology Research Institute of Shengli Petroleum Administration of China Petrochemical Company conducted the solid expansion pipe test for the first time and achieved success [2]. Since then, due to its expanding application scale, it has been favored by more and more scientific research institutions. The core advantages of this technology are: first, it can save the drilling size; Second, it can be used in the whole process of casing repair, completion and oil production; Third, the operation procedure is flexible, with wide applicability and remarkable economy. Therefore, it is praised by the industry as "one of the core technologies of oil drilling and production industry in 2 1 century" [3].
The expansion pipe must have good mechanical properties, that is, high strength and good formability. A large number of tests and mechanical simulation results show that the collapse strength of the expanded casing will be greatly reduced, generally by 30% ~ 50%. In order to improve the operation safety and expand the application scope, an expansion pipe with higher anti-collapse strength is needed. In view of this, around the core of improving the collapse strength of expansion pipe, this paper first puts forward the main basis for selecting high-performance expansion pipe materials from the internal factors. Secondly, the main external factors affecting the collapse strength of the expansion pipe are analyzed, and the sensitivity of the main external factors to the collapse strength is explained through experimental analysis, microscopic theoretical analysis and simulation calculation. In this paper, the essence of previous research in this field at home and abroad is absorbed, a new type of expansion tube material is proposed, and the future development direction of expansion tube technology is preliminarily explored. The purpose is to improve the performance of expansion tube and expand its application field.
1 material performance requirements of expansion pipe
Looking back on the research of tube expanding materials, it took about 6 years abroad to systematically study the effects of tube, tube expanding method and post-expansion heat treatment on the mechanical properties, residual stress and extrusion resistance of tube expanding [4 ~ 7]. The commonly used materials for expansion pipes include ordinary low-carbon alloy steel, high-pressure boiler steel and special materials for expansion pipes, such as N80, L80 and K55. Some data show that high strength pipeline steel X-95 and casing material P 1 10 can also be used as expansion pipe materials. In order to simulate and predict the extrusion resistance of expanded steel pipe, the compression test of expanded steel pipe was carried out according to ASTM E9-89 standard. Figure 1 reflects the influence of expansion on the compressive yield strength and hardness of steel pipes made of different materials. As can be seen from the figure 1a, the yield strength of P 1 10 and X95 decreases most obviously, about 30%. The reason is that the work hardening effect is not obvious (figure 1b), which cannot make up for the decrease of yield strength caused by Bauschinger effect. The yield strength of K55, L80 and N80 changed little before and after expansion, presumably due to the cancellation of work hardening and Bauschinger effect. Fig. 2 shows the change curve of impact toughness of steel pipe before and after expansion. After expanding, the impact toughness of different brands of steel pipes decreased, but they all met the requirements of API standard.
To sum up, the hardness, yield strength, tensile strength, yield ratio and elongation of tube expanding materials have been studied in detail by foreign scientific research institutions, and the basic properties of tube expanding materials have been summarized. In order to meet the requirements of large plastic deformation of expansion pipe, the pipe material of expansion pipe should meet the following requirements: (1) good plastic deformation ability; (2) High tensile strength; (3) Low yield strength; (4) Higher work hardening index; (5) The mechanical properties (expansion rate is generally 10% ~ 25%) of the expanded pipe shall meet the requirements of API. This puts forward the basic principles of material selection for the use of expansion pipes in different expansion processes and different working environments, and has important reference significance for further improving the material properties of expansion pipes and improving the strength of the pipe body.
Fig. 1 Effect of expansion on yield strength and hardness of pipes with different steel grades.
Study on adaptability of expansion process of expansion tube 2
In order to study the influence of expansion technology on the properties and strength of pipes, a foreign research institution conducted a special study. In this paper, the influence of expansion loading mode on the collapse strength of pipe body is emphatically analyzed and studied. Proprietary C-Mn steel 50 steel pipe was used in the test, with an outer diameter of 193.7mm, a wall thickness of 9.5mm and an expansion rate of 15%.
See table 1 for the results of dimensional change, residual stress and collapse strength of pipes after different expansion joints.
The test results show that the collapse strength of the expanded tube has decreased by 47% ~ 55%. The tube expanding mode has a significant effect on the residual stress, diameter-thickness ratio and collapse strength of the expanded tube. D mode, the diameter-thickness ratio and residual stress after expanding are at a medium level in several expanding modes, and the collapse strength after expanding is the highest. E-type has lower residual stress, higher diameter-thickness ratio and lowest collapse strength. Type F has the highest diameter-thickness ratio, lower residual stress and higher collapse strength after expansion. This test provides a feasible expansion process for conditional use of expansion pipe. In order to ensure the good performance of the expansion pipe, lubrication measures should be taken to make the expansion pipe as centered as possible and the expansion speed appropriate (the best expansion speed is 7.6 ~ 18m/min).
Fig. 2 Effect of expansion on impact toughness of pipes with different steel grades
Table 1 Effects of expansion and loading methods on the size, residual stress and collapse strength of expanded pipe.
Note: A is before expansion; B is the compressive load applied just before the expansion cone; C is hydraulic expansion behind the expansion cone, and compressive load is applied before the expansion cone; D, fixing the pipe end behind the expansion cone, and pulling the expansion cone to expand; E is the hydraulic expansion behind the expansion cone; F is the hydraulic expansion after fixing both ends of the pipeline and expanding the cone.
Analysis of key factors affecting the collapse strength of expansion pipe
Due to the particularity of the technology of the expansion pipe itself, the unfavorable factors of the collapse strength become more complicated. In this paper, the effects of expanding structure, yield strength, strain aging and Bauschinger effect on the collapse strength of expanded tubes are studied, and the sensitivity of each factor is analyzed.
3. 1 Influence of complex geometry expansion tube structure
In the process of manufacturing, transportation or operation, the expansion pipe is more vulnerable to external pull and impact than the thick-walled casing, and it is inevitably worn by the drill string during the operation. The calculation shows that the existence of wear defects on the inner wall of the expansion tube reduces the extrusion strength of the expansion tube itself. In addition, if the outer stratum is rock salt layer, the stiffness of the expansion pipe body in different radial directions will change due to wear, which will lead to more serious and uneven stress distribution on the outer wall of the expansion pipe. The existence of uneven load and inner wall wear defects aggravated the destruction of the extrusion strength of the expanded tube. For the casing with model P 1 10 and wall thickness of 12.7mm, it is assumed that the maximum thickness of inner wall defects is 0, 0.5, 1, 1.5, 2 and 2.5mm respectively, and the collapse strength of the casing after wear is calculated. In addition, ellipticity is also one of the important factors affecting the collapse strength of expanded tubes. In the process of tube expanding, ellipticity and wall thickness unevenness will continue to maintain or even increase, and the increase of diameter-thickness ratio after tube expanding is also the reason for the decrease of collapse strength of tube expanding.
Fig. 3 Effect of Wear on Collapse Strength of Expansion Tube
3.2 Influence of yield strength of tube expanding material
As mentioned above, the yield strength of P 1 10 and X95 materials decreased obviously after expansion, about 30%, especially in the initial stage of expansion (the expansion rate was within 10%). The reason is that the work hardening effect is not obvious, which can not make up for the decrease of yield strength caused by Bauschinger effect. Because work hardening counteracts Bauschinger effect, the yield strength of K55, L80 and N80 changes little before and after expansion. Therefore, the higher the work hardening rate of materials, the smaller the decrease of compressive yield strength after expansion. At the same time, according to the Hall-Page equation, namely
Oil and gas accumulation theory and exploration and development technology: Proceedings of Postdoctoral Academic Forum of China Petrochemical Petroleum Exploration and Development Research Institute 20 1 1. 4
Where σy is the yield stress; σ0 is lattice friction; K is a constant; D is the particle diameter.
The empirical relationship between work hardening index n and grain diameter of low carbon steel is obtained by experiments, that is,
Oil and gas accumulation theory and exploration and development technology: Proceedings of Postdoctoral Academic Forum of China Petrochemical Petroleum Exploration and Development Research Institute 20 1 1. 4
The relationship between yield stress and work hardening index is as follows:
Oil and gas accumulation theory and exploration and development technology: Proceedings of Postdoctoral Academic Forum of China Petrochemical Petroleum Exploration and Development Research Institute 20 1 1. 4
It can be seen from Formula (3) that the higher the work hardening index of the material, the lower the yield strength. Therefore, on the premise of keeping the tensile strength basically unchanged, it is of positive significance to reduce the yield strength of the expanded tube material and improve the work hardening index of the material. Considering the nonlinearity, loading history, ellipticity and wall thickness inhomogeneity of the material, it is considered that the yield strength of the material is not the key factor affecting the collapse strength of the expanded tube when the diameter-thickness ratio is greater than 20.
3.3 Effect of strain aging
Strain aging mainly occurs in low carbon steel and low alloy steel. It refers to a mechanical metallurgical phenomenon that interstitial solutes (C, N) in solid solution interact with dislocations during or after plastic deformation, pinning dislocations to prevent deformation, resulting in increased strength and decreased toughness. Strain aging has an important influence on the collapse strength of expanded tube. Here, L80 expanded tube is taken as an example for quantitative analysis. Table 2 shows the effects of expansion and strain aging on the compressive yield strength of L80 casing. Because the expansion pipe expands underground at the temperature of 50 ~ 350℃, the strain aging effect of the expansion pipe at a certain temperature is studied experimentally. As can be seen from Table 2, when the pipe expands by 20%, the compressive yield strength decreases from 632MPa to 505MPa, which is about 20%, and the so-called reverse load softening phenomenon appears. After aging at 65438 050℃ for 65438±0.5h, the yield strength damage recovered about 65438 04%. After aging at this temperature for 5 hours, the compressive yield strength increased to 639MPa, which was equivalent to the yield strength of the pipe before expansion. When the aging time is 5h and the temperature rises to 175℃, the yield strength will not continue to increase. This phenomenon shows that the external pressure strength of the expansion pipe will be greatly reduced in the initial stage of expansion during the underground use. But after a period of aging, in a certain range, the compressive strength will be improved.
Table 2 Effect of Expansion and Strain Aging on Yield Strength of L80 Casing
3.4 Bauschinger effect in the process of expansion
During the service period, the expansion pipe will be subjected to internal pressure, external extrusion pressure and tension. The direction of internal pressure is the same as the expansion direction, but the direction of external extrusion pressure is opposite to the initial expansion direction, so the Bauschinger effect will lead to the increase of internal pressure resistance and the decrease of external extrusion resistance of the expansion pipe. The influence mechanism is as follows:
The magnitude of Bauschinger effect is closely related to the plastic deformation of metal materials. In a certain range, the Bauschinger effect increases with the increase of plastic deformation. However, when the plastic deformation exceeds the slip zone, because of its proliferation and difficult redistribution, the Bauschinger effect can be quickly reduced or even disappeared when the dislocation is loaded in the opposite direction. Therefore, it is of great significance to study the relationship between Bauschinger effect and plastic deformation of metal materials to improve the collapse strength of expanded pipes.
3.5 Residual stress in the tube after expansion
In the process of tube expanding, plastic deformation occurs in both circumferential and axial directions, which is generally uneven. Uneven deformation produces additional stress in the tube, which remains in the tube after expansion to form residual stress. Zhang Jianbing et al. measured and analyzed the residual stress of 35CrMo steel pipe and J55 casing after expansion joint. The results show that there is a significant circumferential residual compressive stress (its value is about 200MPa) after expansion joint, which is equivalent to directly reducing the transverse compressive yield strength of the pipe, and is an important factor leading to the reduction of the collapse strength of the pipe after expansion joint.
3.6 Sensitivity Analysis of Influencing Factors of Collapse Strength of Expansion Pipe
Both mechanical simulation analysis and laboratory tests show that the compressive strength of the expanded pipe body is greatly reduced due to the joint action of residual stress and Bauschinger effect. After expansion, the tube wall becomes thinner to some extent. Therefore, the original damage or possible wear caused by subsequent operations will seriously affect the collapse strength of the expanded pipe. For the expansion pipe with diameter-thickness ratio greater than 20, the influence of wear depth is greater than the wear radius; The yield strength of the material has little effect on the extrusion strength of the expanded tube. Other factors, such as Bauschinger effect and residual stress, and their influence on casing collapse strength need further quantitative analysis and research.
4. Explore new ideas to improve the anti-collapse strength of expansion pipe.
In order to improve the safety of expansion pipe operation and obtain good comprehensive performance of expansion pipe, a new idea of strengthening extrusion strength is put forward from the perspectives of material heat treatment, pipeline structure optimization and pipeline expansion molding.
4. Optimization of heat treatment process of1expansion pipe material
Generally speaking, with the increase of steel grade, the work hardening rate decreases and the yield strength ratio increases, which will lead to a significant decrease in the collapse strength after expansion. The mechanical properties of ordinary casing can be improved by proper heat treatment. Such as low-carbon steel or low-alloy steel casing with ferrite+pearlite structure, can be subcritical quenched, that is, the material is heated to the two-phase region between austenite and ferrite (between AC 1-AC 3), and quenched after heat preservation to obtain ferrite and martensite two-phase structure [9, 10]. The steel with this structure has the characteristics of high strength, low yield point, continuous yield, high work hardening rate and high elongation [1 1]. The research shows that the expanded ferrite+martensite dual-phase structural steel has higher collapse strength than the ordinary ferrite+pearlite structural steel [12].
4.2 Optimization of pipeline structure before and after expansion
As mentioned above, the existence of defects in the expansion pipe greatly reduces its collapse strength, and the adverse effects of corresponding defects are more serious after expansion. Therefore, before selecting the expansion tube, stricter standards should be formulated and the forming strength should be increased to obtain an approximately ideal circular expansion tube. During the service of the expansion pipe, the process measures should be optimized to prevent the drill string or other operation strings from wearing the inner wall of the expansion pipe.
4.3 Eliminate the residual stress after tube expansion.
Due to the influence of processing and operation technology, there is a certain residual stress in the expansion pipe, which will adversely affect the collapse strength of the expansion pipe. Therefore, active measures should be taken to minimize the residual stress. The technology and method of eliminating residual stress before entering the well are mature. The elimination of residual stress after downhole expansion is a relatively new understanding at present. Among them, ultrasonic impact is an effective method. The lower part of the expansion core is connected with an ultrasonic device, which moves and rotates from bottom to top with the expansion process. Using high-frequency and high-power ultrasonic waves above 20kHz, the surface of the expansion tube is greatly compressed and plastically deformed, which can effectively reduce the residual stress and improve the comprehensive performance of the expansion tube.
Research prospect of new materials for expansion pipe
As mentioned above, the expansion pipe material needs good strong-plastic matching and excellent work hardening ability, and the strong-plastic product (the product of tensile strength and elongation) can be used as an index to measure the performance of the expansion pipe material. LSX80 is the first expansion pipe steel introduced by Shell Company in the world, and it has a strong plastic product of 30GPa%. Advanced automobile steel has similar performance requirements for expansion tube materials, which can be introduced into the development of new expansion tube materials. TWIP steel and austenitic stainless steel in the second generation automobile steel belong to the category of high alloy steel, and their microstructures are mainly soft austenite. Using the TWIP effect of austenite, the strength of steel is increased by 800 ~ 1000 MPa, and the plasticity reaches 50% ~ 80%, so its strength-plasticity product reaches 50 ~ 70 GPA% [13]. At present, the third-generation automobile steel independently developed in China has prepared a dual-phase composite structural steel containing about 30% metastable austenite and ultra-fine grain matrix through alloying design and reverse austenite transformation of medium manganese carbon steel. The tensile strength at room temperature is 0.8 ~ 1.6 GPA, the elongation at break is 30% ~ 45%, and the strong plastic product is 30 ~ 48 GPA% [14]. Introducing these advanced materials into petroleum industry has broad prospects as high-performance expansion pipe materials in the future.
6 conclusion theory
1) By analyzing the hardness, yield strength, Bauschinger effect and their interaction of steel pipes P1/kloc-0, X95 and K55 before and after expansion, the basic properties of expanded pipe materials are given, which provides the main basis for developing high-performance expanded pipe materials.
2) Combined with five different loading modes of expansion test, the influence of expansion technology on the structure and comprehensive performance of expansion pipe is analyzed. Based on this, combined with the actual working conditions, it can provide an important reference for selecting the appropriate expansion process, so as to obtain an expansion pipe with good comprehensive performance.
3) The distribution and magnitude of residual stress after expansion joint have changed greatly due to different expansion joint processes, which not only seriously affects the extrusion strength of expanded pipe, but also directly affects the efficiency of other subsequent processes. The method of eliminating underground residual stress proposed in this paper provides a new idea for improving the strength of pipe body.
4) The manufacturing and processing of the expansion pipe may cause the deformation of the pipe body in different degrees. Even after entering the well, the subsequent operation may also cause the abrasion of the inner wall of the expansion pipe, which will seriously affect the anti-extrusion strength of the expansion bellows, and the influence of this defect on the strength will become worse after expansion.
5) If other factors are not considered, the lower the yield strength ratio of tube expanding material, the higher the work hardening rate and the smaller the collapse strength loss after tube expanding. The collapse strength of ordinary casing can be improved by making suitable heat treatment process (such as sub-temperature quenching).
6) Introducing the second-generation automobile steel and the new third-generation automobile steel into the technical field of expansion pipes in petroleum industry has innovated the theoretical research ideas in this field, supplemented and expanded the original material selection range of expansion pipes, which has important enlightening significance and guiding role.
refer to
Meng Qingkun, Xie, Feng Lai, et al. Overview of expandable casing technology [J]. Drilling and Production Technology, 2003,26 (4): 67 ~ 68,74.
Li zuohui Research on key technology of expansion pipe and its first application [J]. Petroleum drilling and production technology, 2004,26 (3):17 ~19.
[3]Dupal K K, Campo D B, Lofton J E, et al. Industrial experience of solid expansion pipe technology [J]. World Petroleum, 200 1, 222:7~8.
[4]Filippov A, Mack R, Cook L, et al. Expansion tube solution [J].SPE 56500, 1999.
[5] Mark R&D, Terry McCaughey and Lev Lin. How does field expansion affect casing and tubing performance [J]. World petroleum, 1999, 220:69~7 1.
[6]Mack R, Filippov A, Kendziora L, et al. On-site expansion of casing and tubing-effect on mechanical properties and sulfide stress cracking resistance [J]. Corrosion, 2000, 3:26~3 1.
Mack R.D. Effect of tube expansion on mechanical properties and properties of selected OCTG-laboratory research results [J].OTC 17622, 2005.
Zhang Jianbing, Han Jianzeng, Chen Jianchu, et al. Residual stress of expansion casing [J]. Petroleum drilling and production technology, 2005,27 (2):18 ~ 20.
Zhang Shukun, Zhang Limin. Subcritical Quenching Toughening Process for 36Mn2V Steel Oil Casing [J]. Steel Pipe, 2005,34 (3): 20 ~ 22.
[10] Wang jiheng, Li Hui, Xie chunsheng, et al. study on the structure and properties of subcritical quenching of 35CrMo steel [J]. hot working technology, 2009,38 (6):144 ~146.
[11] Avaghoa diagram, Wu Baorong. Dual phase steel-physical and mechanical metallurgy [M]. Beijing: Metallurgical Industry Press, 2009.
[12]Pavlina E J, Van Tyne C J, hertel k. hydraulic bulging test of dual-phase steel pipe produced by new process route [J]. journal of materials processing technology, 2008,201:242 ~ 246.
[13]Frommeyer G, Brux U, Neumann P. Ultra-ductile and high-strength manganese TRIP/TWIP steel for high energy absorption purposes [J].ISIJ International. ,2003,43(3):438 ~446.
[14] HanDong, Cao Wenquan, Shi Jie, et al. Microstructure and properties control technology of the third generation automobile steel [J]. Steel, 20 1 1, 46 (6): 1 ~ 1.