Application and research of titanium alloy oil drill pipes

The complex and harsh drilling environment has put forward higher requirements and standards for future drilling tools. Compared with other drill pipes, titanium alloy drill pipe has a series of characteristics such as high specific strength, excellent corrosion resistance, fatigue resistance, and non-magnetic, making it be the first choice of material for oil and gas drilling in unconventional deep well, deep sea and other harsh environments. This paper discussed the design and microstructure properties of titanium alloy drill pipes in terms of alloy composition, service performance as well as pipe body processing and joint connection. Combined with the development of the application and standards of titanium alloy drill pipe, the prospects of the development and application of titanium alloy drill pipe in unconventional drilling operations such as deep wells and ultra-deep wells were foreseen, in order to provide support for promoting the application and development of titanium alloy drill pipe in the oil fields.


Introduction
With the exploitation of energy for meeting the demand of the rapidly development of human society, oil and gas resources that are easy to exploit in the shallow surface are increasingly scarce, the water content of old oilfields is getting more higher, and the mining environment is getting worse.The development of new drill pipe materials has become an urgent task.
For the drill pipe, steel drill pipe, aluminum drill pipe [1] , and titanium drill pipe are common, among them steel drill pipe is the most widely used drill pipe currently.Although some thin-walled, lightweight ultra-high strength steel drill pipe can be used to solve the drilling problems of extended reach wells, ultra-deep wells and deep water drilling, high strength steel is brittle at low temperature, and the impact toughness and tensile strength of drill pipe do not match with the environment [2] .In complex wells, the drill pipe needs to be cycled at a high stress level for a long time under a small curvature radius.Conventional steel drill pipe will sharply shorten their life or even fail due to the fatigue cracks, wear and high bending stress, which will bring safety hazards to drilling operations.Therefore, light and high strength drill pipe should be selected in deep wells, ultra-deep wells, horizontal wells and other unconventional oil and gas wells.According to the research about Li Ruizhe et al [3][4] , it is also found that titanium alloy drill pipe has high hardness and specific strength, which is beneficial to the corrosion resistance and fatigue resistance, and the fatigue life of titanium alloy drill pipe still has significant advantages under the multi-factor coupling of H2S, mud and temperature.
The strength of pure titanium is only about 265 ~353MPa.The addition of alloying elements can change the structure of the material to improve the material properties.The main alloying elements of titanium alloy drill pipe materials are Al, V, Mo, Sn and Zr, which can play a role in solid solution strengthening, aging strengthening and precipitation strengthening [5][6] .Among them, the content of Al and Mo is the main influencing factor.Zhang et al. [7] believed that high-strength titanium alloys can be prepared by increasing the content of Al and Mo following the Al-Mo equivalent design criterion.In the case of a certain alloy composition, heat treatment is the most effective method to control the microstructure of titanium alloy.However, since heat treatment can only play a role in adjusting the structure, the strengthening heat treatment of titanium alloy is limited by the composition of the alloy phase.Among them, the titanium alloy with α structure cannot be strengthened by heat treatment because the martensitic transformation will not change the performance of the titanium alloy, so the yield strength of the α titanium alloy is low, and the maximum is only about 689 MPa.It is mainly used in ground pipelines and cannot be used in unconventional oil well drill pipes.A small amount of metastable β-type titanium alloys, such as Ti-3Al-8V-6Cr-4Zr-4Mo and Ti-6Al-2Sn-4Zr-6Mo alloys, can be strengthened by heat treatment to control the morphology and size of β phase and secondary α phase.The smaller the size of α phase is, the more the phase boundary is, and the dislocation pile-up at the phase boundary improves the strength of the titanium alloy material.The strength of β-type titanium alloy after heat treatment can be as high as 1140 ~1242MPa [8] .However, due to the poor thermal stability of β titanium alloy, it is not suitable for use in high temperature drilling environment.Therefore, the most widely used structural titanium alloys are near α-type and α + β-type dual-phase titanium alloys.The properties of near α-type and α + β-type titanium alloys can be improved by heat treatment strengthening.During the heat treatment process of titanium alloys, structure of β transformation during the cooling process increases with the increase of solid solution temperature in the two-phase region, and structure of β transformation improves the strength of titanium alloys by solid solution strengthening.In addition to the effect of solid solution strengthening in the subsequent aging treatment process, its strength is also further improved due to the precipitation strengthening effect of α phase precipitated from β phase [9][10] , such as TC4 and TC4-Ru, which are now widely used in the oil and gas industry.After heat treatment, although the tensile strength and yield strength decrease slightly, the impact energy increases exponentially.Wu Guicheng et al [11] .studied the effects of solution aging and pre-stretching deformation on the microstructure evolution and strength-toughness of Ti-5.5Al-2Zr-1Mo-2.5Valloy tubes.It was found that the pre-stretched tubes had lamellar α fracture spheroidization after heat treatment and the introduction of a large number of dislocations promoted the nucleation of secondary α, which led to the obvious improvement of material yield and tensile effect.Li Ruizhe [12][13] et al. found that the strength of TA15X titanium alloy drill pipe increased by 200 MPa after single solid solution treatment, but the elongation decreased, and the microstructure changed from the original lamellar structure to the near-net-basket structure.Feng Chun et al. found that the microstructure of Ti-5Al-3V-1Mo-1Zr titanium alloy drill pipe after two-stage solid solution and aging at the phase transition point is a fully lamellar transformation structure.The lamellar width increases with the increase of aging temperature, and the impact energy increases with the increase of the width of α lamellar in the cluster.Therefore, it can be seen that the fine control of the inter-cluster orientation relationship and the lamellar width in the cluster by heat treatment can further improve the impact performance of titanium alloy drill pipe materials.Table 1 shows the microstructure type and mechanical properties of TA15X titanium alloy drill pipe after different heat treatment processes [14] .Table 1.Mechanical properties of TA15X lamellar and basketweave microstructure [14] .

Service performance
Corrosion and fatigue are the main reasons for the failure of drill pipe.The excellent corrosion resistance and fatigue resistance of titanium alloy make titanium alloy pipe present a good application prospect in the field of drill pipe with high performance

Anti-fatigue mechanism and performance
Under the service condition of titanium alloy drill pipe, one of its failure modes is high cycle fatigue failure caused by high frequency vibration.The fracture surface is smooth and flat, and there is no obvious plastic deformation characteristic.The research shows that the fatigue crack includes the initiation of fatigue crack, the stable propagation of fatigue crack and two stages.The crack initiation stage occupies most of the life of the crack [15] .The equiaxed α phase in titanium alloy can effectively curb the initiation of cracks.Otherwise, the addition of Al to the titanium alloy changes the critical shear stress and promote the slip system starting of α phase, thereby improving the yield strength and fracture toughness of the material [16] .The lamellar secondary α phase is generated in structure of β transformation.The crack angle of lamellar structure changes and the number of cracks increases when the crack propagates in the β grain to the direction of grain boundary α, which leads to the tortuous path of crack propagation.The increase of total crack length and crack bifurcation makes the strain field at the crack tip relatively relaxed, and the energy absorbed by crack propagation is large, which leads to the decrease of crack propagation rate.By comparing V150, Ti, S135 and G105, Xianbo Peng [17] found that titanium alloy drill pipe has the highest specific strength, and titanium alloy drill pipe still has significant fatigue life advantages under both single factor and multi-factor coupling.Jackie E. Smith [18] modeled titanium alloy drill pipe to analyze the performance of drill pipe in ultra-deep and deep directional drilling.According to laboratory tests, when the cyclic stress is between 30000psi and 40000psi, the fatigue life of titanium is 10 times greater than that of steel.The titanium alloy drill pipe can handle the high torque and abrasive formation that can tear the steel drill pipe.

Corrosion resistance mechanism and performance
In addition to the excellent fatigue resistance of titanium alloy itself, fatigue corrosion and stress corrosion cracking (SCC) are important causes of oil drill pipe failure because of the sulfur-containing situation in China's oil and gas fields.The service environment of drill pipe has long been subjected to complex stress conditions such as tension, bending and torsion.Under the condition of alternating stress, the existence of corrosive medium will accelerate the initiation and propagation of cracks at defects, resulting in fatigue corrosion failure.Partridge et al. [19] found that in inert medium, fatigue crack initiation life accounts for 90 % of the total life, but under the action of corrosive medium, fatigue crack initiation life only accounts for about 10 %.The existence of corrosive medium makes the life of crack propagation stage play a crucial role in the life of corrosion fatigue.The β phase in the α + β dual-phase titanium alloy can prevent the crack from expanding in the sensitive ɑ phase, so that the titanium alloy has good fatigue corrosion resistance.Song [20] compared the corrosion fatigue life of TA15X titanium alloy drill pipe with that of S135 drill pipe under simulated drilling fluids, and found that the corrosion fatigue life of S135 steel drill pipe material was significantly lower than that of titanium alloy drill pipe material under the same stress amplitude.The chemical properties of titanium alloy are active, and it is easy to form dense TiO2.Under the action of stress, the cracking of the passivation film leads to pitting or crevice corrosion.The stress concentration in the pitting pit produces cracks and develops in depth, and stress corrosion cracking occurs.However, by adding different alloying elements, the fatigue corrosion resistance and stress corrosion resistance of titanium alloy in different media can be significantly improved.When Pd is added to the alloy, it will undergo redox reaction with H + in the environment and lose electrons.Thus inhibiting the corrosion of titanium alloy [21] ; the addition of Ru element changes the structure of the passivation film on the surface of titanium alloy and increases the surface potential of the passivation film, thus enhancing the corrosion resistance of titanium alloy and the thermal stability of the passivation film [22] .Schutz [23] added Pd element to β-C titanium alloy and found that the added β-C titanium alloy still had good SCC resistance at 260℃.In addition, a large number of studies have also proved that platinum group alloy elements Pd and Ru and precious metal Mo can significantly improve the stress corrosion resistance of titanium alloys [24][25] .In summary, titanium alloy has good service performance under the service condition of drill pipe.However, the fatigue performance degradation and failure mechanism of titanium alloy under some complex dynamic load conditions and the effect of titanium alloy microstructure on the corrosion resistance of titanium alloy are still unclear, which need further study.

Manufacturing processes
The structure of titanium alloy drill pipe is the same as that of traditional drill pipe, which mainly includes two parts: pipe body and joint.The two parts are mainly connected by friction welding to reduce the probability of galvanic corrosion.

Pipe body
Titanium alloy drill pipe body is made of seamless tube [26] , and its main production process is divided into five parts: (1) Melting and casting: First, the vacuum consumable arc furnace (VAR) or the more advanced electronic cooling bed furnace (EB furnace) is used to form the billet after secondary melting; (2) Rough forging of pier: The forging of titanium alloy tube can control the microstructure of the tube after forging by forging temperature.(3) Extrusion: After upsetting and forging, the titanium alloy ingot is processed into a bar billet and then drilled, and then the titanium alloy tube billet is coated with copper or coated with copper and iron for hot extrusion.Due to the serious oxygen absorption of titanium alloy at high temperature, it is necessary to ensure that the residence time of the billet at high temperature is as short as possible.In order to make the metal flow more uniform, the extrusion temperature should be slightly lower than the β phase transition point.(4) Rolling: Cold rolling is generally used.In the rolling process, due to the mandrel inside the tube, the lubricant containing molybdenum disulfide is used for full lubrication, and finally straightening.( 5) Heat treatment: The heat treatment process includes annealing, solid solution + aging treatment, which is selected by the manufacturer or according to the contract.(b) (a)

Joint
Titanium alloy drill pipe uses two connection structures: drill pipe + steel joint or drill pipe + titanium joint.The joint of titanium alloy is connected with the pipe body by assembly or friction welding [27] .The main manufacturing process of titanium alloy joint is forging + drilling+ annealing + machining, and the annealing temperature is generally low.The annealing system of Ti-Al-V-Mo-Zr titanium alloy drill pipe joint is 720°C/1h air cooling [14] .Feng Chun et al. report that the mechanical properties of Ti-Al-V-Mo-Zr titanium alloy drill pipe joint after the above process are shown in Table 2.The structure is equiaxed structure + lamellar structure.The equiaxed structure has high strength and the lamellar structure has good impact resistance.Therefore, the strength and toughness of titanium alloy joint are well matched.

Application
Since the American company produced the first commercial titanium alloy product in the 1950s, the large-scale industrial production of titanium alloy began worldwide.Up to now, hundreds of titanium alloys have been developed and applied in more and more fields.The United States is the first country to apply titanium alloy drill pipes to the field of oil and gas exploration and development.In the 1980s, Ti-3Al-8V-6Cr-4Mo-4Z titanium alloy was first used by Unocal Oil in oil wells in the Salton Lake area of California where the working environment temperature exceeded 300°C [28] .It is proved that titanium alloy can be applied to complex oil engineering fields.At present, the RMI company in the United States is also the most mature technology and application in this field [8] .In order to meet the requirements of titanium alloy pipes in oil and gas field production, the company developed GR.28 titanium alloy-a titanium alloy pipe that meets the API standard of 110 steel grade in the 1990s.And to meet the requirements of oil and gas field production, taking into account the production cost and production and processing difficulties.Table 3 shows the typical materials of titanium alloy tubes and equipment used in foreign energy sources summarized by Feng Chun et al [29] .Table 3.Typical materials of titanium alloy tubes and equipment used in energy development [29] .Titanium alloy drill pipe, tubing, heat exchanger tube -In addition to Unico and RMI, Chevron has also used Ti-6Al-4V-Ru (Grade 29), Ti-3Al-8V-6Cr-4Zr-4Mo and other oilfield titanium alloys for offshore drilling pipelines and subsea oil drilling in order to solve some of the harsh environments of ultra-high temperature and pressure in the Gulf of Mexico [30] .

Company
In the 1990s, Grant Prideco and RTI Energy Systems have jointly produced titanium alloy drill pipes.
In 1999, Torch Drilling Technology Service Co., Ltd.used the combination of titanium alloy drill pipe and steel drill bit to drill a short-radius horizontal well in Greeley County, Kansas State.Later, in 2000, with the help of titanium alloy drill pipe, three old wells with high-angle directional drilling were completed in Ector County.The yield strength of the drill pipe was 840 MPa, the strength-to-mass ratio was 1.54 times that of the S-135 steel drill pipe, and the fatigue life was 10 times that of the steel drill pipe.This is the first application of titanium alloy drill pipe in the oil industry.Over the past 20 years, Russia [31] has also developed several α and near α titanium alloys, such as Ti-5.3Al-2Mo-0.6Zr,Ti-5.5Al-1.5V-1.4Mo,etc. for oil and gas fields.For example, Russia's VSMPO-Avisma has successfully developed titanium alloy drill pipe components and successfully applied them to Norway's North Sea drilling platform.At present, the independent research and development of titanium alloy drill pipes in China is still in its infancy.Among them, China Tubular Goods Research Institute of CNPC is in a leading position in this research field and has made outstanding work: In 2016, CNPC Tubular Goods Research Institute to carry out the selection and evaluation methods of 105 ksi titanium alloy drill pipes under the support of the national oil and gas major project "Efficient and rapid drilling technology and equipment for deep and ultra-deep wells".The high-strength titanium alloy drill pipe with independent intellectual property rights and yield strength exceeding 724 MPa has been successfully developed.
The special threaded joints and the key performance indexes of some titanium alloy pipes have been studied and developed, which provides a technical basis for the further development of highperformance domestic titanium alloy drill pipes.In 2017, the current practice of titanium alloy drill pipe connecting titanium alloy pipe body + steel joint by cold assembly to form drill pipe was innovatively changed.The joint and pipe body of titanium alloy were connected by advanced friction welding, and the world 's first batch of friction welded all-titanium alloy drill pipe was successfully developed.In 2020, Feng Chun et al [32] .improved the plasticity of TA15X titanium alloy by adding αstable element Al, neutral element Zr and β -stable elements Mo and V on the basis of TA15; the TA15X titanium alloy drill pipe product was developed, and a good matching of strength and toughness was achieved through the heat treatment process.The tensile strength and yield strength respectively reached 1145 MPa and 950 MPa [33] .On June 19, 2020, the TA15X titanium alloy drill pipe completed the first field test in the well TS3CX in the Tahe oil area of the Northwest Oilfield.After a total of 160,000 drills in the well of 6000 meters deep, the drilling tool was intact, reaching the world's advanced level.It has promoted the lightweight of drilling tools and made up for the gap in the use of titanium alloy drill pipes in the field of deep wells in China.
In terms of standardization, in the international scope, the standard system of the American Society for Testing and Materials (ASTM) is widely used in titanium alloy standards.There are corresponding titanium alloys in different countries, and different titanium alloy standards are stipulated as shown in Table 4. ASTM of the United States regulates 38 kinds of titanium alloys, and China also regulates 76 kinds of titanium alloy standardized materials.China's "GB / T3624-2007" stipulates titanium and titanium alloy pipes, and the United States B861-2002 and B862-2002 respectively stipulate titanium, titanium alloy seamless pipes, titanium and titanium alloy welded pipes.Russia's "ΓOCT 21945-82" and "ΓOCT 22897-86" respectively stipulate cold-rolled seamless titanium tubes and hot-rolled seamless titanium tubes.But for titanium alloy drill pipe, in addition to the "ISO15156-3" anticracking corrosion resistant alloy and other alloys mentioned [34] , there is no specific international standard for titanium alloy drill pipe.For decades, China's titanium alloy drill pipe has experienced from scratch at the beginning, to the later imitation, technology introduction to the present independent design and production.The types of titanium alloy pipes are constantly enriched, the standards are constantly improved, and the technology is constantly improving.However, the research on strength-toughness matching and fracture mechanism is not perfect.The research on the influence of multiphase microstructure of titanium alloy on the properties of materials is not deep enough.Moreover, the high cost of titanium alloy and the lack of alloy system also restrict the promotion of titanium alloy drill pipe.For the application of titanium alloy drill pipe under more severe conditions, there is still a lack of corresponding supporting system in China.These are the next research direction of titanium alloy drill pipe.(2) At present, the biggest problem is that the price of titanium alloy pipes used in harsh oil and gas exploitation environment is higher and the types are less.It is necessary to improve the titanium alloy system and develop more oil drilling using titanium alloy systems.(3) On the basis of developing more oil drilling using titanium alloy systems, it is also necessary to establish a corrosion resistance and antifatigue system for titanium alloy drill pipes.
2022, the National Standard for Manufacturing and Inspection of Friction Welded Titanium Alloy Drill Pipes for Oil and Gas Drilling Engineering: 'GB/T 41343-2022', which was drafted by the TGRI, was released.It includes the delivery technical conditions, manufacturing process, material requirements, inspection and test procedures, as well as the marking, packaging and storage of titanium alloy drill pipes used in oil and gas drilling and production operations.It made up for the domestic gap and began to be implemented on October 1, 2022.

1 )
Compared with other commonly used iron-based, aluminum-based, nickel-based drill pipe, titanium alloy has better development and application prospects in the current world oil and gas field exploitation to deep wells, ultra-deep wells, deep sea wells and other unconventional and

Table 4 .
Comparison of Chinese, American and Russian titanium alloy standards.