Application Of Titanium Alloy Materials in The Petroleum Industry
Titanium is an important structural metal developed in the 1950s. Titanium can form alloys with other metal elements to enhance and improve its mechanical properties and physical properties to meet different needs. Titanium and its alloys have high strength, low elastic modulus, excellent corrosion resistance, good biocompatibility and other characteristics. They are widely used in aerospace fasteners, petroleum industry, engineering components, medical pacemakers and other fields.
The titanium alloy materials used in oil well pipes abroad mainly include UNS R56404 (Ti-6Al-4V-0.08Ru), UNS R55400 (Ti-5.5Al-4.3Zr-5.7V-1.3Mo-0.10O-0.06Pd), UNSR58640 (Ti-3Al-8V-6Cr-4Zr-4Mo), UNS R56260 (Ti-6Al-2Sn-4Zr-6Mo), etc. With the advancement of technology, TC4 (Ti-6Al-4V) titanium alloy with good corrosion resistance and high specific strength is gradually used in my country's oil well pipes.
Most of my country's traditional oil well pipes are made of carbon steel, such as G105, L80, N80, P110, etc. However, the oil and gas development environment in my country is relatively harsh. Oil well pipes not only face the challenges of high temperature and high pressure when serving underground, but also have to withstand the combined effects of corrosive environments such as H2S, CO2, high-concentration brine/completion fluid, elemental sulfur and strong acid. Traditional carbon steel oil well pipes can no longer meet the harsh service environment.
Titanium alloy oil pipes and casings will be subjected to large external pressure loads underground. Liu et al. established a new titanium alloy oil casing strength prediction model based on the strength and compression criterion model, and calculated and compared the compressive strength of titanium alloy pipes and steel pipes with different key parameters. The test results show that the compressive strength of titanium alloy pipes is less than that of steel pipes of the same specifications. Therefore, the compressive strength of titanium alloy oil pipes and casings is crucial to the safety of the pipe string underground.
Foreign scholars Schutz et al. exposed UNS R56404 titanium tubes to liquid mercury, and conducted high stress, continuous tensile and cyclic loading tests under relevant oil and gas production conditions. They found that in liquid mercury at 232 ℃, titanium tubes can completely resist liquid metal embrittlement and other forms of environmental degradation.
They also studied the acidic use of UNS R55400 titanium alloy under the ANSI/NACE MR0175/ISO 15156 standard. UNS R55400 belongs to the α+β type titanium alloy, which is a new type of high-strength titanium alloy specially developed for high-temperature and high-pressure oil and gas production services. They found that UNS R55400 titanium alloy can be used in acidic brine environments with up to 198 g/L chloride (saturated), and can be used in acidic conditions with a maximum hardness of 41 HRC, a maximum temperature of 288 ℃, with or without elemental sulfur, and withstand at least 3.45 MPa H2S partial pressure and 6.9 MPa CO2 partial pressure.
Therefore, the corrosion resistance of titanium alloy oil well pipe is better than that of traditional carbon steel oil well pipe. Secondly, it can be strengthened through aging treatment to enhance its own mechanical properties, and has excellent characteristics of resisting liquid metal embrittlement, sulfur and acid resistance. However, in a service environment with a large external pressure load, the compressive strength of titanium alloy oil well pipe is far inferior to that of carbon steel oil well pipe. There is little research in my country on how to improve the compressive and bending strength of titanium alloys. Therefore, it is of great significance to study how to maintain or even improve the strength of titanium alloy materials under high stress conditions.
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