8613480627251The exploration and development of oil and gas resources are divided into onshore and offshore extraction. Approximately 60% of oil and gas reserves are located in onshore formations, while 40% are buried in the seabed of offshore areas such as the Bohai Sea and the South China Sea. Offshore oil and gas extraction also holds significant development value.
Before the industrialized large-scale application of titanium alloys, the casings of logging tools were mostly made of 45 steel, 40Cr steel, and 30MoVNb steel, which were difficult to meet the needs of modern high-end logging operations. High-tech oil and gas exploration and development require that the casing of logging tools have a pressure-bearing capacity greater than 98 MPa and resistance to corrosion from crude oil and various formation media. At the same time, the casing should have low absorption of natural γ rays and should not form a shield against the geomagnetic field, ensuring the normal conduct of magnetic logging. Titanium alloys precisely meet the above comprehensive performance requirements.
In the mid-1980s, China introduced six sets of comprehensive logging tools equipped with Ti-6Al-4V titanium alloy housings from Schlumberger of France and Dresser of the United States. At that time, there were no titanium alloy logging tool housing products in China. Ti-6Al-4V is a titanium alloy with mature technology, excellent comprehensive performance, and outstanding cost-effectiveness. Since then, Baoji Nonferrous Metal Processing Factory, the predecessor of Baotai Group, and other units have conducted systematic and in-depth research on the extrusion of tubes made from this alloy.
The casing of the logging tool is a thick-walled, elongated cylindrical structure designed to withstand external pressure loads. The higher the pressure-bearing capacity of the casing, the greater the depth of the formation that the logging operation can reach. Comparing the pressure resistance test results of Ti‑6Al‑4V titanium alloy extruded tubes and 40Cr steel tubes: the Ti‑6Al‑4V tubes did not undergo unstable failure under both room temperature conditions at 112.7 MPa and high temperature conditions at 117.6 MPa; whereas the 40Cr steel tubes exhibited unstable failure at 80.4 MPa at room temperature, with a critical failure pressure range of 78.4 MPa to 80.4 MPa. When the external pressure exceeds the critical value, the casing will collapse, wrinkle, and damage the internal detection components. Therefore, the critical failure pressure directly determines the operational safety and reliability of the logging tool. The mechanical properties and critical failure pressures of the two materials are detailed in Tables 2 and 3.
The titanium alloy casing for deep-well logging tools introduced from China has a maximum operating pressure of 137.2 MPa and an allowable working temperature range of 150°C to 175°C. Test data indicates that selecting Ti-6Al-4V tubing with appropriate specifications (diameter, wall thickness) can meet this pressure resistance index. Currently, China has the production capacity for a full range of high-performance titanium alloy extruded tubing. The tubing produced by Baotai Group is not only supplied to the domestic market but also exported to other countries. The domestic Ti-6Al-4V alloy has a pressure resistance of over 120 MPa at 175°C, meeting the domestic demand for high-end logging. Within the conventional casing size range, the theoretical maximum critical failure pressure of this alloy can reach 248 MPa, and the casing made from it can withstand pressures exceeding 137 MPa.
The detection of natural γ-rays in strata and energy spectrum analysis are core methods for obtaining oil-bearing information in oil and gas well logging. The lower the absorption of γ-rays by the casing, the higher the detection sensitivity and the more reliable the data. The comparative test results of Ti-6Al-4V alloy and 40Cr steel for Co60 and Cs137 γ-rays are shown in Table 4.
The well logging operating conditions are harsh, necessitating that the housing material possesses excellent corrosion resistance. Relevant research has confirmed that Ti-6Al-4V alloy exhibits strong corrosion resistance in crude oil, sulfide, and chloride media, with a corrosion rate of less than 0.017 mm/a. This can effectively prevent the risk of pressure capacity reduction, instability, and failure due to corrosion-induced thinning of the housing over an extended period.
Some well logging projects require magnetic parameter measurements. If the magnetic susceptibility of the casing is too high, it can shield the geomagnetic field and interfere with the operation of magnetic logging equipment. Therefore, the lower the magnetic susceptibility of the casing material, the better. Ti-6Al-4V is a non-ferromagnetic material with a magnetic susceptibility of only 3.2×10⁻⁶ cm³/g. At the same time, it has good processing and forming properties, making it an optimal material for the casing of magnetic logging tools.
