Rapid detection of the vacuum failure of logging tools based on the variation in equivalent thermal conductivity

Abstract

The logging tool is the key downhole facility used for oil and gas exploration. A vacuum flask is indispensable for protecting the inside electronics of the logging tool from extreme working environments. During transport, assembly, or working processes, vacuum flask failure can lead to the destruction of the logging tool. The existing test methods are unable to detect vacuum failure quickly and accurately. To solve this problem, a rapid test method based on the variation in equivalent thermal conductivity before and after vacuum failure was proposed. First, the principle of this method was illustrated in detail, and a simplified 1D heat transfer model was calculated by the finite difference method. We found that the temperature rise ratio between the insulator and heat source (defined as P) can be used as the testing indicator with a 1 h testing time. Subsequently, a rapid test device was designed, and numerical simulations were performed to determine the critical values of the testing indicator for actual vacuum flasks. The simulated results showed that independent of the heating power and initial temperature, the P values for vacuum flasks in normal and abnormal states are 0.391 and 0.299, respectively, with a critical value of 0.351. Finally, experimental tests were conducted to test the effectiveness of the rapid test method and testing indicator. The experimental results showed that the measured P values of normal and abnormal vacuum flasks were above or below the critical value (Pe) under different test conditions, respectively, indicating that the proposed rapid test method can quickly and accurately detect vacuum failure. The maximum error between the simulated and experimental results was only 8.7%.