Surface treatment technology of downhole water cut sensor

Based on the structure, working principle, and working conditions of conductance water cut sensor, it is revealed that the early failure of the metal electrode of the sensor is due to the comprehensive influence of well fluid erosion, electrochemical corrosion, and oil pollution during its long-term service in the downhole. A technology for electrode surface treatment is proposed using boron-doped diamond (BDD) films to improve the service performance of the modified electrode. The hot wire chemical vapor deposition method was adopted to fabricate BDD film, the boron doping concentration and deposition time were optimized, and fluorination treatment was applied to improve the wear resistance, electrochemical corrosion resistance, and oleophobic property of the BDD film comprehensively. The results showed that BDD film with boron doping concentration of 6×10-3 exhibited high wear resistance and good electrochemical corrosion resistance, and endowed the modified electrode with superior erosion resistance and corrosion resistance. The friction coefficient and wear rate of BDD modified electrode were 92% and 78% lower than those of Invar alloy, also, the low-frequency impedance modulus value of the modified electrode was higher than 1×104 Ω·cm2. The BDD film prepared with a deposition time of 8 h had a favorable micro-nano structure owing to small grain size and uniform distribution. Such morphology was conducive to enhancing the oleophobic performance of the modified electrode, and its contact angle in the simulated well fluid was high to 102°. The engineering applicability of BDD film modified electrode under simulated working conditions indicated that, the modified electrode had excellent comprehensive performances of erosion resistance, electrochemical corrosion resistance and oil adhesion resistance, and can realize the long-term stable operation of the conductance water cut sensor under harsh downhole conditions.