Abstract
Detecting and localizing the inner damage of the downhole casing is crucial for oil and gas production. The current methods for damage detection suffer from polarization effect, capacitance effect, and difficulty in measuring weak direct current signals. As an alternative, we propose a new instrument design based on a six-electrode array in this study. The basic detection principle is introduced, and the key factors that affect the detection performance are studied through modeling and simulation. Besides, the entire system of the instrument design consisting of several modules is presented. In addition, an experiment is conducted to demonstrate the detection capability of the proposed design. It is found that the detection dynamic of the instrument is 93 dB and a signal-to-noise ratio is 73 dB. It can detect damage when salinity is lower than 120 g/l and can identify the location of damage with high resolution in the longitudinal and radial directions.
Highlights
Changqing Oilfield located in northwest China has high underground mineralization, and the resulting electrochemical effects cause the casing wall corrosion to be extremely serious
The deformation of the downhole casing can lead to the peeling of the inner casing coating, and the physical scraping between the instrument and the borehole wall will cause the damage of the inner casing coating, resulting in the decrease of the anti-corrosion effect of the inner casing coating [6]
Downhole electromagnetic thickness measurement technology can not produce electromagnetic signal feedback on thin coating [8]; Multi-arm caliper logging tool can detect the change of inner diameter of oil casing, but it can not detect the quality of coating, and the attachments on oil casings will affect the measurement accuracy [9]; Optical imaging logging technology is affected by the surface cleanliness and limited space perspective of downhole oil casing, so it is difficult to accurately judge the specific situation of the inner coating of oil casing; the ultrasonic imaging logging tool can be applied to casing quality evaluation, its eccentricity has a great influence on the measured value, and it has not been used to detect the damage of inner coating [10]
Summary
Changqing Oilfield located in northwest China has high underground mineralization, and the resulting electrochemical effects cause the casing wall corrosion to be extremely serious. The impedance of double-layer capacitance Cx and lead distributed capacitance Cp will affect the measurement results, and the size of Cx can often reach more than 1000 times of Cp. According to the calculation formula of capacitive reactance, when an AC signal with a frequency of 1KHz is used as an excitation source, it is assumed that the capacitance of Cx is 100 μF and that of Cp is 10pF. From the perspective of engineering realization, only two sinusoidal signals of different frequencies are used as excitation sources and the effective value of the voltage difference between the two ends of the sampling resistor is calculated and the real well fluid resistance can be obtained by substituting it into formula (7). We choose the dual-frequency sinusoidal signal measurement method and realize the accurate measurement of the real resistance of the well fluid [31]
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