A Simulation Study On Electrode Characteristics Of Plasma Impulse Sound Source While Drilling

More accurate detection results and further detection distance are required in oil exploitation.As the emission source of deep detection acoustic logging while drilling apparatus,the plasma impulse sound source can make up for the deficiency that the current sound source can’t take into account the high and low frequency and the emission power is limited.In this thesis,a new electrode structure is proposed based on the simulation study of the electrode characteristics of plasma sound source while drilling.As the core component of sound source,the electrode structure can generate shock wave with high intensity and wide frequency band,and its size can meet the matching conditions of common borehole and drill collar size,which provides a basis for the design of sound source of LWD deep detection acoustic logging tool.Firstly,the generation mechanism of plasma impulse sound source while drilling is analyzed in this thesis.The pre-breakdown time is calculated based on the temperature of boundary water reaching 773 k and the average electric field intensity more than 8k V/cm.The shock wave intensity is calculated by the plasma control equations.Secondly,the characteristics of needle-plate electrode are analyzed by the finite element method and compared with the experimental result,the accuracy of the numerical simulation is verified.At the same time,different electrode structures are constructed for comparative analysis with the effective simulation model,and it is considered that the microporous electrode is more suitable for the electrode structure of plasma impulse sound source while drilling.Thirdly,the effects of electrode gap,hydrostatic pressure and temperature on the characteristics of microporous electrode are studied.The best electrode gap is 7.5mm.At the optimum electrode gap,the pre-breakdown time increases exponentially and the shock wave peak decreases with the increase of hydrostatic pressure.Temperature mainly affects the prebreakdown process of sound source.Finally,the microporous electrode is optimized under the optimal electrode gap.Compared with the microporous electrode,the pre-breakdown time of the optimized electrode is reduced by 37%,and the shock wave intensity is increased by 17%.The shock wave at the center of the optimized electrode can reach more than 300 d B within 10 k Hz and 290 d B within100 k Hz.The shock wave intensity at 17 cm from the center of the electrode is 1.541 MPa.