Design And Implementation Of Electronics For Downhole Circumferential Scanning Magnetic Resonance Imaging Tool

This paper first introduces a new type of downhole circumferential scanning nuclear magnetic resonance imaging instrument.The instrument uses an array of magnets to form a gradient-type magnetic field with circumferential uniformity around the well.The structure of the array antenna enables the excitation of downhole NMR RF magnetic fields from axial,radial and circumferential three-dimensional.The design of the circumferential scanning NMR electronic circuit system illustrates the method of using the array antenna to transmit high-power RF pulse signals into the formation,exciting the formation pore magnetics to generate nuclear magnetic resonance phenomena,and collecting echo signals using the array antenna.The instrument achieves circumferential zone measurement capability in the wellbore formation by switching different combinations of array antennas,using a tuning circuit to obtain radial slices of different depths of formation.Through the acquisition and processing of NMR echo signals from different azimuths,the empirical circumferential pore structure and fluid properties can be obtained,and the three-dimensional volume detection of nuclear magnetic resonance logging is realized in the true sense.The main difficulty in the design of the electronic circuit of the downhole circumferential NMR logging tool is how to realize the real-time switching of the high-power RF pulse transmission and the weak signal acquisition system in the array antenna under the premise of ensuring the short echo interval.In this paper,we focus on the following parts of nuclear magnetic resonance logging methods with circumferential resolution:(1)high-power pulse signal transmission circuit design;(2)weak signal acquisition circuit design;(3)high-voltage protection circuit design;4)Design of fast antenna recovery circuit for shortening echo interval;(5)Design of interface circuit for real-time switching of array antenna transmission acquisition;(6)Design of multi-frequency multi-slice tuning circuit for different detection depths;(7)System master control Circuit design.This paper introduces a multi-stage antenna high-voltage bleeder circuit,which can change the antenna quality factor Q step by step,can reduce the antenna quality factor to a certain extent in a shorter time,shorten the dead time of antenna recovery,and achieve short Echo interval acquisition system optimization.The electronic circuit system realizes multi-antenna nuclear magnetic resonance measurement by means of time division multiplexing.For the requirements of system measurement,it is possible to select different azimuth measurement antennas in real time and control the radial detection depth of the antenna in real time.Through the optimization design of each functional module,the paper finally realizes the application of the underground magnetic ray logging electronic circuit prototype in the downhole circumferential partition.Finally,in the partition type water tank,the self-made circumferential scanning probe test electronic circuit system prototype was successfully collected,and the effective echo signal and accurate echo string data were successfully collected.Facing complex nuclear reservoirs with high heterogeneity can effectively realize three-dimensional imaging of formation fluids by collecting nuclear magnetic signals from different circumferential orientations.