Research On Multi-frequency Micro-resistivity Acquisition Circuit System Of Oil-based Mud

Micro-resistivity imaging logging technology can clearly show the distribution of stratum fractures and rock formations.This type of information helps geologists analyze geological lithology and sedimentary rock structures,and provides a data basis for oil and gas assessment.Therefore,the technology is of great significance in the field of oil and gas exploration.major.Traditional micro-resistivity logging tools are applied to waterbased muds.Water-based muds have strong electrical conductivity,and the detection signals flow through the muds and formations as conductive currents,and then into the collection electrodes.In recent years,oil-based muds have gradually replaced waterbased muds because of a series of advantages such as high temperature resistance,corrosion resistance,protection of the well wall and improved drilling efficiency.However,oil-based muds are weakly conductive muds,and the original measurement circuit has been damaged.Therefore,the AC excitation signal needs to be used to reconstruct the loop in the form of displacement current.Based on the above background,this paper designs and implements a microresistivity acquisition circuit system in an oil-based mud environment.The acquisition system realizes the time-sharing measurement of the amplitude and phase of the three frequency current signals and the signal acquisition of 90 sampling electrodes in 25 ms.At the same time,the system range covers the current signal of 50nA~1.5mA amplitude,and the dynamic range is about 90 db It has good linearity in the range.In order to simplify the complexity of circuit design,the design converts multiple measurement frequency signals to a fixed frequency before performing sampling operations and other operations.The focus of this work is the detection of weak signals.The main factor affecting signal measurement is the noise mixed in the link.The article analyzes the sources of noise in the circuit system,mainly including resistance thermal noise,operational amplifier inherent noise,analog switching noise,analog-to-digital converter noise,and external noise.Through the analysis and simulation of the mathematical model of noise,this paper presents a series of methods to suppress noise and improve the signal-to-noise ratio.Subsequent circuit and program designs follow this design method,which makes the circuit system more accurately provide the resistivity information of the formation.At the same time,the digital phase sensitive detection technology is used in the algorithm.The digital phase sensitive detection can effectively filter out the uncorrelated noise in the signal to be measured.It is found through experiments that the algorithm can extract the amplitude and phase information of the useful signal under low signal-to-noise ratio.Combined with the index requirements of scientific research projects,this article introduces the hardware and software logic design of the circuit system.The acquisition circuit system is divided into two parts,the main processing board and the acquisition board.The main processing board is responsible for signal detection,calculation,and communication functions.The acquisition board implements multi-channel signal conditioning and switching functions.In order to increase the configurability and flexibility of the circuit system,the main processing board control unit selects the cooperative processing scheme of FPGA and single-chip microcomputer.Finally,this paper builds a relevant experimental platform to test the working characteristics of the acquisition circuit system,including linearity,noise floor and repeatability,and verifies the correctness and rationality of the system design to meet the design index requirements.