The Research Of Under Well Digital Acquisition System Of Litho-density Well Logging

Energy is the lifeblood of the country’s development, in the National “TwelfthFive-Year” Technology Development Plan, it was clearly stated that the mineralresources exploration technology should be vigorously developed. In the field of oilexploration, nuclear logging has become an indispensable part of oil well logging. Atpresent, foreign logging instruments, especially the nuclear well logging instrument,are featured with good stability and high precision. The domestic instrument in suchkind, however, has not only low accuracy but also poor temperature stability.Therefore, the domestic well logging companies basically purchase the foreigninstruments or rent them at high-price from abroad.The litho-density logging tool is one of the most important instruments in nuclearwell logging technology, mainly used to determine the formation lithology andcalculate the effective porosity of the formation. The difficulty in increasing themeasurement accuracy and stability of the litho-density logging instrument is thenuclear pulse data processing and acquisition underground. Based on analysis ofproblems on the traditional spectroscopy system, this paper puts forward the digitalprocessing system scheme which suggests acquisition first and then processing.With the digital signal processing technology developing rapidly and digitalprocessors, high-speed ADC and high-temperature devices coming into the market, itis feasible to design the digital nuclear spectrum measurement system withhigh-stability. Digital spectrometer can be stable at high temperature, flexible insystem upgrading, low in price, and easy to maintain. But there exist many technicaldifficulties in the process of producing the instrument, which are mainly shown in thefollowing aspects. (1) Stabilizing the spectrum at high temperature. Oil well logging is carriedout under a high temperature and high pressure environment, multi-channelenergy spectrometer will make spectrum drift easily at high temperature, leadingto measurement errors.(2) Pulse shaping. Traditional analog circuits shaping is very slow, and it iseasily influenced by the change of environment parameters, especially in welllogging.(3) Pile-up rejection processing. This is a problem that all the nuclearspectrum of multi-channel analyzers needs to solve. In this case, the mostimportant thing is to solve the problem of the dead-time recording and how toreduce the leakage of counting.(4) Litho-density measurement. The main purpose is to solve the γ-rayproblem in low energy counting, apart from taking the instrument material intoconsideration, we need to pay more attention to working out the method to recordthe low energy pulse.In response to the above-mentioned issues, the following solutions are proposedin this paper.(1) Building new nuclear pulse processing scheme. In the pulse processing,unlike the traditional analog multi-channel spectrum measurement system whichuses the method of processing first and then acquisition, this scheme adopts themethod of acquisition first and then processing. The discretization of detectoroutput waveform signal is mainly by high-speed ADC, and then takes a digitalsignal processing algorithm which suits logging environment to complete thescreening of the signal by a programmable way, so we can reslve the problemswhich are difficult to deal with for analog technology.(2) Selecting the optimal pulse shaping algorithm. This paper has selectedthree typical algorithms based on the discussion of several common shapingalgorithms for simulation and modeling, finds the most suitable model for thehigh temperature environment under the well to improve the stability and thecount rate by modifying the parameters, and then tests the correctness of theselected algorithm by experiment.(3) Using twin peaks "feedback spectrum stabilization correction" forspectrum stabilization. Considering the shortcomings of the single-source multienergy-windows processing for high-voltage high-pressure spectrumstabilization, this paper proposes the method of twin peaks "feedback spectrum stabilization correction" for spectrum stabilization, in which programmable gainand numerical high voltage are controlled by dual-source (Am-241and Cs-137)and multi energy-windows.(4) Linear processing of spectrum drift. The ratio of two elementscharacteristic peaks channel and the corresponding energy can be expressed as alinear function which has an intercept. The linear processing of spectrum driftuses the method of dynamic function approximation, which achieves the purposeof spectrum stabilization by fixing the intercept of the function.Based on analysis of the shortcomings of traditional nuclear pulse multi-channeltechnology, this paper proposes a digital pulse processing system which is based onhigh-speed ADC and high-end FPGA, solves baseline drifting and pile-up separatingand nonlinear spectrum distortion. The characteristics of the nuclear detector pulsecan be extracted, and complete the underground electrical systems as well as the dataof high-speed nuclear pulse acquisition and process accurately and in real-time. Thetesting results of the standard well and the production well show the precision of thesystem which has reached the international level of the congeneric products.In this study, the high accuracy and temperature stability of the litho-densitylogging are improved by digital processing method, it provides a guarantee foridentification and evaluation of complex lithology logging; in addition, it canestablish a foundation for the feasibility of nuclear well logging digital processingapplications, so it has practical value and scientific significance.