The Study Of Forward Simulation And Inversion Method For Pulsed Neutron Formation Element Logging

Pulsed neutron formation element logging uses pulsed neutron generator instead of isotope source,which reduces pollution and damage to environment and staff during the process of use,transportation and preservation of the tool.Meanwhile,pulsed neutron formation element logging can get elemental concentrations and mineral concentrations in the formations,identify lithology,acquire the total organic carbon and brittleness index,and play an important role in logging interpretation and evaluation of unconventional oil and gas.This paper mainly focuses on two aspects of the forward simulation and inversion method of the pulse neutron formation logging.In the forward simulation,the spectral response characteristics of the different scintillation,the time and space distribution of inelastic and capture field around tool,and the investigation depth of inelastic and capture gamma spectrum detection depth were studied by Monte Carlo method.The signal-to-noise is improved by optimizing the design parameters of the tool.The results show that the borehole shield thickness of 1.2 cm can shield more than 60%gamma-ray from borehole,Boron-10 surface density of 0.065g/cm~2 can eliminate the capture gamma background from tool material.In the aspects of inversion of element concentrations in pulsed neutron formation elements logging,we design a set of timing sequence for pulsed neutron generator,and establish the spectral processing steps of inelastic spectra and capture spectra.The data preprocessing algorithm of pulsed neutron formation element logging was studied,and the actual test shows that the two peaks drift correction have better applicability than single peak drift correction and interpolation drift correction.The inversion method of elemental concentrations from the pulsed neutron formation element logging based on active-set algorithm is proposed,the new method is capable of constraining the solution variables within reasonable boundary limits,and thus the high-precision elemental yields can be determined by the iterative processing.The conversion of elemental yields into elemental concentrations is accomplished via an oxide closure model and a pseudo-capture spectrum method.By inverting the spectra simulated by Monte Carlo in the simple formation,complex lithology formation and unconventional formation models,and processing a field logging data by using the active-method,it is found that the precision and accuracy of elemental concentrations using the new method is better than that of the elemental concentrations based on the weighted least squares method.The interpretation method of pulse neutron formation element logging is also studied.The element response parameters of the common minerals are obtained,and a inversion method of the mineral concentrations based on a sequential quadratic programming method is proposed.The new method adds constraints to variables,and improves the accuracy of mineral concentrations inversion.In the end,the methods of obtaining the density and permeability of the formations,calculating the total organic carbon and the brittleness index by using the formations element logging are discussed.