Applied Basic Research Of A Three-detector Pulsed Neutron Elemental Well Logging Method

With the growing demand for oil and gas resources,unconventional reservoirs are playing an increasing significant role in world energy structure and have become the new oil and gas growth point.The differences between unconventional and conventional reservoirs in hydrocarbon forming,accumulation and preserving conditions present new challenges for nuclear well logging technology.The study of a three-detetcor pulsed neutron well logging method is carried out for the key issues such as the determination of elemental contents and total organic carbon,and the evaluation of gas saturation.The aim is to provide theoretical guidance and technical support for the development of new nuclear logging instruments,processing methods of measured logging data and their application in reservoir evaluation.The variation of the spatial distributions of fast neutron,thermal neutron,inelastic scattering and capture gamma ray under the formation with different porosities,lithologies and gas-bearing properties are studied.The changes of the measured gamma ray spectra are analyzed when formations composition is different.The results show that both gamma and neutron information can reflect lithology and gas-bearing property of the formation.In order to achieve the evaluation of elemental content and gas saturation,a multi-detector measurement system composing of a controlled neutron source,one LaBr₃ crystal and two He-3 tubes,is designed.Gamma ray energy spectra,neutron time spectra,total count of neutron and gamma ray are recorded by a specific pulse emission and information acquisition time sequence.Measuring method of the three-detector pulsed neutron well logging is formed.The key parameters such as spacings between source and detectors,and shield of the instrument are optimized using numerical simulation method.The new three-detector pulsed neutron well logging instrument is developed and ground tests have been completed.Based on the standard spectra libraries,elemental yields can be calculated by the analysis of measured spectrum.Then the yields areconverted to elemental content.Elemental standard spectra are the basic data for gamma-ray spectrum analysis.Based on the combination of experimental data and numerical simulation,the standard spectra of elements under different conditions are established,which improves the calculation accuracy of elemental yields.The application of several different algorithms in spectrum analysis is studied.Considering noise interference and statistical fluctuation of the spectrum,an algorithm based on maximum likelihood method is proposed to determine the element types and to calculate elemental yields.By using the oxide closure model,elemental yields can be converted to elemental contents.The accuracy of elemental content is influenced by the selection of the oxides.A model combining inelastic elemental yields,ratio of near to far neutron and bulk density is introduced to determine elemental content and it overcomes the dependence of elemental content calculation on oxide closure model to some extent.The neutron and gamma ray information recorded by the three-detector pulsed neutron well logging instrument can be utilized to reflect the gas-bearing property of the formation.The responses of neutron ratio,neutron to gamma ray ratio and other combined information to gas saturation are studied by numerical simulation.A method using the combined information of neutron and gamma rays is proposed to evaluate gas saturation and the new method has advantage in determining gas saturation when porosity is low.The effect of lithology,natural gas density,borehole diameter,types of borehole fluid,formation water salinity,and clay fraction conditions on the response of gas saturation measurement are studied,which provides guidance on the evaluation of gas saturation based on the three-detector pulsed neutron elemental well logging.