A Method for Improving the Evaluation of Elemental Concentration in Neutron-Induced Gamma-Ray Spectroscopy Logging

Abstract

The determination of elemental concentrations, mineralogy and lithology is essential for the evaluation of unconventional plays. Several instruments utilizing D-T generator and gamma ray detectors have been developed to determine elemental weight fractions. The measured gamma ray energy spectrum can be approximated by a linear combination of the standard spectra of individual elements. Each geochemical well logging instrument has its unique set of elemental standards, which can be obtained by experimental approaches. The variations of the shapes of elemental standards are not considered under different conditions and it affects the accuracy of the elemental concentrations. A decomposition of measured spectrum is carried out based on the elemental standard spectra to determine the relative elemental yields, which represent the contribution of gamma rays emitted by each element to the measured spectrum. Then the elemental yields can be converted to elemental weight fractions by using oxide closure model. The standard gamma ray spectra of individual elements are calculated by using Monte Carlo numerical simulation. The responses of standard spectra under different conditions such as different borehole sizes and drilling mud types, are simulated. The reasons for the change of the standard spectra shape are analyzed. A model based on gamma ray attenuation is introduced to compensate for the effects of logging conditions on the changes of standard spectra. The simulation results show that variation of logging conditions have impacts on the shapes of the standard libraries. Therefore, if only one set of fixed standard spectra is utilized in processing measured spectra, the accuracy of elemental concentrations is affected. To address this problem, a model for compensating for the variations of the shapes of standard spectra is proposed. A field example of pulsed neutron spectroscopy logging is illustrated to validate the proposed method and results show that the agreement between computed elemental weight fractions and core analysis values is improved. In the processing of geochemical logging data, changes in the shapes of the standard spectra were not considered. One set of fixed elemental standards are always utilized. In our study, a specific Monte Carlo simulation code is used to study the changes of elemental standards due to the variation of logging conditions and a method is proposed to improve the accuracy of elemental concentration measured by geochemical well logging.