An Adaptive Spectra Fitting Method for Elemental Measurement Using a Pulsed-Neutron Tool
Authors:   University of Electronic Science and Technology of China, Ge Yi , Zhang Qiong ,   University of Electronic Science and Technology of China, Tang Wei ,   University of Electronic Science and Technology of China, Jin Ya ,   China Oilfield Services Limited, Lv Liming ,   China Oilfield Services Limited, Qu Wenbo ,   China Oilfield Services Limited, Zhang Quanwen ,   China Oilfield Services Limited
Published: November 2024

Compared with traditional radioactive sources, pulsed-neutron sources can emit high-energy neutrons in well-logging tools and are controllable. The detection of neutron-induced gammas leads to the estimation of multiple formation properties, which include elemental concentrations derived through the analysis of gamma spectroscopy, leading to the identification of mineral composition for complex reservoir evaluation. However, in the field test, due to the diversity of the environment, the extraction of the net inelastic energy spectra is difficult, and the energy resolution of the spectra is unstable. As a result, the accuracy of multiple elemental concentrations inversion is reduced. In order to improve accuracy and enhance the robustness of results, an adaptive method for obtaining elemental concentrations is introduced in this paper. The proposed method is developed by combining accurate net inelastic acquisition and adaptive energy resolution matching into a framework of elemental concentration computing. It composes of two steps: 1) Firstly, the falling edge of the pulse is automatically found on the gamma time spectra by computing the gradient of time spectra. The proportion of capture gammas in the neutron burst period is then calculated based on the position of the falling edge using a double exponential fitting iteratively of the time spectra. Even if the falling edge is not fixed due to the hardware conditions of the pulsed-neutron generator, a net inelastic gamma spectrum can be acquired. 2) The hyperparameter grid search for hyperparameter is introduced into the framework, and the optimization of energy resolution parameters is carried out while minimizing the objective function. It aims to achieve adaptive matching of energy resolution under external electronic noise and temperature change in the surrounding environment. After the operations of introducing the first and second steps, the robustness of the elemental concentration measurement is enhanced, and more accurate results are computed stably in a variety of environments based on a pulsed-neutron elemental logging tool using a BGO detector. The proposed method is validated for complex downhole environments. In several simulations of different pulsed-neutron timing schemes and formation environments, the concentration of 14 elements, such as Al, Si, Ca, etc., has been successfully obtained to verify the effectiveness of the method. And the accuracy of the method is verified in five test pits, including silica, calcium carbonate, etc. This method is applied to a well in the Bohai Sea, which contains complex metamorphic rocks, such as granite gneiss and biotite schist, with complex mineral composition. The elemental concentration calculated by this method is in very good agreement with X-ray fluorescence (XRF), which verifies its reliability. Future work will expand to mineralogy and focus on more complex reservoir evaluations in unconventional formations.