A Method for Determining the Porosity of Pulsed Neutron by Combining Gamma Energy and Time Spectrum
Authors:   School of Geosciences, China University of Petroleum (East China), Zhang Hui , Zhang Feng ,   School of Geosciences, China University of Petroleum (East China), Qiu Fei ,   School of Geosciences, China University of Petroleum (East China), Zhang Xiaoyang ,   School of Geosciences, China University of Petroleum (East China), Yu Yiming ,   School of Geosciences, China University of Petroleum (East China), Wang Han ,   School of Geosciences, China University of Petroleum (East China)
Published: November 2024

Pulsed-neutron gamma energy spectroscopy and neutron lifetime measurement systems enable the evaluation of the elemental content, saturation. And recent studies have demonstrated that neutron porosity can be determined using a measurement system consisting of a pulsed-neutron generator (PNG) with a gamma detector. However, the measured neutron gamma porosity is not the same as the compensated neutron porosity of openhole wells using Am-Be chemical sources (φCN). Algorithms introduced in the past have included matching methods for porosity using superthermal neutron counting ratios that use density to eliminate source energy differences between instruments. This study mainly focuses on the difference in the action process between the pulsed-neutron gamma instrument and the conventional chemical source instrument. The new porosity evaluation parameter Rm is constructed by extracting and combining the measurement information of gamma energy and time spectrum. The parameter uses its own measurement information while eliminating the effect of porosity differences between two measurement systems with different source energies and different detection types. It achieves the reproduction of φCN in cased wells for different instrument design structures, which is of great economic and practical value for solving the lack of openhole data in actual logging and obtaining multiple parameters of the formation. The measurement system uses a D-T source (14 MeV) with multiple gamma detectors. The fast neutrons emitted by the pulse source will first have inelastic scattering with the formation, and then elastic scattering will occur. Thus, there is a difference in the deceleration length of fast neutrons emitted by high-energy pulsed sources and conventional chemical sources. In this case, the count ratio obtained from the pulse source measurement is less sensitive to the hydrogen content index of the formation. Also, due to the different processes of production between thermal neutrons and scattered gamma rays, differences can exist between the neutron porosity determined by the gamma-to-thermal neutron counting ratio. This paper focuses on the problem of neutron porosity response mismatch caused by the dual difference of action processes between pulsed-neutron gamma and conventional compensated neutron measurement systems by extracting the information of multiple parameters in the gamma energy spectrum and time spectrum. Combining inelastic count ratios (Rin) and sigma with the capture count ratios, it is used to eliminate the effects of the differences in source energy and detection type with φCN. Thus, the measurement sensitivity is greatly improved, and the reproducibility of φCN logging response is achieved. To construct a porosity evaluation model based on the structure of the detection system, the Monte Carlo simulation method is used to model the D-T source instrument for gamma detection and the chemical source instrument for thermal neutron detection. We change the formation to different minerals and clay, then record the gamma energy and time spectrum information from multiple gamma detectors. The two auxiliary parameters, sigma and Rin, were obtained by the forward method to construct the porosity evaluation parameters (Rm). Finally, the neutron porosity evaluation model is established using Rm. Furthermore, the inelastic count ratio measured by multiple detectors constitutes an Ib parameter to eliminate the effect of the borehole on neutron porosity measurement. Finally, we processed two sand-shale logs with different diameters in Shanxi, China. Neutron porosity evaluated by our method is compared with φCN. In a low hydrogen-bearing formation (φCN < 20 p.u.), the absolute error of neutron porosity matching is reduced from 3 to 10 p.u. to less than 1 p.u. A high hydrogen-bearing formation (φCN > 20 p.u.) is reduced from the original 10 to 15 p.u. to less than 1.5 p.u. The logging example further verified the effectiveness of this method in evaluating formation neutron porosity by using a pulsed-neutron gamma detection system in cased hole. It also verified the accuracy of matching with the openhole compensated neutron porosity. This paper focuses on the problem of neutron porosity response mismatch caused by the dual difference of action processes between pulsed-neutron gamma and conventional compensated neutron measurement systems by extracting the information of multiple parameters in the gamma energy spectrum and time spectrum. Combining inelastic count ratios (Rin) and sigma with the capture count ratios, it is used to eliminate the effects of the differences in source energy and detection type with φCN. Thus, the measurement sensitivity is greatly improved, and the reproducibility of φCN logging response is achieved.