A New Method of Porosity Determination by D-T Neutron Generator and Dual CLYC Detector

Porosity is one of the essential parameters in conventional oil and gas reservoir evaluation, as well as plays an important role in the calculation of formation saturation and reserves estimation. Neutron-porosity logging based on the Am-Be source is the most commonly used method to obtain formation porosity. However, due to the restriction and safety issues of the Am-Be neutron source, which has been gradually replaced by the D-T source, the energy of neutrons emitted from D-T sources is significantly higher than that of the Am-Be source, resulting in reduced sensitivity of the instrument. At present, related studies have combined information from multiple neutron detectors or gamma detectors to improve the sensitivity of porosity measurement. The double particle detector CLYC (Cs2LiYCl6:Ce) can detect both gamma and equivalent thermal neutron information, which has been demonstrated in previous studies to be used for information detection in pulsed-neutron logging instruments. In this paper, based on a pulsed-neutron measurement system consisting of a D-T source and dual CLYC detectors, a new method of determining porosity is introduced, which can effectively improve the sensitivity of pulsed-neutron logging in high-porosity formations. The new pulsed-neutron measurement system consists of a D-T neutron source and a dual CLYC detector. The fast neutrons with 14MeV energy emitted by the D-T source occur inelastic scattering and elastic scattering with the formation nucleus and release inelastic gamma rays, gradually slowing down to thermal neutrons and eventually captured by the formation nucleus. Inelastic gamma rays and equivalent thermal neutron information are recorded by the new detection system with dual CLYC detector, including inelastic gamma count and equivalent thermal neutron count of the near detector as well as the far detector. The conventional neutron-porosity logging method used the count ratio of epithermal neutrons from near to far to establish a relationship with formation porosity. But compared to the Am-Be source, the energy of 14 MeV fast neutrons emitted by D-T exceeds the inelastic scattering threshold of the main elements in the formation, leading to the probability of inelastic scattering increases between high-energy fast neutrons and the formation nucleus, which weakens the proportion of elastic scattering of hydrogen nucleus in the neutron moderation process, resulting in a decrease in the sensitivity of the slowing-down length to determine porosity. Therefore, inelastic gamma rays are used by the new method of determination of porosity to describe the moderation process of high-energy fast neutrons and establish a self-saturation correction factor for porosity evaluation. Then, a new porosity evaluation parameter was combined with the self-saturation correction factor and thermal neutron count ratio, and a new porosity calculation model was established by using the parameter to achieve the purpose of improving the measurement sensitivity of high-porosity formations. On this basis, the numerical calculation model of the detection system is built by using the Monte Carlo simulation method to simulate the inelastic gamma rays and equivalent thermal neutron responses in different lithology and porosity formation so that the method of pulsed-neutron porosity by dual CLYC detector is verified. The results show that the sensitivity of the new method of determining porosity is significantly better than that of the epithermal neutron counting ratio. When the porosity is 25%, the sensitivity of the new method of determining porosity is 2.37 times that of the epithermal neutron counting ratio. At the same time, the standard deviation of porosity is 17% of the method of epithermal neutron counting ratio, effectively improving the sensitivity and accuracy of D-T source neutron porosity in high-porosity formations.