A method for obtaining nuclear measurement data includes measuring a burst gate source intensity. The method also includes measuring at least one of burst gate neutron count rate or a capture gate neutron count rate at a neutron detector arranged proximate a gamma ray detector. The method further includes determining, based at least in part on the burst gate source intensity and at least one of the burst gate neutron count rate or the capture gate neutron count rate, a normalized neutron count rate. The method also includes determining at least one of an inelastic spectrum or a capture spectrum. The method includes determining based at least in part on the normalized neutron count rate, at least one of a corrected inelastic spectrum or a corrected capture spectrum.

Methods of using capture gamma-ray spectroscopy for analyzing gravel-packs, frac-packs, and cement are disclosed herein. The methods can include distinguishing particles placed in a borehole region from particles placed in a subterranean formation outside of the borehole region, by utilizing a slurry comprising a liquid, particles, and a thermal neutron absorbing material to place the particles into the borehole region. The methods can also include obtaining first and second data sets by lowering into a borehole traversing the borehole region a pulsed neutron logging tool comprising a pulsed neutron source and a detector, emitting pulses of neutrons from the pulsed neutron source into the borehole region at intervals of one pulse per about 1,000 μsec for the first data set and about one pulse per about 100 μsec for the second data set, and detecting capture gamma rays resulting from nuclear reactions in the borehole and the subterranean formation.

Methods and tools for determining one or more parameters of an earth formation using time-of-flight (TOF) measurements of fast neutrons through the formation are disclosed. The disclosed tools feature a neutron source capable of emitting a population of fast neutrons having a distribution of neutron energies and one or more neutron detectors. The TOF of the fast neutrons travelling from the neutron source to the detector(s) and traversing a portion of the formation is measured and binned as a function of TOF (which is a function of neutron energy). By determining which neutron energies are attenuated by the intervening formation, the composition of the intervening formation is determined.

Embodiments of the present disclosure include a downhole inspection system including a neutron generation unit operable to emit neutrons toward a target in a wellbore. The system also includes a neutron detection unit fixed relative to the neutron generator and operable to detect thermal neutrons from the target. The system includes a shielding arrangement forming at least a portion of the neutron detection unit, the shielding arrangement blocking at least a portion of the thermal neutrons, from penetrating beyond a predetermined radial location within the neutron detection unit.

Estimating parameters of interest of a formation, including density, porosity, and fluid saturation. Methods relate to gamma ray energy spectra calibration for a radiation detector including generating a calibration radiation spectrum using measurements of radiation with the detector in a time interval wherein the radiation comprises predominantly gamma rays emitted by decay of radionuclides produced by neutron activation reactions resulting from neutron irradiation, the time interval following a prior time interval corresponding to thermal neutrons produced from the irradiation; making at least one other radiation measurement with the detector outside the time interval; and producing a calibrated radiation measurement from the at least one other radiation measurement using the calibration radiation spectrum. The measurements may be taken in the time interval by conveying the radiation detector in the borehole at high speed and using a background gate of the detector.

A nuclear logging tool has a housing, one or more neutron sources, one or more shields, and two or more detectors disposed about the housing. Each of the one or more neutron sources is configured to generate neutrons in pulses or continuously and each of the two or more detectors is operable to detect neutrons and gamma rays. The two or more detectors include a first detector disposed at a first distance from a first neutron source and a second detector disposed at a second distance from the first neutron source. The first distance is shorter than the second distance. The first distance and the second distance is measured in the longitudinal direction of the housing. Each shield is operable to absorb neutrons and gamma rays and is disposed inside the housing between one of the one or more neutron source and one of the one or more detectors.

Formation porosity is measured using a logging tool that has a pulsed neutron generator and multiple dual-function detectors that detect both neutrons and gamma rays. Ratios of thermal neutrons, epithermal neutrons, and capture gamma rays from multiple detectors are utilized to obtain multiple neutron porosities and multiple gamma-ray porosities within different depth of investigations. The neutron porosity and the gamma-ray porosity may be further corrected by excluding peak areas attributable to hydrogen and/or chlorine to reduce the shale effect and/or the chlorine effect. The neutron porosity and the gamma-ray porosity may be combined to provide improved porosity evaluations within different depth of investigations into the formation in the entire porosity measurement range (0-100 p.u.).

A method for determining formation hydrogen index includes using as input to a computer measurements of numbers of burst gamma rays (gamma rays detected during operation of a pulsed neutron source) and numbers of thermal neutron capture gamma rays made at two different axial spacings from the pulsed neutron source. A ratio of the numbers of burst gamma rays and a ratio of the numbers of thermal neutron capture gamma rays is determined. A corrected ratio of the numbers of thermal neutron capture gamma rays using the ratio of numbers of burst gamma rays is determined. The formation hydrogen index is determined from the corrected ratio.

A method and system for creating a pulsing scheme. The method may include selecting a pulsing scheme for downhole measurements using a pulsed neutron logging tool and selecting a neutron burst width for the pulsing scheme based at least in part on a neutron tube utilized by the pulsed neutron logging tool during the downhole measurements. The method may further include selecting a carbon-oxygen ratio (CO) neutron burst train for the pulsing scheme to be utilized by the neutron tube, selecting a Sigma decay time for the pulsing scheme, and performing one or more measurements with the pulsed neutron logging tool using the pulsing scheme. The system may include a pulsed neutron logging tool. The pulsed neutron logging tool may include a neutron tube disposed in a pulsed neutron generator, one or more gamma ray scintillator detectors, and one or more thermal neutron detectors.

A well logging instrument includes a radiation generator and a high voltage power supply functionally coupled to the generator. The generator and the supply are longitudinally separated by a distance sufficient for emplacement of a radiation detector. At least a first radiation detector is disposed in a space between the generator and the supply. The instrument includes an electrical connection between the supply and the generator.