A distance of a gas flow path and a velocity of the gas flow therein are calculated using pulsed neutron data and noise data. The gas saturation and distance to flow path obtained from the pulsed neutron data and gas velocity and distance to flow path obtained from the noise data are compared with each other to obtain a first calculated distance and a first calculated velocity. The distance and the velocity of the gas flow are calculated using Doppler data. The distance and velocity values are compared with the first calculated distance and first calculated velocity to obtain a second calculated distance and velocity values. The distance and the velocity of the gas flow are calculated using temperature data. The distance and velocity values are compared with the second calculated distance and velocity to determine a distance of a cement interface and a velocity of a gas flow therein.

Methods and systems for determining formation salinity using pulsed neutron (PN) tools are described. Embodiments of the described methods involve binning chlorine yields or chlorine count rates arising from capture events into early and late capture regimes, which may be used to attribute the events to either the borehole or the formation.

Methods and systems for determining bulk density and/or neutron porosity of a formation are described herein. The methods and systems use a pulsed neutron (PN) tool and may be performed with a tool having a single gamma detector though tools with multiple detectors may be used as well. The PN tool may be a geochemical logging tool. The methods and systems involve partitioning the time spectrum into pluralities of bins that are indicative of non-clay mineral elements and of shale/clay to the overall bulk density.

Wellbore logging apparatus include a positioning mechanism that couples one or more well logging devices included in a well logging tool to a downhole tractor. The positioning tool allows the logging tools to be moved at a different rate of travel through a wellbore compared to the rate of travel of the downhole tractor through the wellbore.

A neutron porosity tool having an electronic neutron generator arrangement and a control mechanism used to provide voltage and pulses to an electronic neutron tube is provided, the neutron generator arrangement including: at least one vacuum tube; at least one ion target; at least one radio-frequency cavity; at least one high-voltage generator; at least two neutron detectors; at least one pulser circuit; and at least one control circuit. A method of controlling a neutron porosity tool having an electronic neutron generator arrangement and a control mechanism that provides voltage, and pulses to an electronic neutron tube, the method including at least: controlling a bipolar neutron tube to produce two distinct neutron reactions; using a control circuit to modify the output of a pulser circuit; and using a plurality of neutron detectors to determine formation response offsets.

Disclosed herein are methods and system for determining an elemental composition of a downhole formation using neutron-induced gamma ray spectroscopy and doping the borehole fluid with a neutron absorber to remove at least part of the borehole thermal neutron flux before it can capture borehole nuclei and emit gamma rays within a spectral range of interest. For example, a method for determining the elemental composition includes adding a dopant to a borehole fluid, wherein the dopant absorbs thermal neutrons in the borehole, lowering a logging tool comprising a gamma ray spectrometer, emitting neutrons into a downhole environment, generating neutron-induced gamma rays comprising borehole gamma rays and formation gamma rays, detecting gamma rays from the downhole formation, and extracting the elemental composition of the downhole formation. The dopant may not emit gamma rays above 0.5 MeV or their peaks background should be predictable and/or consistent to be removed during data processing.

The present disclosure is directed to methods for evaluating a gravel pack, a frac-pack, or cement in a wellbore. In at least one embodiment, a method for evaluating a gravel pack, frac-pack or cement in a wellbore, includes pumping a first material into the wellbore, wherein the first material comprises a first tracer that is not radioactive. The method includes pumping a second material into the wellbore, wherein the second material comprises a second tracer that is not radioactive. The method includes obtaining a set of data using the downhole tool in the wellbore after the first and second materials are pumped into the wellbore. The method includes obtaining a baseline using the downhole tool in the wellbore in a depth interval without the first or second material. The method includes comparing the set of data with the baseline.

A variety of methods and apparatus are disclosed, including, in one embodiment, a pulsed neutron logging tool for a borehole in a subterranean formation, including: a pulsed neutron generator to broadcast neutrons into the subterranean formation; and a gamma ray scintillator detector comprising cerium bromide (CeBr.sub.3), wherein the pulsed neutron logging tool (e.g., as lowered into the borehole) is configured to detect neutron-induced gamma rays from the subterranean formation and natural gamma rays from the subterranean formation.

The techniques as described herein enhance the accuracy and precision of mineralogy analysis in elemental spectroscopy logging by utilizing oxide standards and/or rock-forming mineral compounds as reference materials that provide more representative and realistic signatures of geological formations. A method comprises logging a wellbore with a tool and fitting reference spectra from oxide compounds and/or rock-forming mineral compounds against measured gamma spectra obtained with the tool, to characterize a subterranean formation that the wellbore extends through.

A system and a method for a pulsed neutron logging tool. The pulsed neutron logging tool may include a pulsed neutron generator for generating one or more neutrons, one or more gamma ray scintillator detectors disposed in a detection area, a shielding block that separates the pulsed neutron generator from the one or more gamma ray scintillator detectors, and a shielding layer. The pulsed neutron logging tool may further comprise an information handling system communicatively connected at least in part to the pulsed neutron logging tool. The information handling system is configured to create a gamma ray spectrum from the one or more gamma rays detected by the one or more gamma ray scintillator detectors, remove a background from the gamma ray spectrum to form a peaks-only spectrum, and compare one or more peaks-only elemental standards to the peaks-only spectrum to determine an elemental weight and yield.