A method for determining a material in an annulus between two objects disposed in a borehole includes positioning logging tools in the borehole, each of the logging tools being oriented at a different angle. The method also includes measuring a property of the material in the annulus using the logging tools and determining the material in the annulus based on the measured property.

An x-ray based cement evaluation tool for measurement of the density of material volumes within single, dual and multiple-casing wellbore environments is provided, wherein the tool uses x-rays to illuminate the formation surrounding a borehole, and a plurality of detectors are used to directly measure the density of the cement annuli and any variations in density within The tool uses x-rays to illuminate the casing surrounding a borehole and a plurality of multi-pixel imaging detectors directly measure the thickness of the casing The tool includes an internal length having a sonde section, wherein the sonde section further includes an x-ray source; a radiation shield for radiation measuring detectors; sonde-dependent electronics; and a plurality of tool logic electronics and PSUs. Other systems and subsystems appropriate for carrying out the foregoing are also disclosed, as are a plurality of example methods of use therefor.

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.

Systems, methods, and devices for determining a neutron-gamma density (NGD) measurement of a subterranean formation that is accurate in both liquid- and gas-filled formations are provided. For example, a downhole tool for obtaining such an NGD measurement may include a neutron generator, a neutron detector, two gamma-ray detectors, and data processing circuitry. The neutron generator may emit neutrons into a formation, causing a fast neutron cloud to form. The neutron detector may detect a count of neutrons representing the extent of the neutron cloud. The gamma-ray detectors may detect counts of inelastic gamma-rays caused by neutrons that inelastically scatter off the formation. Since the extent of the fast neutron cloud may vary depending on whether the formation is liquid- or gas-filled, the data processing circuitry may determine the density of the formation based at least in part on the counts of inelastic gamma-rays normalized to the count of neutrons.

A method for determining a corrected neutron gamma density of a formation includes emitting neutrons into a formation using a neutron source to generate gamma-rays. Additionally, the method includes detecting a first count rate of gamma-rays and a gamma-ray spectrum using at least a gamma-ray detector of the downhole tool. The method also includes detecting a second count rate of neutrons using a neutron detector. The method includes using a processor to perform a gamma-ray spectroscopy analysis on the formation based on the gamma-ray spectrum and determining a correction based on results of the gamma-ray spectroscopy analysis. The method includes applying the correction to the first count rate or the second count rate and determining a neutron gamma density of the formation based on a first corrected count rate of gamma-rays or a second corrected count rate of neutrons. The method also includes outputting the determined density of the formation.

Sonic data, ultrasonic data, density data, cased-hole neutron data, and open-hole neutron data of the wellbore are obtained. The sonic and ultrasonic data provides the amplitude, frequency, and phase of the altered sonic and ultrasonic waves. The far counts, near counts, and energy spectrum are obtained from density data, cased-hole (CH) neutron data, and open-hole (OH) neutron data. The amplitude, frequency, and phase provide the interface densities of the first, second, and third interfaces. The hydrogen index (HI) of the formation and the cased wellbore are obtained from the CH and OH neutron data. The widths of the second and third interfaces are obtained from the HI's and the densities of the second and third interfaces.

Systems and methods for estimating absolute elemental concentrations of a subterranean formation from neutron-induced gamma-ray spectroscopy are provided. In one example, a system for estimating an absolute yield of an element in a subterranean formation may include a downhole tool and data processing circuitry. The downhole tool may include a neutron source to emit neutrons into the formation, a neutron monitor to detect a count rate of the emitted neutrons, and a gamma-ray detector to obtain gamma-ray spectra deriving at least in part from inelastic gamma-rays produced by inelastic scattering events and neutron capture gamma-rays produced by neutron capture events. The data processing circuitry may be configured to determine a relative elemental yield from the gamma-ray spectra and to determine an absolute elemental yield based at least in part on a normalization of the relative elemental yield to the count rate of the emitted neutrons.

Various embodiments include apparatus and methods to conduct measurements on a formation using a dual purpose detector. The dual purpose detector can be disposed on a neutron tool in a borehole, where the dual purpose detector operatively detects neutrons and gamma rays in response to the formation being probed using a neutron source of the neutron tool. Porosity of the formation can be determined from data based on the neutrons and gamma rays detected by the dual purpose detector. Additional apparatus, systems, and methods are disclosed.

Systems, methods, and devices for inelastic gamma-ray logging are provided. In one embodiment, such a method includes emitting neutrons into a subterranean formation from a downhole tool to produce inelastic gamma-rays, detecting a portion of the inelastic gamma-rays that scatter back to the downhole tool to obtain an inelastic gamma-ray signal, and determining a property of the subterranean formation based at least in part on the inelastic gamma-ray signal. The inelastic gamma-ray signal may be substantially free of epithermal and thermal neutron capture background.

A measurement tool, primarily for use in a bore hole, has a neutron source located within a housing and a neutron-based detector located within the housing. At least one of the neutron source and the neutron-based detector offset from a longitudinal axis of rotation. The axially offset neutron source or neutron-based detector allow directional measurements to be made of the formations surrounding the bore hole by rotating the tool within the bore hole.