Methods and systems for determining the elemental composition of formation rock are disclosed. The systems include a pulsed-neutron geochemical logging tool that is conveyed in a borehole traversing the formation. The pulsed-neutron geochemical logging tool can collect inelastic and capture neutron spectra. The inelastic and capture spectra are processed to provide the elemental composition of formation rock after removing contributions due to elements in the borehole and in the pores of the formation.

A first example azimuthal neutron porosity tool for imaging formation and cement volumes surrounding a borehole is provided, the tool including at least an internal length comprising a sonde section, wherein said sonde section further comprises one sonde-dependent electronics; a slip-ring and motor section; and a plurality of tool logic electronics and PSUs. An alternative azimuthal neutron porosity tool for imaging formation and cement volumes surrounding a borehole is also provided, the tool including at least a far space detector; a near space detector; and a source located within a moderator shield that rotates around an internal tool axis.

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.

Embodiments of the present disclosure include a method that includes activating a neutron generation unit operable to emit neutrons toward a target for a first period of time. The method also includes recording first measurement data, via a detection unit, during the first period of time. The method further includes deactivating the neutron generation unit after the first period of time. The method also includes processing at least a portion of the first measurement data after the first period of time, the first measurement data being correlated to burst gate. The method includes recording second measurement data, via the neutron detection unit, during a second period of time, the second measurement data being correlated to a capture gate.

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.

A system that may identify when a pulsed neutron generator is operating while disposed in an undesirable environment, such as in air, may include a pulsed neutron generator designed to emit neutrons in an environment. The system may also include a radiation detector designed to take measurements of the neutrons. The system may also include data processing circuitry designed to determine if the environment surrounding the pulsed neutron generator is air based at least in part on a neutron signal obtained by the radiation detector. The determination may include comparing one or more characteristics of the neutron signal with corresponding reference characteristics.

A method for evaluating induced fractures in a wellbore includes obtaining a first set of data in a wellbore using a downhole logging tool. A first proppant is pumped into the wellbore, after the first set of data is captured. The first proppant includes a first tracer that is not radioactive. A second proppant is also pumped into the wellbore, after the first proppant is pumped into the wellbore. The second proppant includes a second tracer that is not radioactive, and the second tracer is different than the first tracer. A second set of data is obtained in the wellbore using the downhole tool after the first and second proppants are pumped into the wellbore. The first and second sets of data are compared.

A method for determining a fractional volume of at least one component of a formation includes entering into a computer a number of detected radiation events resulting from imparting neutrons into the formation at an energy level of at least 1 million electron volts (MeV). The detected radiation events correspond to at least one of an energy level of the imparted neutrons and thermal or epithermal energy neutrons. A measurement of at least one additional petrophysical parameter of the formation is made. The at least one additional petrophysical parameter measurement and at least one of a fast neutron cross-section and a thermal neutron cross-section determined from the detected radiation events are used in the computer to determine the fractional volume of the at least one component of the formation. In another embodiment, the fast neutron cross-section and the thermal neutron cross-section may be used on combination to determine the fractional volume.

Embodiments of the present disclosure include a method that includes activating a neutron generation unit operable to emit neutrons toward a target for a first period of time. The method also includes recording first measurement data, via a detection unit, during the first period of time. The method further includes deactivating the neutron generation unit after the first period of time. The method also includes processing at least a portion of the first measurement data after the first period of time, the first measurement data being correlated to burst gate. The method includes recording second measurement data, via the neutron detection unit, during a second period of time, the second measurement data being correlated to a capture gate.

A method for evaluating induced fractures in a wellbore includes obtaining a first set of data in a wellbore using a downhole logging tool. A first proppant is pumped into the wellbore, after the first set of data is captured. The first proppant includes a first tracer that is not radioactive. A second proppant is also pumped into the wellbore, after the first proppant is pumped into the wellbore. The second proppant includes a second tracer that is not radioactive, and the second tracer is different than the first tracer. A second set of data is obtained in the wellbore using the downhole tool after the first and second proppants are pumped into the wellbore. The first and second sets of data are compared.