Casing condition is an important concern to oilfield operators. Systems and methods are disclosed herein for using neutron logging tools to measure casing condition, using windows in the gamma energy spectrum that are sensitive and insensitive to casing condition to obtain a ratio having a value that ranges between one extreme representative of completely absent casing and an opposite extreme representative of casing in good condition. The sensitive (“divergence”) window may be positioned at or near 7.65 MeV, the characteristic energy of gamma rays from a neutron capture event by an iron nucleus. The insensitive (“consistency”) window is preferably adjacent to the divergence window with a comparable size to the divergence window. A suitable division point between the windows may be about 6.25 MeV.

A distance of a water flow path and a velocity of the water flow is calculated using data obtained from both a pulsed neutron sensor and distributed acoustic sensors. The two distance and velocity values are compared to obtain a first calculated distance and velocity. The distance of the water flow path and the velocity of the water flow are calculated using the Doppler data obtained from distributed Doppler sensors. 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 of the water flow path and the velocity of the water flow are calculated using temperature data obtained from distributed temperature sensors. 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 water flow therein.

Aspects of the subject technology relate to determining holdup compensated formation saturation while refraining from calculating holdup. Inelastic gamma spectrum data for an inelastic gamma spectrum generated downhole in a wellbore can be accessed. Further, capture gamma spectrum data for one or more capture gamma spectrums generated downhole in the wellbore can be accessed. A model that accounts for holdup measurement can be applied to both the inelastic gamma spectrum data and the capture gamma spectrum data to identify a compensated oil saturation for a formation surrounding at least a portion of the wellbore based on both the inelastic gamma spectrum and the one or more capture gamma spectrums.

A method for making natural gamma ray measurements includes deploying a logging while drilling tool including an electrical neutron output of the electrical neutron source is modulated and the natural gamma ray sensor is used to make a corresponding gamma ray measurement during at least a portion of the modulation. The gamma ray measurement is processed to obtain a corrected total natural gamma ray measurement of the subterranean formation.

An associated particle-based inspection apparatus is described. The apparatus includes a grounded target region and a neutron generator that produces a neutron and one or more corresponding charged particles. The apparatus further includes an associated particle imaging (API) detector comprising a particle detector that detects the one or more corresponding charged particles, wherein the particle detector comprises at least one particle detector element that facilitates determining a trajectory, origination time, and a velocity of the neutron based upon a detection, by a particular one of the at least one particle detector element, of the corresponding charged particles.

An example apparatus includes an outer housing and a drive shaft at least partially within and rotationally independent from the outer housing. A drill bit may be coupled to the drive shaft. At least one gamma ray sensor may be rotationally coupled to the drive shaft within the outer housing. In certain embodiments, the apparatus further includes a housing rotationally coupled to the drive shaft, wherein the housing comprises at least one pressurized cavity; and the at least one gamma ray sensor is located within the at least one pressurized cavity

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.

A distance of a water flow path and a velocity of the water flow is calculated using pulsed neutron data and noise data. The two distance and velocity values are compared with each other to obtain a first calculated distance and a first calculated velocity. The distance of the water flow path and the velocity of the water 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 of the water flow path and the velocity of the water 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 water flow in the cement interface.

A method for determining a formation thermal neutron decay rate from measurements of radiation resulting from at least one burst of high energy neutrons into formations surrounding a wellbore includes determining a first apparent neutron decay rate in a time window beginning at a first selected time after an end of the at least one burst, a second apparent decay rate from a time window beginning at a second selected time after the burst and a third apparent decay rate from a third selected time after the burst. The second time is later than the first time. A thermal neutron capture cross section of fluid in the wellbore is determined. A decay rate correction factor is determined based on the first and second apparent decay rates and a parameter indicative of the wellbore capture cross-section. The correction factor is applied to the third apparent decay rate to determine the formation thermal neutron decay rate.

A method for estimating a drilling fluid flow rate in a subterranean wellbore includes processing an acquired gamma ray spectrum in combination with standard elemental spectra and at least one standard oxygen activation spectrum to compute an oxygen activation yield. The oxygen activation yield is further processed to estimate a drilling fluid flow rate and or to infer a borehole diameter change or a lost circulation event. The gamma ray spectrum is acquired using a logging string employing a neutron source and a natural gamma ray sensor.