Abstract: In some aspects of the invention, a computer-implemented method of obtaining sigma values from pulsed neutron capture decay curve is disclosed. The method can include selecting spectra collected by pulsed neutron instrument disposed down a borehole traversing a rock formation, the spectra including capture interactions and inelastic interactions; and modeling the selected spectra using a nonlinear model to obtain sigma values relating to one or more reservoir properties.

Abstract: In some aspects of the invention, a computer-implemented method of obtaining sigma values from pulsed neutron capture decay curve is disclosed. The method can include selecting spectra collected by pulsed neutron instrument disposed down a borehole traversing a rock formation, the spectra including capture interactions and inelastic interactions; and modeling the selected spectra using a nonlinear model to obtain sigma values relating to one or more reservoir properties.

Abstract: An apparatus for measuring the density of a subterranean formation from within a wellbore, especially a cased wellbore. The apparatus has an energy source configured to generate 14 MeV of neutrons in pulses of 20 microseconds or less. The apparatus has at least three detectors for detecting gamma rays which are produced as a result of the neutron pulse. The detectors and energy source are aligned along a central axis with the energy source being at one end. Shielding is disposed between each of the three detectors and between the end detector and the energy source adjacent to it. The detectors are configured to measure gamma rays below 700 KeV and generate a signal in response thereto. The signals may then be compared to predetermined characteristic signals or computer simulations to determine the density, and hence porosity, of the formation.