Charles H. Neuman1
(1) Chevron Petroleum Technology Company (Retired), Slidell, LA
Abstract: Investigation of density derived from pulsed neutron capture measurements
Each service company that offers pulsed neutron logs includes measurements based on the gamma rays from inelastic scattering. These measurements are influenced by formation density and are thus useful to recognize gas. One company offers a trace that represents density from the gamma rays due to inelastic scattering. However, the process of deriving the density involves the very empirical step of subtracting different portions of the gamma rays attributed to factors other than inelastic scattering until a satisfactory approximation to density is achieved. This paper presents a method for predicting the gamma rays from thermal neutrons to be subtracted from the total gamma rays measured during the neutron pulse. It includes representative results from many wells for which the density-like trace was compared with open-hole density logs recorded in Cabinda, Angola, Africa. The method presented for subtracting gamma rays due to capture is based on representing those gamma rays by two exponential decays. The fraction of gamma rays due to capture is different at different times during the pulse, but the overall effect is predicted from the amplitudes and decay times that fit the data used to represent sigma. While this process determines the fraction of capture gamma rays subtracted to create a density-like trace, the amplitude of the trace is adjusted empirically to fit expected density.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana