The Neutron Lifetime Log has been recorded on digital magnetic field tape. Present known methods of Neutron Lifetime interpretation have been programmed for the IBM 1401 magnetic tape computer. The computer locates the chosen depth interval and computes apparent capture cross section ?a. Capture cross section for the rock ?r is computed as a linear interpolated value between the values for a clean water sand ?cr and a shaly water sand ?sr. Capture cross section of the fluid ?f is computed as a function of ?a, ?r and porosity. Water saturation SW and hydrocarbon HYD are computed. All values are tabulated every foot of borehole and HYD values are plotted. This machine interpretation program is compared with methods of graphs, nomographs and slide rule. Some Gulf Coast logs are used for the comparisons.
Interpretation of the Neutron Lifetime Log (NLL) is similar in procedure to interpretation of resistivity logs or most any other type of log. Analytical or empirical equations containing measured and desired parameters are solved for the desired parameters. Solution may be by charts, graphs, slide rule, or electronic computers. Our Research Department recorded the NLL in the field on digital computer type tape; and solution for this paper was made on an electronic computer. Normal and lateral borehole logging curves are measurements of electrical volume resistivity. Electrical current, artificially conducted into the formation near the borehole, produces a potential difference at two measuring electrodes. This potential, with proper calibration, is recorded as apparent resistivity Ra. Interpretation of Ra indicates various parameters of the formation. Similarly, the Neutron Lifetime Log is a measurement of the thermal neutron capture cross section, or its reciprocal, neutron lifetime. Neutrons are artificially produced and radiated into the formation, near the borehole, where they are moderated to thermal energy and absorbed. Gamma radiation, produced by absorption of the neutrons, is detected. The rate of decay of this radiation is a function of the apparent capture cross section ?a. Interpretation of ?a indicates various parameters of the formation.
