Pulsed neutron capture logging tools utilize a 14 MeV generator source and two detectors to measure the die-away rate of the induced thermal neutron capture gamma rays from the boreholes and surrounding earth formations. From the observed die-away information, one can deduce an estimate INT FM) of the intrinsic macroscopic thermal neutron capture cross section of the formation. None of the calculated exponential decay constants, however, is exactly equivalent to the true formation absorption decay constant rTRUE (=4545/ TRUE). This paper outlines the physical neutron transport factors and additional systematic error factors that are responsible for the observed differences. These factors include:

  1. decay of neutrons thermalized in the borehole erroneously being attributed to formation decay,

  2. neutron diffusion within the formation,

  3. neutron communication between the borehole fluid and formation regions, and

  4. cement effects.

The last three of these factors, and the way they affect log measurements, are discussed in the paper. Also presented is an improved method for estimating INT FM, given the measured exponential decay constants and a knowledge of borehole parameters. The correction technique is based on several hundred test pit runs using a TMD (mark of Halliburton) pulsed neutron tool in formations with a wide range of borehole conditions. The data indicate that with proper processing methods, satisfactory TMD corrections can be achieved for almost all realistic formation/borehole combinations.

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