Abstract
Summary Time-dependent, three-dimensional (3D), coupled-neutron/gamma-ray Monte Carlo calculations were made for both thermal neutron decay time (TDT-K™ and TDT-M™) logging systems. Calculations were made for different borehole sizes, casing sizes, and different formation and borehole macroscopic capture cross sections (Σ). Good agreement was obtained between our calculations and laboratory TDT measurements. Monte Carlo calculations were made to separate the contributions to the detector count rates resulting from gamma rays produced in various regions, such as the borehole fluid, casing, cement, and those produced in the formation. The decay rates resulting from these various components show how the measured Σ may differ from the intrinsic formation Σ and lead to a better understanding of the relationship between the near- and far- detector measurements. The calculations also provide information on the bed-thickness response of the TDT.
Published Version
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