THE SPECIFIC PURPOSE MONTE-CARLO CODE MCENL FOR SIMULATING THE RESPONSE OF EPITHERMAL NEUTRON LIFETIME WELL LOGGING TOOLS

A new specific purpose Monte Carlo code called McENL for modelling the time response of epithermal neutron lifetime tools is described. The code was developed so that the Monte Carlo neophyte can easily use it. A minimum amount of input preparation is required and specified fixed values of the parameters used to control the code operation can be us ed. The weight windows technique, employing splitting and Russian Roul ette, is used with an automated importance function based on the solut ion of an adjoint diffusion model to improve the code efficiency. Comp lete composition and density correlated sampling is also included in t he code and can be used to study the effect on tool response of small variations in the formation, borehole, or logging tool composition and density. An illustration of the latter application is given here for the density of a thermal neutron filter. McENL was benchmarked against test-pit data for the Mobil pulsed neutron porosity (PNP) tool and fo und to be very accurate. Results of the experimental validation and de tails of code performance are presented.