Correlated sampling in the McDNL and McPNL codes for neutron porosity and neutron lifetime log corrections

A unique correlated sampling approach has been incorporated in the McDNL and McPNL Monte Carlo codes which enables their use in correcting neutron porosity and neutron lifetime log responses for a number of important effects. The composition and/or density can be changed from that of a reference sample in any number of the geometrical zones that are used to simulate the sample and logging device in a number of comparison samples that are evaluated simultaneously. For ten additional samples calculated in this way, the computation time only increases by about 10% over that for the reference sample alone. Relative standard deviations for these samples are typically two or more orders of magnitude less than the absolute standard deviation. A modified method for obtaining results from the previously used method is suggested for more general application. Correlated sampling results are given and compared to independent calculations for porosity variations in the formation and to a simulated invasion problem