Use of MCNP perturbation feature for nuclear well logging Monte Carlo sensitivity calculations

In the oil industry a log is a record of the measurements taken by the instrument, called a sonde, in an attempt to determine a geology through which a drill is passing. Nuclear sondes probe the formation with very energetic neutrons and gamma rays. The sonde is typically about half the diameter of the borehole and contains a source, detectors and shielding. In this work the sensitivity of detector response to the borehole fluid, detector helium gas and formation matrix density changes has been tested with well logging benchmark, containing a neutron source and two 3He neutron detectors. The MCNP4C code standard perturbation feature, using the first- and second-term Taylor series expansion of perturbed quantity, has been applied for differential sampling. The volume averaged neutron flux and (n,p) reaction rates were the tallying quantities. Calculation results show that the MCNP perturbation feature can be fairly successfully implemented as initial step of well logging sensitivity studies. However, one should always keep in mind the limitations resulting from a two-term Taylor series expansion, currently used in MCNP. (author)