ABSTRACT

Nuclear borehole instruments are ubiquitous in well logging—from bare-bones logging-while-drilling (LWD) gamma to conventional triple-combo and more exotic neutron-gamma capture (Σ) and spectroscopy tools. Now, more than ever, nuclear modeling is being used in-stead of physical experiments to design new tools, to re-late nuclear measurements to formation properties and petrophysical parameters, and even to generate "digital" calibration standards.

This paper discusses significant improvements in software and hardware used to model measurements acquired with borehole nuclear instruments. These advances make nuclear modeling more accurate, more efficient, and substantially less expensive to perform.

For qualitative interrogations of complex rock formations, The University of Texas at Austin developed UTNuPro to compute macroscopic nuclear parameters. It is a successor to the venerable SNUPAR program that supports more elements and yields better matches with Monte Carlo calculations. An inversion-based interpretation involves adjusting formation properties until a predicted nuclear log matches what was measured. Parameters computed with UTNuPro can be utilized by some inversion routines to provide first-order trends without requiring proprietary knowledge of the tool geometry.

If the user possesses details of the tool geometry, then nuclear Monte Carlo simulations can provide quantitative results. Wondering whether a nuclear log makes sense in shale when drilling with an unusual mud? Model the shale (or bed boundaries or layers, etc.) before and after invasion. Designing a new tool? It is straightforward to model different nuclear sources and detector types.

This paper explores recent and forthcoming changes in the nuclear Monte Carlo n-particle code (MCNP) that are useful to the industry. Modeling the tool geometry is much easier because MCNP now supports an "unstructured mesh" geometry that can be rendered (almost) directly from a computer-aided design (CAD) file for the tool. An "importance map" of the geometry is constructed simultaneously to speed up simulations. Such maps can also be used for fast, quasilinear modeling to implement reliable inversion-based interpretations in real-time, akin to borehole resistivity, resulting in better rock compositional assessments. New helper tools improve detector processing and particle tracking (MCNPTools), compute more realistic post-processed detector spectra (DRiFT), and produce source descriptions for complicated decay chains, such as those for U and Th necessary for natural gamma modeling (ISC). New physics includes alpha particle tracking (vital for modeling 10B-based neutron detectors) and correlated fission event generators (CGMF and FREYA). Pulse-neutron spectral interpretation is simplified because MCNP can now "tag" each photon with its element of origin.

A comprehensive modeling program requires some type of computer cluster; accordingly, this paper provides best practices for choosing appropriate hardware as well as constructing and maintaining the cluster with mini-mum effort while nonetheless satisfying the strict US Department of Energy requirements for MCNP clusters.

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