Application of a Novel Petrophysical Model for the Development and Validation of Elemental Spectroscopy Log Interpretation

Abstract

An innovative computational approach to obtain elemental standards for isotopes has been developed with the potential to replace experimental measurements by Monte Carlo simulations. Elemental standards are essential for spectral unfolding in formation evaluation applications commonly used for nuclear well logging tools. Typically, elemental standards are obtained by standardized measurements. However, such experiments are expensive, subject to constraints. e g. impurity of the test formations and lack the flexibility to address different logging environments due to its time-consuming nature. In contrast, computer-based Monte Carlo simulations provide an accurate yet much more flexible approach to obtain elemental standards for formation evaluation. Given that Monte Carlo modelling provides a much lower cost and more dimensions of flexibility, the processing of nuclear tool logging data could be enhanced to a new level employing Monte Carlo modelled standards. In the scope of this paper, a Monte Carlo computational model has been developed for the natural gamma ray logging instrument, which is an important formation evaluation sensor and part of the geochemical pulsed-neutron tool system. Monte Carlo simulations have been performed to obtain three elemental standards: Uranium, Thorium, and Potassium. These standards are then compared to the original experimental standards obtained at the API facility test pit with the natural-radioactive source distributed throughout the formation. The Monte Carlo simulation derived elemental standards show a highly accurate match to the experimental elemental standards. Also, these elemental standards have been applied to process actual field logs and satisfactory results are obtained. A few case studies from actual wells are presented and discussed. With the validation of the Monte Carlo approach, it is highly possible to replace current experimental elemental standards with Monte Carlo computed standards for application in the geochemical pulse neutron tool system, which will significantly increase the efficiency while reducing the uncertainties for data processing. Future work will involve a series of case studies e.g. the sensitivity assessment of formation impurities and an extension of the Monte Carlo computed elemental standards to a great variety of logging tool designs.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015