Predicting Nuclear Log Response
Authors: McDonald William J. , Palmatier Everett D. 
Published: December 2007

With the Monte Carlo procedure described here, gamma rays are traced under realistic logging conditions. It is a rapid and accurate method of generating certain spectra that cannot be derived by any other method.

Introduction

To predict the responses of nuclear logging systems and to provide guidance in designing new systems, we have used a Monte Carlo procedure to simulate the behavior of nuclear logging tools under borehole conditions. This approach bas not, to our knowledge, been used previously to develop a comprehensive theoretical model of nuclear logging, even though it has certain highly desirable capabilities not present in more standard approaches. With the Monte Carlo method, we are able to treat rather complex borehole and formation geometries and to follow in considerable detail the intricate series of interactions undergone by nuclear particles before detection.

Exploiting the ultimate capabilities and understanding the exact limitations of nuclear logging processes has been severely hampered by the lack of a comprehensive theoretical model. Heretofore we have relied primarily on empirical studies. The design of nuclear logging tools and the interpretation of data from them usually has been based on the response of the tools in test pits and boreholes under known and controlled conditions. Because of the time and expense involved, it is impractical to apply this empirical approach to every situation of interest. Consequently, to devise a scheme of log interpretation under actual borehole logging conditions, extensive extrapolation of the empirical results is necessary. These extrapolations may be unreliable; hence, they are a matter of importance and concern.

If we are willing to restrict ourselves to a study of only the unscattered particles (as Rhodes and Mott and, in a more comprehensive treatment, Czubec have done in their studies with gamma rays) so that analytical methods are applicable, we can gain a great deal of insight into the significance of various influences such as borehole diameter and bed thickness. Unfortunately, this approach fails to provide adequate understanding of tools such as the gamma-density logs and various chlorine logs, which depend on the observation of scattered gamma rays. Hence, an extension of current technology is needed.

We have found that the Monte Carlo method is particularly useful to treat the scattering of particles particularly useful to treat the scattering of particles in a complex borehole geometry. This technique is not new; it has been used previously for such diverse problems as the study of traffic flow and the design of problems as the study of traffic flow and the design of nuclear reactors. It has also been applied to problems in the petroleum industry, such as risk problems in the petroleum industry, such as risk analysis. To our knowledge, however, no one has previously used the Monte Carlo method for tracing previously used the Monte Carlo method for tracing gamma rays and neutrons under borehole logging conditions.

This paper describes the development and testing of a computer program (we call this program NUCLOG) to simulate the behavior of gamma rays under borehole logging conditions.

JPT

P. 1421