The Compensated Neutron Log And The Effects Of Environment
Authors: Wilson Billy F. , Wichmann P.A. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.

This paper was prepared for the 49th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Houston, Texas, Oct. 6–9, 1974. Permission to copy is restricted to an abstract of not more than 300 words. illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

A large number of laboratory and field data measurements have been made to study the effect of borehole environment on porosity measurements made with a Compensated Neutron instrument.

This data includes the effect on the individual detectors as well as the effect on the ratio of the two detectors. The effects of borehole size, borehole standoff, formation lithology, casing and salinity have been analyzed.

Introduction

Since the first commercial neutron log was run in 1941 by Well Surveys, Inc., the technical development of these instruments has been steady. From the early gross detection of neutron induced gamma rays, through the direct detection of thermal neutrons, through the partial spectral discrimination of the neutron induced gamma rays, to the detection of epithermal neutrons, the emphasis has been on more meaningful - to the actual definition of porosity or hydrogen content - measurement of neutron transmissivity. The development of dual detector neutron devices - in itself no more than a logical extention of the art - and the associated electronic circuitry, is yet another step in this technical development.

The most important advances that the Dresser Atlas dual detector system, or Compensated Neutron (CN)* Log, has contributed to are the lessening of the effects that a multitude of varying borehole conditions can exert on neutron log response. In some cases, most notably borehole size changes, the improvement is dramatic. Lithology effects, on the other hand, are slightly more pronounced, though not nearly so much as those attributed to the other commercial dual detector system described in the literature.

The basic measurement from the system is a ratio of the dual detector counting rates that is converted in the logging panel to limestone porosity units. This measurement is presented on a linear scale, usually with a simultaneously run Densilog.

THE DUAL DETECTOR SYSTEM

The response of any single detector neutron log to porosity may be illustrated as a plot or series of porosity may be illustrated as a plot or series of plots of the counting rates versus porosity. Each plots of the counting rates versus porosity. Each different borehole size produces a different curve.