Neutron Activation Logging for Silicon to Aluminum Ratios
Authors: Wichmann P.A. , Webb R.W. 
Published: December 2007

A technique of neutron activation followed by detection and identification of the activation followed by detection and identification of the activation products has been developed and field tested. Although the results have not been spectacularly quantitative, they have yielded answers of sufficient qualitative significance to encourage further testing of similar methods of nuclear logging, which can ultimately extend the accuracy and scope of elemental assay.

Introduction

Activation logging for various constituent elements in reservoir rocks has been proposed by several authors in the past. Existing commercial tools employing the relatively new pulsed neutron generators have made continuous qualitative measurements for oxygen and silicon. For various reasons, these measurements have not gained wide acceptance in the oil industry.

Because there is oxygen in water and no oxygen in hydrocarbon material, it appears superficially that an oxygen log might be extremely valuable in formation evaluation. Unfortunately, several circumstances combine to make this tool less than perfect. All major reservoir rock constituents (SiO2, CACO3, and CACO3MgCO3) contain approximately one half oxygen by weight. This fact, combined with formidable borehole effects (primarily changes in borehole size), tends to mask almost completely any contrasts that might be observed as a result of variations in formation fluid saturations. The silicon log does provide much useful and reliable information, particularly in distinguishing carbonates from silicates. To determine more subtle changes in silicon concentration (as would be necessary for porosity determination), borehole effects become much more significant and quantitative interpretation has been difficult, if not impossible. Both of these continuous logging approaches depend on the optimum combination of logging speed and source-to-detector spacing to take advantage of the short decay times of the oxygen and silicon decay products (7.35-second half life for the N(16) oxygen products (7.35-second half life for the N(16) oxygen product and 2.3-minute half life for the Al(28) silicon product and 2.3-minute half life for the Al(28) silicon product). Longer-lived activation products than these product). Longer-lived activation products than these would require extremely slow logging speeds or inordinately long source-to-detector spacings for continuous logging. Therefore, logging for aluminum (whose activation product, Mg, has a 9.5-minute half life), employing principles and tools similar to those used in the experimental oxygen and silicon determinations. requires that stationary measurements be made. We do not purport to describe a commercial technique, but rather to report on useful results that may stimulate interest in elemental definition to the point that more sophisticated, commercial techniques may be developed. For example, instruments developed in the future may take advantage of means for determining the individual energy characteristics of the gamma rays from the various activation products so as to identify the specific elements with greater ease and accuracy.

Significance of the Si to Al Ratio

One of the biggest problems facing log analysts today is that of making reliable estimates of shaliness or clay content to use in correcting many of the basic logging parameters.

JPT

P. 201