A New Approach for Log Analysis of the Pulsed Neutron and Resistivity Log Combination

Abstract

JPT Forum articles are limited to 1,500 words including 250 words for each table and figure, or a maximum of two pages in JPT. A Forum article may present preliminary results or conclusions of an investigation that the present preliminary results or conclusions of an investigation that the author wishes to publish before completing a full study; it may impart general technical information that does not warrant publication as a full-length paper. All Forum articles are subject to approval by an editorial committee. Letters to the editor are published under Dialogue, and may cover technical or nontechnical topics. SPE-AIME reserves the right to edit letters for style and content. Introduction A current technique for evaluating the pulsed neutron-resistivity combination involves crossplotting the capture cross-section, Sigma, as read from the log vs true formation resistivity, Rt, on an inverse-square-root scale. The data points must be from clean zones with a limited range of points must be from clean zones with a limited range of porosities. The Sigma - Rt crossplot generates a clean porosities. The Sigma - Rt crossplot generates a clean oil-water straight line with a slope equal to a R w (Sigma w - Sigma hy). Values of water saturation can be determined directly from this straight line.This paper presents a different approach for evaluating clean zones of a single lithology using the pulsed neutron-resistivity combination. The method has the same capabilities as the technique mentioned previously, plus the following advantages. plus the following advantages. 1. This method permits analysis in the presence of porosity variations. porosity variations. 2. The value of the porosity exponent, m, does not have to be assumed as 2.0.3. Under favorable conditions, the method allows us to estimate the value of the water saturation exponent, n, from logs alone.The procedure discussed here is inspired by the pattern recognition approach introduced by Pickett. Theory The fundamental equations for log analysis are (1) (2) (3) The basic equation for quantitative analysis of clean formations from pulsed neutron logs is (4) Solving for porosity we obtain (5) Combining Eqs. 2, 3, and 5 and taking the logarithm of both sides leads to (6) where (7) Analysis of Eqs. 6 and 7 indicates that a log-log crossplot of Rt vs Sigma - Sigma ma for zones that are 100% saturated with water must result in a straight line with a slope equal to -m, provided the values of a, R w, m, Sigma ma, and Sigma w are constant. If the value of Sigma ma is not known, this type of analysis allows us to determine Sigma ma by trial and error. In fact, if the value of Sigma ma is incorrect, it is impossible to obtain the desired straight line, and only curves result.Once the 100% water-bearing trend is established, we can create a family of parallel lines, each representing a value of water saturation. JPT P. 415