Dual Porosity CNL Count Rate Processing

Abstract

ABSTRACT The standard method of computing porosity for the two-detector Compensated Neutron Log (CNL) is to use the ratio of near to far detector count rates. This procedure has the advantage that, to first order, environmental effects tend to cancel, leaving only residual effects to be corrected using a series of departure curves. Recently, a new four-detector Dual Porosity CNL tool has been introduced which uses two epithermal neutron detectors in addition to the two standard thermal neutron detectors. Two separate porosity measurements are thus obtained, one from each pair of detectors. In clean formations, the measured porosities agree. However, in shaly formations containing high neutron absorbers the porosity measured by the epithermal detectors reads lower and agrees more closely with density derived porosity. To maintain reasonable count rates, the epithermal detectors are spaced closer to the neutron source than the thermal neutron detectors. If ratio processing is used for the epithermal measurement, the computed porosity is more sensitive to borehole effects indicating a greater difference in the response of each detector to the borehole environment. As a result of a detailed study of the tool response to many environmental variables, a new data processing technique has been developed using individual detector count rates. The method is analogous to the "spine-and-ribs" analysis developed for the FDC tool. Parameters which have been studied are borehole size, tool standoff, mudcake thickness, mud weight, borehole salinity, formation salinity, formation matrix, formation gas, temperature and pressure. Examples are given of how some of these parameters affect the individual count rates. The new processing method can be used to compute both epithermal and thermal neutron porosity. In the latter case, however, additional input parameters relating to neutron absorbers should be specified. Count rate processing has two important advantages. First, it can provide a value of porosity much less sensitive to variations in borehole parameters, particularly tool standoff for the epithermal measurement. Second, the deviation of a data point from the spine can be a measure of the effective tool standoff. This parameter can be used as a basis for an adaptive filter of the neutron porosity. Examples illustrating the improvements obtained with count rate processing are presented.