Dual-Spacing TDT Applications in Marginal Conditions (includes associated papers 6992 and 6989 )
Authors: Jameson J.B. , McGhee B.F. , Blackburn J.S. , Leach B.C. 
Published: December 2007

The Dual-Spacing TDT log has broadened the application of pulsedneutron-capture logs to include shaly and fresh-water formations thatformerly were considered marginal for cased-hole analytical methods. Accurate saturation determinations are thus made possible in manyadditional reservoirs. An auxiliary technique using count rates of the twodetectors has proven very sensitive to the presence of gas.

Introduction

The Dual-Spacing Thermal Neutron Decay Time(TDT*) tool is an important device for cased-holeformation evaluation logging. It uses two radiation detectors and a simultaneous gamma ray log to providein one logging run the data for determiningformation porosity, saturation, fluid identification, andcorrelation.

The technical details of the measurements have beendiscussed previously. Curves recorded includemacroscopic capture cross-section (Sigma), a ratio curve fromnear- and far-detector count rates, and a display of theactual count rates from near and far detectors. A curveshowing the level of background radioactivity is alsorecorded.

The simultaneous recording of all this information, along with the gamma ray log, provides data for acomplete reservoir evaluation in cased hole. Details of theinterpretation techniques have been discussed.However, until the introduction of the Dual-Spacing TDT in1973, such cased-hole evaluation logs were restricted inpractice to clean formations of medium to high porositycontaining relatively saline waters. The dual-spacing tool, embodying features that considerably broaden itsrange of application, permits reliable cased-holeevaluations even in formations formerly considered marginalfoe pulsed-neutron logging. Moreover, a new fluididentification technique permits the qualitative location of gasunder a wide variety of conditions. Several examplesfrom the Rocky Mountain area Illustrate the usefulness ofthe Dual-Spacing TDT in tight, low-salinity formations.

The Dual-Spacing TDT Tool

The Dual-Spacing TDT tool has a diameter of 1–11/16 in.With the normally combined gamma ray tool, it is 32 ftlong. Its pressure and temperature ratings are 16,500 psiand 300 deg. F (or higher with special equipment). The TDTmeasurements are made at a point 82 in. above the bottomof the tool.

The small diameter of the logging tool allows it to belowered through 2-in. tubing to log the cased hole belowthe packer. Good quality logs usually can be obtainedthrough both tubing and casing above the packer, or even in concentric strings of casing (except in the presence ofa gas-filled annulus - see Example 3).

The basic measurement of the tool is the thermalneutron decay time, tau. This is a measurement of the timerequired for a certain percentage of the thermal neutronpopulation to be captured in the formation. This decaytime is related to the neutron capture cross-sections of theelements in the formation. To make the measurements, bursts of high-energy neutrons are emitted periodicallyfrom a pulsed-neutron source. After each burst, theneutrons scatter throughout the region around the borehole.By successive collisions with the nuclei of elements in thesurrounding media, the neutrons are slowed to thermalvelocities. The thermal neutrons then disappear progressively as a result of capture, mainly by elements in theformation.

JPT

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