METHOD FOR EXTRACTING THE VALUE OF THERMAL CAPTURECROSS-SECTION FROM PULSED NEUTRON TOOL DATA BY NMR-LIKEINVERSE LAPLACE TRANSFORM

摘要

Pulsed neutron capture (PNC) tools measure the rate ofdecay of γ -rays resulting from neutron capture. PNCtools are often used to measure reservoir salt watersaturation in cased-hole conditions from within thecompletion. The various elements of the completion:casing, tubing, borehole fluid, cement sheath quality,and borehole size, all contribute to the borehole signalthat must be separated from the total signal to obtain anaccurate formation thermal capture cross-section, Σ.The physics of the process dictates that this decay isexponential, and thus the measurement is analogous tothe exponential relaxation process studied with pulsednuclear magnetic resonance (NMR) tools. The largebody of knowledge on processing of NMR data can beleveraged to help solve some of the problems associatedwith processing and interpreting the PNCmeasurements. This paper describes how the existingNMR inversion code can be successfully applied toPNC data from different PNC tools.The advantages and disadvantages of this approach arediscussed, and several log examples using differenttools and detectors are presented.The conclusion of this work is that this method issignificantly more effective in separating borehole andformation components than any of the other existingmethods. Since this separation is the principal difficultywith PNC logging today, the extracted value offormation Σ is thus less affected by the boreholeenvironment, requiring smaller and more stableenvironmental corrections. The method also provides amore effective visual quality control of the validity ofthe computed Σ values. This conclusion is supported bylaboratory data and log examples.