Improvements in a Through-Casing Pulsed-Neutron Density Log

摘要

The practice of measuring the formation density with arnstandard pulsed-neutron system has been described in thernliterature 1,2,3,4,5 , and has been successfully applied in manyrnreservoirs. This success leads to the obvious question: Can arnsystem be developed that is optimized for this measurement?rnTo understand which parameters should be measured,rncharacterized, or require improved response, a more exactrnanalytical model is required. The pulsed-neutron densityrnmeasurement is based on the transport of gamma rays createdrnby inelastic scattering of fast neutrons. The current densityrnalgorithm uses a diffusion model for the gamma ray transportrnprocess. This simple model does not include the effect ofrnvariations in fast neutron parameters on the process of gammarnray production. As a result, an ambiguity is introduced in therndensity log that is associated with the dynamic nature of therngamma source. The initial distribution of gamma rays is arnfunction of atom density and hydrogen content and must bernproperly described as a first step toward a more exact transportrnmodel.rnThe pulsed-neutron density measurement is analyzed andrnoptimization is achieved using theoretical models, responsesrnfrom laboratory formations and test wells, and throughrncomputer modeling. Along with data supporting an improvedrnanalytical model of the measurement, data is shown forrnincorporating fast neutron detectors and for optimizingrnspacings of the gamma ray detectors. As a result of this study,rna prototypical detector array is described for improved densityrnresponse.