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Neutron porosity measurements are among the basic well logging services provided to operators. Most commonly, this measurement is performed by well logging tools equipped with chemical neutron sources, such as AmBe or Cf-252, using He-3 neutron detectors. The recent dramatic decrease in the He-3 gas supply and potential governmental restrictions on the use of chemical sources are likely to make the current form of neutron porosity measurements quite impractical. The potential solutions to these problems are to use a pulsed neutron generator (PNG) as replacement of chemical sources and Li-6 glass scintillation neutron detectors as an alternative to He-3 detectors. Well logging tools equipped with PNGs and Li-6 glass detectors have additional advantages, specifically being capable of performing formation Sigma (Σ) measurements because of Li-6 glass detector sensitivity to gamma rays, and controllable neutron output of the PNG. This paper discusses how to extract the neutron and gamma ray signal from the total spectra recorded with Li-6 glass detectors. It also presents results of the neutron porosity measurements obtained with a generic slim cased-hole test article fitted with two Li- 6 glass detectors. Spectral decomposition parameters of the neutron capture spectrum, which are correlated to the thermal neutron flux, show stable and prominent dependence on porosity. Simultaneously measuring the time spectra for each record introduces the advantage of a combined formation Sigma and neutron porosity measurement. The possibility of using time spectra for determining diffusion corrections for measured Σ values is discussed as well. Finally, the example of an openhole neutron porosity log measured using this test article is presented. It will be compared with the neutron porosity values obtained from the measurements made by a chemical source compensated neutron porosity tool.
The reasons to move away from chemical neutron sources, such as the commonly used AmBe, are numerous.
Zhang, Feng (China University of Petroleum (East China)) | Tian, Lili (China University of Petroleum (East China)) | Liu, Juntao (China University of Petroleum (East China)) | Zhang, Quanying (China University of Petroleum (East China)) | Wang, Xinguang (China University of Petroleum (East China)) | Chen, Qian (China University of Petroleum (East China))
In the exploration of shale gas reservoir, the evaluation parameters such as lithology and gas content has the great significant for search of "sweet spot". This article introduces a multi-detectors pulsed neutron logging technology used for determining formation gas saturation and elemental concentration. The response of thermal neutron and gamma count ratio to gas saturation and gamma specturm to elemental concentration under borehole condition was simulated by using Monte Carlo method. A controlled neutron element tool (CNET) based on D-T neutron generator, two He-3 tubes and a LaBr3 detector was developed. This new logging technology was applied in Xinjiang oil field and the elemental interpretation results is consistent with the core analysis result.
Presentation Date: Thursday, October 20, 2016
Start Time: 10:10:00 AM
Presentation Type: ORAL
Refinements in radiation logging techniques during recent years have involved increasing usage of scintillation detectors. These detectors produce voltage pulses whose heights are related to the energies of the gamma rays which initiate them. Analysis of the gamma-ray spectrum, as indicated by the pulse heights, yields information about the chemical elements composing the formations surveyed. Refined scintillation counter techniques can furnish chemical information concerning earth formations in situ, from a study of the gamma-ray spectra emitted by the formation either naturally or as a result of neutron bombardment.
Accompanying the rising interest in gamma-ray scintillation spectroscopy, there has been increased activity in the development of accelerator-type neutron sources (in contrast to encapsulated chemical-mixture sources). Such neutron generators are attractive for several reasons: (1) they greatly reduce radiation hazards to personnel; (2) there is a great reduction in contamination danger if they are lost in the hole; (3) they can produce larger neutron intensities than can conveniently available encapsulated sources; and (4) they are capable of being pulsed, thus permitting new techniques in logging.
In the past, both accelerator and encapsulated neutron sources have been used by others in conjunction with scintillation-detector pulse-height analysis. The results have not been too encouraging, due to the interference among different gamma-ray spectral lines and to the fact that the gamma-ray peaks were not too clearly distinguishable above the large and ill-defined background "noise".
This paper is a status report on laboratory studies of a technique using a borehole accelerator as a neutron source, which gives an improved scintillation spectrum, thus permitting more accurate chemical analyses of the formations penetrated. The Schlumberger-accelerator neutron source is presented; the origins of inelastic and of thermal-neutron, capture gamma rays are discussed, and results are given for some laboratory measurements performed in borehole geometry.
ABSTRACT Pulsed neutron porosity (PNP) logging is a new method of determining formation porosity in which the die-away of epithermal neutrons with time is measured following emission of pulses of neutrons. The pulsed neutron technique offers superior porosity sensitivity and decreased lithology dependence in comparison to steady-state neutron porosity logs. A prototype PNP logging tool has been constructed and tested. The tool contains a neutron tube and pulsing control for production of 14-MeV neutrons, an epithermal neutron detector (3He covered with Gd foil), and a time analyzer for determining the time of neutron detection relative to the production pulse. Epithermal neutrons build up during the production pulse, and then after the pulse they die away at a rate that varies with formation porosity. The porosity sensitivity is superior to that of existing acoustic, density, and compensated neutron techniques. Field tests in several wells have demonstrated excellent repeatability. Comparison to core porosity is very good. KK INTRODUCTION Porosity logging using nuclear techniques has existed for many years. Early tools contained encapsulated neutron sources and single gamma-ray or thermal-neutron detectors.