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使用核技术的孔隙度测井巳开展多年。早期的井下仪包括密封的中子源及单个伽马和热中子探测器。由这些仪器测定孔隙度常常很不理想,因为除地层的孔隙度之外,还有许多变量影响测量。井壁中子孔隙度仪使这些问题减少,它是将源和探测器固定在与井壁接触的压紧装置上,测定超热中子。本文介绍一种新型的中子孔隙度仪。它带有一个脉冲中子发生器,探测超热中子随时间的衰减。这种脉冲中子孔隙度(PNP)仪是基于以下原理工作:超热中子总数的衰减率主要是中子与氢的相互作用,且很少依赖于地层的岩性。PNP法的随时间而变化的测量较之稳态法减小了岩性效应,提高了固有孔隙的灵敏度。尤其,PNP法优于岩性效应较小的井壁中子孔隙度法,并能由观测的超热中子衰减获得偏距校正。脉冲中子俘获(PNC)也使用中子脉冲。但不应将PNP测井方法与PNC混为一谈。
Porosity logging using nuclear techniques has been in existence for many years. Early downhole tools included sealed neutron sources and single gamma and thermal neutron detectors. Determining porosity from these instruments is often less than ideal, as there are many variables that affect the measurement in addition to the porosity of the formation. The borehole neutron porosimeter reduces these problems by immobilizing the source and detector on the compaction device in contact with the borehole wall and measuring the superheated neutrons. This article describes a new type of neutron porosimeter. It comes with a pulsed neutron generator to detect the decay of superheated neutrons over time. This pulsed neutron porosity (PNP) instrument operates on the following principle: The decay rate of the total number of superheated neutrons is primarily the interaction of neutrons with hydrogen and is less dependent on the lithology of the formation. The measurement of the PNP method over time reduces the lithology effect compared to the steady state method and improves the sensitivity of the intrinsic porosity. In particular, the PNP method is superior to the wellbore neutron porosity method with a lower lithology effect and can be offset-corrected by the observed superheat neutron attenuation. Pulsed neutron capture (PNC) also uses neutron pulses. However, PNP logging should not be confused with PNC.