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This tutorial on neutron porosity logging is part of an ongoing series on the general topic of porosity being published in Petrophysics. Using the notion of a generic thermal neutron tool, we describe the phenomenological behavior of the tool. As a basis for a better understanding of the tool response, a brief review of neutron physics is made to introduce some important macroscopic parameters such as slowing-down length and absorption cross section. A Monte Carlo model is used to compute the response of the generic tool and to illustrate the empirical sensitivity of the tool to formation porosity and to the slowing-down length. Following this article dealing with the physics of the measurement, the next will be devoted to the interpretation aspects of thermal neutron porosity logging.
The fundamental nuclear physics principles required for a thorough understanding of gamma spectral logging are discussed in terms which should be especially helpful to persons with limited experience in nuclear techniques. After a brief discussion of radioactivity, a description of the various nuclear particles is given which leads to the discussion of artificial and accelerator produced neutron sources. The interaction of neutrons in materials by elastic and inelastic scattering and capture is treated with particular emphasis on those interactions pertinent to well logging. A brief description of cross sections is included. Neutron detection techniques utilizing B1 O(n,.c)Li7 and Li6(n,.c)H3 capture reactions and (n,p) recoil type reactions are presented.
The neutron-porosity log first appeared in 1940. It consisted of an isotopic source, most often plutonium-beryllium, and a single detector. Many variations were produced exploiting both thermal and epithermal neutrons. In most of the early tools, neutrons were not detected directly. Instead, the tools counted gamma rays emitted when hydrogen and chlorine capture thermal neutrons.
For a number of years radio activity logging devices have been used todefine lithology, bed thickness, permeable intervals, and porosity both behindcasing and within open holes. The need has long existed of defining oil andwater saturation either qualitatively or quantitatively within intervalsalready cased off from the borehole. It is now feasible to differentiate oilbearing from salt water bearing zones behind well casing with a combinationneutron curve and a chlorinilog curve, The two curves fall along the same lineopposite oil filled or fresh water filled zones. Opposite salt water filledintervals the two curves depart from each other in recorded response and thusoffer a graphic means of distinguishing hydrocarbon bearing from nonproduciblehorizons. Within the article below the logging device will be briefly describedand a background of basic theory will be given. It will be pointed out thatseveral factors influence the respective response of the standard neutron curveand the chlorinilog curve and that it is necessary to consider essentiallysimilar zones, with particular reference to such considerations as cementthickness, borehole size, casing weight, borehole fluid type, particularly whenattempting quantitative evaluation. The manner of presentation and a suggestedmethod of evaluating water saturation will be demonstrated. Various currentapplications and uses as well as promising future applications will bediscussed.
The instrument used to record both the "H" curve and the"CL" curve is of the scintillation type. In order to give consistentresults, the equipment is designed for maximum stability and efficiency. It isrefrigerated to allow operation at high temperatures without detrimentaleffects.
There are a number of methods that may be used to determine the presence ofchlorine. Among these are the neutron-neutron method, the neutron-gamma method,and the spectral analysis method. The principle of operation of any of thesemethods depends upon the relatively large thermal neutron capture cross sectionof chlorine. Table 1 gives the abundance and the neutron capture cross sectionof the elements that are of any importance in nearly all types offormations.