Dual spaced, borehole compensated neutron well logging instrument

#1. Field of the Invention

#This invention deals with well logging techniques, in particular with an improved dual spaced neutron well logging tool that is insensitive to borehole effects and insensitive to changes in the operational characteristics of the two neutron detectors.

#2. Description of the Prior Art

#In the drawings, the FIG. 1 illustrates how the borehole influences the porosity measurement of conventional neutron logging tools, and how these borehole effects are eliminated by the present invention. The FIGS. 2 and 3 give more detailed presentations of two specific embodiments of the present invention.

#The advantages and operating principles of the invention are best understood by the principles illustrated in FIG. 1:

#Let us first consider a very basic neutron logging tool comprising only one source and one detector, S.sub.1 and D.sub.1 in FIG. 1. In this example, source S.sub.1 emits neutrons into an adjacent earth formation, and detector D.sub.1 detects backscattered thermal neutrons. It is obvious that the neutrons leaving the source S.sub.1 and entering the formation are slowed down by interfering materials such as the drilling mud or the mud cake. This reaction in the borehole effects significantly the spacial distribution of thermal neutrons in the formation and, thus, the radiation received by the detector D.sub.1. Furthermore, the thermal neutrons diffusing towards the detectors are absorbed in the (normally highly saline) drilling mud surrounding the logging tool. Therefore, the porosity indication is effected by effects at both the source S.sub.1 and the detector D.sub.1.

#In the preceding discussion, thermal neutrons were considered. Similar considerations can be made for epithermal neutrons or for thermal neutron capture gamma rays.