A new dual-spacing neutron tool developed for porosity determination and gas detection in open or cased boreholes is combinable with various other logging tools. Simultaneous recording saves rig time, and an overlay log presentation facilitates wellsite interpretation. presentation facilitates wellsite interpretation. Introduction A two-detector neutron tool has been developed to determine porosity in open and cased boreholes. The response of the tool is determined by the rate at which the thermal neutron population decreases with distance from the source. This rate of decrease is principally determined by the porosity of the formation. The quantity measured is the ratio of counting rate from the near-spaced detector to that from the far-spaced detector. The surface control panel converts this ratio to a porosity index, which is then displayed on a linear scale. Individual counting rates, count-rate ratio, and porosity index may also be recorded on magnetic tape for subsequent machine computation. The dual-spacing measurement provides compensation for the effects of hole conditions - salinity, temperature, mud type, and mud cake. The residual effects are generally small. The log has therefore been named the compensated neutron log (CNL*). The porosity uncertainty caused by statistical variations is diminished through the use of a high-yield chemical neutron source. A unique feature of the tool is that it may be run in combination with other logging tools such as the formation density, thermal decay time. sonic, and induction-spherically focused logs. Such combinations make possible overlay presentations for wellsite interpretation, and offer the advantages of reduced rig time and positive depth matching. Dual-Spacing Neutron Detection Principle Principle Fast neutrons from a point source radiate into an infinite homogeneous medium and are slowed down by repeated collisions until they are in thermal equilibrium with the atoms of the medium. The thermal neutrons continue to diffuse, but, on the average, suffer no further energy loss. Eventually they experience absorbing collisions (capture collisions) and disappear from the system. In the process of slowing down, the neutron "cloud" spreads out spatially. The width of this spatial distribution is frequently characterized by a single property, called the slowing-down length. This is a measure of the size of the cloud of neutrons of a particular energy. It is proportional to the mean particular energy. It is proportional to the mean distance that neutrons travel from the source in slowing down to that energy. An alternative theoretical model emphasizes the so-called group-diffusion theory. In this case, one treats all the neutrons in the medium (usually down to some preassigned energy) as if they had the average energy of the group. This approach, also, leads to the definition of a characteristic parameter, the diffusion length, which is a measure of the width of the spatial distribution of the neutrons in the group. P. 1073

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Depths of investigation of a Density tool and several versions of Neutron tools hate been measured experimentally at a porosity of 22 percent in quartz sandstone. Comparisons with earlier results in 35-percent-porosity sandstone indicate little change in depth of investigation over this porosity range. The results also illustrate dependence of depth of investigation on casing and cement, neutron-source energy, relative change in hydrogen content, and presence of thermal-neutron absorbers. presence of thermal-neutron absorbers.

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