The measurement of slow neutrons with a proportional counter as a means of measuring saturation in laboratory flow experiments



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For many years a need has existed within the petroleum industry for a method of measuring oil saturations in a porous medium in which flow experiments are being conducted involving flow of gas and oil within the medium. Such a method should be able to measure saturation without disturbing flow patterns and should perform satisfactorily at elevated pressures which more nearly simulate reservoir conditions. The use of neutron moderation as a method for determining oil saturation is not new. Brunner and Mardock used this method when they activated rhodium plates with slowed neutrons and then counted the induced beta activity of the plates. In the present work the method of detection has been changed to simplify the operation and improve the counting rate. The rhodium plates have been replaced by a boron trifluoride proportional counter, and counts may be taken directly (thus avoiding the loss of any activity during the time previously required for transporting the plates from the point of activation to the counting location). The other existing methods used to measure saturation, such as resistivity, X-ray absorption, and internal tracers are reviewed, and the relative merits of each system are considered. Reasons for adopting neutron moderation are presented as well as reasons for adopting the simpler system of counting slowed neutrons instead of the induced beta activity used by Brunner and Mardock. The accuracy of data, obtained at two positions on a flow system, is evaluated and is found to be within 5 percent throughout the range of saturation. A saturation detector with such accuracy will find use in many of the-flow-experiments being conducted today. In particular, relative permeability measurements can be improved and extended to cover reservoir conditions with this device.