ABSTRACT

Although epithermal neutrons are less sensitive to thermal absorbers and formation temperature than thermal neutrons, they have not been extensively utilized in dual-detector porosity tools, presumably because of their lower count rates. However, dual-spaced epithermal neutron tools are now available with adequate count rates to provide good porosity measurements in low to medium porosity ranges. They can also provide accurate porosity measurements in gas-filled boreholes, where thermal neutron tools exhibit very little sensitivity to porosity. Since even epithermal neutron tools are strongly influenced by borehole diameter, lithology, standoff, and salinity, it is very difficult to completely characterize tool response with the limited number of test pits that are available. Therefore, it is necessary to have a model which can predict tool response for all combinations of these various conditions. Unfortunately, the processes involved are very complicated. Monte Carlo methods are reliable, but they are too expensive and time consuming to use extensively. Faster computational methods are available, but they cannot handle all aspects of the tool response. Consequently, a simple, semi-empirical model was developed. The primary variables of the model are the slowing down length, which is related to the distance a source neutron travels before it has lost enough energy to be detected, and the removal cross section, which determines how far a detectable neutron will travel before it has lost too much energy to be detected. Other variables include borehole diameter and salinity. The relationship between count rate and these variables is broken into components, each of which represents a different phenomenon. The mathematical representation of each phenomenon is taken to be the simplest function that matches the boundary conditions and the available data. The parameters of the functions are determined from measurements in test pits, which is why the method is called semi-empirical. The model was applied to a dual-detector epithermal neutron tool, and the porosity response functions in liquid-filled and gas-filled boreholes have been obtained. In liquid-filled holes, the measurement is compensated for standoff or mudcake. In gas-filled holes, both a near and a far porosity are calculated. The near porosity is insensitive to the presence of foam in the borehole, whereas the far porosity is more accurate elsewhere. Log examples are presented to illustrate the effectiveness of the model.

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