SCALING NEUTRON FIELDS IN WELL LOGGING

Neutron logging tool readings (both for a single detector and for compensated tools) can be correlated with a General Neutron Parameter (GNP). The GNP is a specific combination, for every tool design, of apparent neutron parameters. Apparent neutron parameters are analytical functions of the real neutron parameters of borehole and formation materials, borehole size and tool diameter. They are defined as those neutron field parameters which are sensed by a logging tool situated inside a borehole. Analytical formulae are presented for calculating the apparent neutron parameters. The method of determining values of the GNP is demonstrated for four real tools. All examples relate to the variation of porosity in the following conditions: a single lithology with a variable borehole diameter for fresh water saturation (single detector tool); single borehole diameter for a variable lithology for fresh water saturation (dual detector tool), a single lithology saturated with fresh water with variable borehole diameter inside which brine of variable salinity exists (dual detector tool); a variable lithology for a single borehole diameter when both borehole and formations are filled with brine or fresh water (dual detector tool). By a scaling procedure one general calibration curve is obtained, where all experimental tool readings fall in one relationship, irrespective of the nature of the calibration facility (the borehole diameter, lithology, fluid salinity, etc.). Further application of scaling by the GNP parameter permits the porosity calibration curves to be established for all borehole and formation conditions.