Depth of investigation of the dual-spacing thermal neutron decay time logging tool

The depth of investigation of the Dual-Spacing Thermal Neutron Decay Time logging tool was measured in a 22-percent-porosity-sand formation. The laboratory formation consisted of an 8-in. borehole surrounded by nine separate concentric annuli 2-in. and 4-in. wide, each filled with a mixture of sand and pebbles as required to give the 22-percent porosity. Formation diameter was 48 inches. Thermal neutron decay times were recorded and depths of investigation determined for both near and far detectors in cased and uncased borehole. Measurements were made with all annuli initially saturated with fresh water, and after successively flooding each annulus in turn with saturated salt water, starting with the outermost. This simulates in reverse the case of freshwater invading a saltwater-filled formation. Similar measurements were made to simulate the case of saltwater invading a formation initially filled with freshwater