Abstract
The weak dependence of the results of pulsed neutron logging on the presence of the oil well borehole suggests that thermal neutron die‐away measurements in an open hole may be capable of “seeing” beyond a limited amount of borehole fluid invasion. In order to obtain an estimate for the depth of investigation which can be expected from pulsed neutron logging when some invasion has occurred, multigroup neutron diffusion theory was used to calculate thermal neutron lifetimes for the open‐hole logging geometry. The transient, three‐group equations were solved with the aid of an IBM 7090 computer. Two cases of open‐hole fluid invasion were investigated: Fresh‐water filtrate invading a salt‐water‐saturated formation, and salt‐water filtrate invading a formation saturated with fresh water (or oil). In both cases the formation is assumed to be a 20 percent porosity sandstone and the fluid‐filled borehole is taken to be 20 cm in diameter. It is shown that for any given depth of borehole fluid invasion, up to the point of total masking, it is much easier to detect the presence of oil behind salt‐water invasion than to detect salt water behind a zone invaded by fresh water.
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