ABSTRACT

Monte Carlo modelling is used to investigate environmental effects on compensated neutron instruments, Borehole salinity; formation salinity, thermal neutron absorbers, mud weight, mudcake thickness, temperature, and pressure effects are examined. The effect of thermal neutron absorbers is inversely related to the intrinsic capture cross section of the formation matrix. Equal amounts of thermal neutron absorbers affect the dolomite response more than the limestone response, despite the similarity in the response of the two formations with no absorbers. Formation salinity effect is also related to the matrix capture cross section. Up to about 120 kppm, formation salinity affects the instrument response in a similar way to thermal neutron absorbers. As the salinity increases further the moderating power of the formation decreases and the effect rapidly diminishes. The mudcake effect varies significantly, depending on the capture cross section and density of the mud and the thickness of the mudcake. Of particular interest are the large effects of hematite mudcake compared to barite and calcite mudcake. The differences in mud-cake effects stand somewhat in contrast to the much smaller differences in the effects of the various drilling fluids themselves. An increase in formation temperature results in a decrease in formation density and an increase in thermal diffusion length, leading to a lower apparent porosity.

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