Improved environmental corrections for compensated neutron logs

The basic openhole responses and environmental correction algorithms for CNL logs have been updated. The improved processing is based on an extensive set of laboratory formation measurements to which mathematical modeling calculations have been added. In all, the new algorithms include basic responses for the three principal formation matrix types and corrections for seven environmental effects and formation fluid salinity. A total of 467 laboratory formation measurements have been augmented with 245 data points generated through mathematical modeling. This data base has been used to define more accurately the effects on the tool response of variations in logging conditions from those considered standard in the laboratory. More accurate corrections for the effects of formation pressure, temperature, mudcake, natural or barite mud, and borehole salinity have been defined. Certain other effects are dependent on more than one parameter. For example, the effect of formation salinity is somewhat matrix dependent, and therefore the corrections are handled differently for sandstone, limestone, and dolomite. The effect of tool standoff is dependent on the borehole size, and the standoff correction is consequently larger boreholes. The porosity crossplots and environmental correction charts based on the new algorithms represent a significant evolutionary improvement over previous techniques. Theymore » should be an important aid to the use and interpretation of neutron logs.« less