The determination of reservoir porosity through casing has become a vital concern as more old fields undergo detailed re-evaluation. Many times the prevailing situation is that there is no reliable porosity data and the well has tubing in place. In such cases, pulsed neutron capture (PNC) logs are often relied on for both formation porosity and capture cross-section information.

Environmental effects on the pulsed neutron count rate ratio curve can be quite large and can have a dramatic effect on porosity calculations. This paper discusses the empirical modeling of typical downhole conditions that affect the TMD* ratio log and introduces updated corrections that can be applied to compensate for them. Included in the modeling are effects due to varying borehole fluid salinity, casing and tubing diameter, and cement. A lithology dependent ratio-to-porosity transform makes this conversion more consistent with density, neutron and acoustic techniques. When wellbore conditions are known, these computations can be applied either real-time or post-log at the wellsite to allow immediate reservoir evaluation.

Both test formation and field log data are presented to demonstrate the accuracy and utility of the environmental corrections and porosity transform.

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