Abstract

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 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 porosity transform makes this conversion more 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 presented to demonstrate the accuracy and utility of the environmental corrections and porosity transform.

Introduction

The lack of porosity Information in old fields drilled prior to the availability of reliable porosity logs has frequently been a problem for log analysts. Due to the numerous operating company mergers of the recent past, many fields have changed hands, and the detailed reservoir characterization and reserves estimates needed to Incorporate these fields with existing production are, In many cases, sketchy or non-existent.

Aggressive cased-hole logging and Infill drilling programs are often under taken to furnish the programs are often under taken to furnish the needed reservoir data. In some cases this can be effective, but In areas where operating economics are borderline or tubing Is In place In existing wells, these types of reservoir characterization programs are impractical. What is needed then Is programs are impractical. What is needed then Is a reliable through-tubing porosity log, preferably one that can be used for stand-one basic formation evaluation. Pulsed neutron logs fit these criteria quite well as long as care Is exercised to ensure the calculation of accurate formation parameters. Borehole effects on sigma logs have been well documented; however, environmental effects on the count rate ratio curve, and thus the porosity calculation, are lust now being addressed In a truly quantitative manner.

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