Advances in Density Log Processing Deliver Enhanced Precision and Operational Efficiency

Abstract

This paper introduces a new density log processing technique for wireline compensated density logging tools and demonstrates the improvement in statistical precision and repeatability that is achieved using this technique.

Wireline density logging tools generally consist of a chemical gamma ray radioactive source and two detectors, near and far spaced. The density measurement is generally ‘compensated’ for the presence of mud cake by combining together the measurements from the two detectors with the greater weight applied to the far spaced.

The production of gamma rays from a radioactive source and their reaction with the formation are events conforming to Poissonian statistics. This statistical nature results in the log lacking exact repeatability. Historically, tool design and low logging speeds have been used to improve the precision of the statistical count rate recorded at the detectors by increasing the number of detected counts available in a given depth increment.

The statistical noise on any radiation counting system is related to the square root of the number of detected counts. Therefore, in a dual-spaced detector system most of this noise arises from the far-spacing detector that has a relatively low counting rate due to its distance from the radioactive source. Since the two detectors in the density log are independent, their statistical errors are combined and amplified by the compensation process; thus, the compensated density is noisier than either of its components. A new combination processing technique has been developed to significantly reduce this amplification. Results are presented that show the improvement in statistical precision and repeatability of the density logs. The results demonstrate that the density log can be recorded at a significantly increased speed, which improves operational efficiency, achieving the same statistical precision as that from legacy processing.

The density log is a key formation evaluation measurement throughout the well logging industry. It is run in virtually every well that is logged. The reduced-noise, compensated density curve can be used with increased confidence in quantitative analysis of logged density measurements, which in turn, reduces reservoir uncertainty. This is especially true in higher density formations where the count rates are lower.