Post-drilling open-hole log measurements made by wireline tools are most commonly transmitted to surface via wireline, but the trend is towards increased use of memory-capable tools with greater operational flexibility. The primary purpose of the original memory tools was to operate on wireline in intermediate and TD sections while offering a low-risk alternative to conventional tools in horizontal wells, bad hole conditions and other challenging well scenarios.

This wide operational envelope required the tools to be small-diameter to allow protected conveyance inside drill pipe, but it also required that measurement performance and associated environmental characterization be on a par with conventionally sized tools that generated the majority of the historical record.

The extent to which these goals have been achieved, using evidence from the over 150,000 wells logged with the most highly developed of the small-diameter tool types, is reviewed.

Standard response modelling is essential but insufficient to ensure optimum measurement performance. Findings from the performance in thousands of wells have been used to refine models, hardware designs and processing algorithms, including more precise control on the position of tools in the wellbore.

Log data from test wells and commercial wells are provided showing the measurement quality from small-diameter tools matches that from conventionally sized tools for the large majority of operations.

Log data is also provided from wells that had presented insurmountable challenges for traditional logging tools which demonstrate the conveyance flexibility and the reduced acquisition risk in using the small-diameter tools.

The significance of the findings is that the performance of the most highly developed small-diameter tools has evolved to the point where measurement quality is not a differentiating factor for the large majority of operations, allowing job planning to be driven by the other considerations, in particular the minimization of acquisition risk.

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