Historically, return permeability testing has become a standard to establish minimally damaging fluid properties. This single number is typically focused upon for its simplicity, however the testing procedure and intermediate data provide much more than just this value. The process provides insight into formation damage mechanisms and their overall impact on potential well performance. The variable nature of procedures and apparatus as well as interpretation of data is regularly limited to the percent return permeability - a potentially misleading value in fluid selection. A thorough review of existing literature and experience on recent projects details the recognized potential for variability as well as misperceptions in interpreting results. Examples of test data are reviewed to capture damage mechanisms where fluid formulations require further optimization. In many cases conclusions are drawn and fail to identify why differences in the percent return permeability yields a particular result. Prior published work assessing formation damage tends to focus upon formation characteristics. This paper details the results and data typically provided with these analyses. These data include: fluid loss, initial return permeability, and techniques used to determine the damage mechanism. Test results are reviewed to illustrate where the risk of unwarranted conclusions occurs. Recommendations for appropriate analysis of both final return permeability and intermediate data are provided for optimized fluid design. Through the techniques discussed, testing procedures have the potential to provide the maximum insight into fluid-formation interactions on the core material available.

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