This is a just a brief note, updating a small-study series (2017, 2019) concerning the ability of conservative artificial tracers, in inter-well forced-gradient flow, to ‘detect’ and quantify the properties of ‘nearby’ fractures or faults (non-intersected by the wells involved in the test). In an earlier short note (2015), we criticized the ‘bad habit’ of predicating ‘tracer test failure!’ from the objective datum of a very long residence time (a property intrinsic to fluid turnover in the georeservoir subject to the given forced-gradient flow). We now revisit a particular reservoir setting in the Upper Rhine Rift Valley, to find that a long residence time for circulating fluids may indeed be associated with ‘tracer test failure’, but in a different sense than previously incriminated. Again, this is not a test design issue, but a more general problem posed by a georeservoir class we may deem ‘very large’ in terms of fluid turnover volume under the typical conditions of a geothermal heat extraction project. With ever increasing georeservoir dimensions, the usefulness of artificial tracers in inter-well circulation may indeed become questionable, especially in the presence of large-scale faults.

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