We describe field tests carried out to show that salt tracers injected into the mud at the surface during drilling can be used to measure the extent of hydrodynamic dispersion during well-bore circulation. The results of one test using lithium bromide are described in detail, together with the procedures for tracer injection, sampling of return mud, and chemical analysis by ion chromatography. Test data are analysed in terms of residence time theory, and the residence time density function, cumulative distribution function and washout function are computed. Results suggest that circulation is well-mixed and is close to plug flow. The observed washout function is compared with the predictions of several simple models. The anion was completely recovered in the filtrate but cation losses were observed which are attributed to ion exchange with clay solids during circulation. In tests with salt mixtures, cation losses reflect the affinity of ions for clay cation exchange sites.

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