Preflushes are often used as part of the sequence of fluids pumped in primary cementing. Usually two functions are served by preflushes: I) to wash the drilling fluid ahead, by a combination of turbulence and chemical reaction; II) to provide a chemically compatible spacer between the lead slurry and the drilling mud. In some cases a wash precedes a spacer, but often only a single preflush is used. We consider well parameters typical of surface casing cementing in North Eastern British Columbia. Using a two-dimensional model of annular displacement flows, we show that the wash concept is flawed. In particular, in a sequence of simulations varying from intermediate density to low density we show that the wash progressively advances ahead of the lead slurry, channeling rapidly up the wide side of the annulus. Even when fully turbulent, it is ineffective at displacing mud from around the annulus, invalidating the motivation of chemical cleaning through contact time. Furthermore, the advance along the wide side of the annulus drains the volume of fluid separating the cement from drilling mud. Thus, the idea that the wash provides a barrier between slurry and and mud is invalid.

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