Some drilling muds create downhole emulsions in the form of mud filtrate that contains large amounts of emul-sifiers and drill solids oil wetting agents. Drilling mud filtrate can also contain finely ground weighting solids, which allow the filtrate to quickly begin emulsifying reservoir fluids. This very "tight" emulsion behind pipe or in the formation creates both a high-viscosity emulsion and an oil-wet state on the formation sand. It is also possible for well treatment fluids, especially acid, to react with crude oils to create an acid/oil emulsion that cannot be pumped down the clean oil pipeline and, thus, can delay oil sales until the emulsion can be broken. In rare cases, contamination with drilling fluid filtrate in the "flushed zone" around the well bore or oil/brine incompatibility can cause an emulsion to be formed between completion or workover brine and crude oil.
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