Formation-water displacement optional

A water displacement stage ahead of the standard acid preflush is also optional. The recommended fluid is a 2%-8% ammonium chloride (NH Cl) solution, with 5% NH Cl preferable for most applications. The recommended volume is 15-75 gal/ft, and a typical volume is 25-50 gal/ft. The purpose of this stage is to displace formation water containing appreciable bicarbonate and sulfate ions. A concentration greater than 1,000 parts per million (ppm) of these ions is considered appreciable. 

By including a non-acid water displacement stage, the precipitation of calcium carbonate and calcium sulfate scales can be avoided. These scales form when spent HCl (calcium chloride in solution) mixes with formation brine. 

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