Trace amounts of metal chelant additives, commonly used in stimulation and fracture fluids, have been shown to have a debilitating effect upon the performance of widely used barium sulfate scale inhibitors. EDTA and citric and gluconic acids render examples of some commonly used scale inhibitors (phosphonates, polycarboxylates and phosphate esters) completely ineffective at concentrations as low as 0.1 mg/L. Such low concentrations may be expected to return from formation stimulation treatments for many months and would appear to jeopardize any scale inhibitor program in place. The testing was based upon a simulated North Sea scaling system at pH 4.0 and 6.0. Scale inhibitor concentrations studied were 50 and 100 mg/L The large negative effect of the organic chelants was observed at pH 4.0 and 6.0. The only scale inhibitors studied which remained ambivalent to these interferences were large polymeric vinylsulfonates.

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