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Abstract

A method for inhibiting the formation of scale in waters having a high barium content and low pH comprises the addition of an aqueous solution of a phosphonate of formula: ##STR1## wherein Z=--CHR.sup.1 PO.sub.3 R.sub.2 R=H, CH.sub.3, C.sub.2 H.sub.5 or M R.sup.1 =H, CH.sub.3, CR.sub.3, C.sub.6 H.sub.5, SO.sub.3 H.sub.2 M=alkali metal or ammonium ion n=2-6, preferably 2-4 m=2-6, preferably 2-4 a=2-10, preferably 2-4 b=2-10, preferably 2-6 x=0-6, preferably 0-3 y=0-6, preferably 0-2 provided that when y=0 at least one of n or m is greater than 2. The phosphonates may be present in the form of a fully or partially neutralised salt and may be injected into production fluids obtained from an underground reservoir or injected into the reservoir via a wellbore as part of a squeeze technique.

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