Method for inhibiting scale formation

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: wherein Z = - CHR PO3R2 R = H, CH3, C2H5 or M R = H, CH3, CR3, C6H5,SO3H2 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=o 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.