Treating wells with ion-exchange-precipitated scale inhibitor

A well treating process for inhibiting scaling by fluid which is produced from a clay-containing subterranean reservoir which tends to exchange cations with those contained within an injected aqueous solution, comprising:

• dissolving within an aqueous solution of relatively neutral pH (a) at least one nitrogen-containing phosphonate scale inhibiting compound which contains scale-inhibiting anions that form multivalent cation salts which are only slightly soluble at the pH of the solution and (b) enough of at least one substantially neutral salt to both provide a ratio of monovalent-to-multivalent cations of at least about 100 and to substantially saturate the solution with respect to multivalent cation salts of the scale-inhibiting anions;
• injecting said solution into a portion of said reservoir which (a) has a significant tendency to exchange cations with those contained within an injected aqueous solution and (b) contains a significant proportion of adsorbed multivalent cations;
• and,
• controlling the rate at which said solution is injected so that at least some portions of that solution remain relatively near the well long enough to induce a significant precipitation of scale inhibitor salt due to the ion-exchange-induced addition of multivalent cations to the solution.