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Sealing a zone in a subterranean well-formation is effected with an aqueous silicate solution containing an alkali metal silicate, at least one activator and a mixture of a chelating agent and a phosphonate. This composition greatly increases the divalent ion tolerance of the silicate gelling system

Sealing a zone in a subterranean well-formation is effected with an aqueous silicate solution containing an alkali metal silicate, at least one activator and a mixture of a chelating agent and a phosphonate. This composition greatly increases the divalent ion tolerance of the silicate gelling system.

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1. A composition for increasing the gel formation tolerance of aqueous alkali metal silicate solutions for divalent ions present in subterranean formations, which comprises: (a) aqueous alkali metal silicate solution; (b) a thermally responsive gelation activator; and (c) a mixture of phosphona

1. A composition for increasing the gel formation tolerance of aqueous alkali metal silicate solutions for divalent ions present in subterranean formations, which comprises: (a) aqueous alkali metal silicate solution; (b) a thermally responsive gelation activator; and (c) a mixture of phosphonate and a chelating agent selected from the group consisting of ethylene diamine tetraacetic acid, propane diamine tetraacetate, diethyltriamine pentaacetate, N-hydroxyethylene diamine triacetate, hydroxyethylimino diacetic acid, nitrilo triacetate and 1,3-propanediamine tetraacetate. 2. The composition of claim 1, wherein the alkali metal silicate is sodium silicate. 3. The composition of claim 1, wherein the phosphonate is selected from the group consisting of sodium phosphonate, phosphonic acid, diethylenetriamine pentamethylene phosphonic acid, methylenephosphonate and their derivatives. 4. The composition of claim 1, wherein the concentration of the phosphonate is about 40 to about 200 ppm. 5. The composition of claim 1, wherein the concentration of the chelating agent is about 0.5 to 3.0%, by weight. 6. The composition of claim 1, wherein gelation occurs at a temperature of about 100° F. to about 350° F. 7. The composition of claim 1, wherein gelation occurs at a temperature of about 120° F. to about 200° F. 8. A method of plugging or sealing a zone in a subterranean formation with an aqueous alkali silicate solution which has an increased gel formation tolerance to divalent ions present in said formation, which comprises: (a) introducing into said zone an aqueous alkali metal silicate composition comprising, (i) an aqueous alkali metal silicate solution, (ii) a thermally responsive gelation activator; and (iii) a mixture of a phosphonate and a chelating agent selected from the group consisting of ethylenediamine tetracetic acid, propane diamine tetracetate, diethyltriamine pentaacetate, N-hydroxyethylenediamine triacetate, hydroxyethylimino diacetic acid, nitrilo triacetate and 1,3-propanediamine tetraacetate; and (b) activating said gelation activator in response to a thermal change of said composition within said formation whereby said silicate composition forms a gel in said zone. 9. The method of claim 8, wherein the alkali metal silicate is sodium silicate. 10. The method of claim 8, wherein the phosphonate is selected from the group consisting of sodium phosphonate, phosphonic acid, diethylenetriamine pentamethylene phosphonic acid, methylenephosphonate and their derivatives. 11. The method of claim 8, wherein the concentration of the phosphonate is about 40 to about 200 ppm. 12. The method of claim 8, wherein the concentration of the chelating agent is about 0.5 to 3.0%, by weight. 13. The method of claim 8, wherein gelation occurs at a temperature of about 100° F. to about 350° F. 14. The method of claim 8, wherein gelation occurs at a temperature of about 120° F. to about 200° F.