Well Treatment with Complexed Metal Crosslinkers

A method of treating a subterranean formation penetrated by a well bore, the method comprising:

• mixing into an aqueous mixture a crosslinkable, hydratable polymer and a delayed crosslinking agent, in amounts effective to obtain an aqueous mixture that can form a persistent gel at a crosslinking condition comprising an onset temperature of from 40° C. to 60° C., the aqueous mixture being formed at a pH where a pH sensitivity slope of 5° C. per pH unit to 25° C. per pH unit can be observed for at least one pH unit range within a pH range from 8.5 to 12.5,
• wherein the crosslinking agent comprises a complex comprising
• a metal in a concentration between 10 and 500 ppm by weight of a liquid phase of the aqueous mixture,
• a ligand selected from the group consisting of amino acids, phosphono acids, and salts or derivatives thereof,
• wherein the hydratable polymer is present in the aqueous mixture in an amount of from about 1.2 g/L (10 ppt) to about 7.2 g/L (60 ppt) based upon a weight of the liquid phase of the aqueous mixture, and
• injecting the aqueous mixture into the well bore;
• and
• gelling the aqueous mixture at the persistent crosslinking condition.