Methods for breaking viscosified fluids

A method of fracturing a subterranean formation penetrated by a wellbore wherein the static temperature of the formation adjacent said wellbore is above about 100.degree. F., comprising injecting into the wellbore and into contact with the formation at a rate and pressure sufficient to fracture the formation an aqueous fluid comprising:

• a) an aqueous liquid containing a source of potassium ions present in a concentration of at least about 0.05 molar;
• b) a gelling agent comprising at least one member selected from the group of galactomanans, modified or derivatized galactomanans, and cellulose derivatives present in an amount sufficient to increase the viscosity of said aqueous liquid;
• c) a crosslinking agent for said gelling agent present in an amount sufficient to effect crosslinking of said gelling agent in said aqueous liquid to further increase the viscosity of said aqueous liquid.
• d) a breaker comprising at least one member selected from the group of alkali metal chlorites and hypochlorites present in an amount sufficient to effect a controlled reduction in the viscosity of the crosslinked gelling agent of the aqueous liquid after a predetermined period of time;
• and
• e) a breaker activator comprising ammonium ion or a compound capable of producing ammonium ion upon dissolution in said aqueous liquid present in an amount sufficient to activate said breaker to provide said controlled reduction in viscosity.