A delayed gelling system useful in conformance control in the production of petroleum from subterranean formations is disclosed. The gelling system comprises an acidic aqueous solution of acid-soluble or cationic polyacrylamide, an at least partially neutralized acid aluminum salt, an activator comprising a hydroxyl donor, and an optional gel modifier. The gelling system may be pumped into formations with excessive water production and thermally activated in the formation at downhole conditions to form a co-gel of polyacrylamide interspersed in an inorganic gel network to reduce water production.