Hydraulic fractures which extend between a hydrocarbon fluid producing zone of an earth formation and a water-producing zone are sealed with respect to the water-producing zone by simultaneously injecting a non-sealing fluid such as crude oil, diesel fuel or brine into the portion of the fracture extending within the hydrocarbon producing zone and injecting a settable gel composition such as a polyacrylamide polymer cross-linked with inorganic transition metals into the portion of the fracture extending within the water-producing zone. The flow rates of the simultaneously pumped fluids are controlled in accordance with the ratio of the respective fracture heights and the viscosity and density of the respective sealing and non-sealing fluids. One fluid is typically pumped through the well annulus while the other fluid is pumped through a tubing string which is sealed from the annulus by a conventional packer disposed at the interface between the hydrocarbon and water-bearing zones.