The instant invention is directed to shear thickening fluids which comprise a water-swellable material (clay), present in a sufficient concentration so as to be capable of forming a stiff paste upon interaction with the water used, and water wherein the clay and water are kept separated by an intervening hydrocarbon-surfactant composition. The intervening oil phase prevents the interaction between the water and the clay phases and results in a stable, nonreacting, pumpable composite until such time as the oil envelope is ruptured by application of a sufficiently high shear force. Upon such rupture, the materials interact rapidly forming a semi-rigid stiff paste. Various well-control problems, such as oil and gas with blowouts, can be controlled by use of the above-described composite. The composite is pumped down the well pipe. Exiting the orifices of the drill bit or a nozzle supplies the shear force needed to rupture the oil envelope thereby permitting the interaction between the clay and the water resulting in the formation of a stiff paste which can stop or prevent unwanted flow in or near the wellbore.