Polymer flooding provides a better mobility ratio than water flooding, for a better sweep especially in viscous oil reservoirs that have been under water flood. During polymer injection, larger fractures may be induced compared with water injection. The geometry and propagation behaviours of fractures caused by the injection of viscous fluids in sands under bi-axial stress that simulated unconsolidated formation were studied. The results showed that the geometry of fractures induced by viscous fluids in an unconsolidated formation is a dominantly planar fracture, although it is very tortuous. In view of isotropic stress in the horizontal plane, multiple fractures were induced in several directions. Similar fractures were observed when the injection fluid viscosity was varied by a factor of 3, while keeping the mobility ratio constant. The tests confirm that the injection pressure should exceed the minimum stress by a factor of 2.5 to induce fracture propagation. Below this pressure only matrix flow occurred, although the permeability was enhanced by the injection.