Because the vertical stress changes with depth, the orientation of the hydraulic fractures also changes as the
fractures move closer to the surface. In sufficiently shallow rock, the direction fractures propagate can leave the vertical plane and propagate on an angle and could even approach the horizontal plane. So as fractures propagate
into shallower rock above, they become less likely to propagate vertically and more likely to extend laterally
into the rock. The depth at which this occurs varies by rock type and thickness and by the change in related pressures and is difficult to estimate