The stress within a rock can be resolved into three principal stresses (Figure 20). A formation will fracture when the pressure in the borehole exceeds the least of the stresses within the rock structure. Normally, these fractures will propagate in a direction perpendicular to the least principal stress (Figure 20). The direction of the least principal stress in any particular region can be predicted by investigating the fault activity in the area (Figure 21).
To initiate a fracture in the wall of the borehole, the pressure in the borehole must be greater than the least principal stress in the formation. To propagate the fracture the pressure must be maintained at a level greater than the least principal stress.