What is the propagation direction of a hydraulic fracture?

• A. Perpendicular to the minimum principal stress
• B. Parallel to the minimum principal stress
• C. Perpendicular to the maximum principal stress
• D. Parallel to the maximum principal stress

Answer: (A)

Hydraulic fracture growth is driven by the fluid pressure inside the crack, which must overcome the rock’s confining stresses. In a typical three-dimensional stress state, the smallest compressive resistance comes from the minimum principal stress. The fracture tends to extend in the direction where confinement is weakest, which is orthogonal to that minimum principal stress. In other words, the crack advances along a path perpendicular to the minimum principal stress. (In real rocks, layering and anisotropy can steer the path, but the general tendency is to grow perpendicular to the minimum principal stress.)