Fracture Initiation and Breakdown
Fracture initiation refers to the moment when a hydraulic fracture first forms in the rock due to fluid injection. As fluid is pumped into the wellbore, the pressure increases until it exceeds the stresses and tensile strength of the surrounding rock. At this point, the rock fails and a fracture begins to form. The pressure required to initiate this fracture is known as the breakdown pressure.
Fracture Initiation Near the Wellbore
Fracture initiation usually occurs near the wellbore wall, where the injected fluid pressure acts directly on the formation. The stress state around the wellbore is influenced by the in-situ stresses and the geometry of the borehole.
A fracture forms when the pressure inside the wellbore becomes large enough to overcome the minimum principal stress and the tensile strength of the rock.
Breakdown Pressure
Breakdown pressure is the maximum pressure reached during injection before the fracture opens. Once the rock fails and the fracture begins to propagate, the pressure typically drops slightly because fluid can now flow into the newly created fracture.
Fracture Propagation Pressure
After fracture initiation, the pressure required to keep the fracture growing is usually lower than the breakdown pressure. This pressure is known as the fracture propagation pressure.
Propagation pressure depends on several factors, including:
- in-situ stresses
- fluid viscosity
- leakoff into the formation
- fracture geometry
