Breakdown and Propagation
Learning objectives
- Read a hydraulic-fracturing pressure-time record: pump-up, breakdown, propagation, and shut-in
- Compute the breakdown pressure and see the fracture initiate at that peak
- Explain why the fracture opens against Shmin and propagates perpendicular to it
- Distinguish the breakdown peak from the lower steady propagation pressure near closure
Pumping Past the Ceiling
Section 7.1 set the ceiling; this section pushes through it deliberately. Pump fluid into a sealed interval and the pressure climbs until the wall goes tensile and a fracture initiates, at the breakdown pressure from Part 6.3: , which for the canon well is 50.7 MPa. That is the peak of the record, the moment the intact rock fails in tension. Once the fracture exists, keeping it growing takes far less pressure, because the tensile strength is a one-time toll: the fracture now merely has to hold open against and overcome the friction of pushing fluid down its length. So the pressure drops sharply after breakdown to a lower, roughly steady propagation pressure just above , around 46 to 48 MPa on the canon well.
Run the injection in the figure. The pressure ramps up along the fluid-filling line, spikes at breakdown, then falls to the propagation plateau, and the map view shows the fracture growing. Its direction is fixed by the stress field, not by the well: a hydraulic fracture opens against the least stress and propagates perpendicular to it, which means it grows in the plane containing and , spreading along the direction. In a normal-faulting field like the canon's, where is largest, that plane is vertical and the fracture is a tall vertical sheet aligned with . This is why fracture azimuth is a stress compass, exactly like the DITFs of Part 6.3, and why knowing the stress direction before stimulating tells you which way the frac will go.
The Record Is a Stress Measurement
The shut-in part of the record is the payoff already met in Part 5.4: stop the pumps and the pressure bleeds down to closure, which equals . So a single fracturing operation delivers three stress-related numbers, the breakdown that involves and , the propagation near , and the closure that is , which is why a deliberate small fracture, the minifrac of the next section, is run as much to measure the stress as to stimulate. The two intents that section 7.1 distinguished converge here: the same operation that opens a producing fracture also reads the stress that controls it. The difference between drilling and fracturing is only which side of the breakdown pressure you choose to be on, and this section has crossed to the far side, where the next two sections do their work: measuring precisely with a minifrac, then growing a fracture at production scale.
References
- Hubbert, M. K., & Willis, D. G. (1957). Mechanics of hydraulic fracturing. Transactions of the AIME, 210, 153-168.
- Haimson, B., & Fairhurst, C. (1967). Initiation and extension of hydraulic fractures in rocks. SPE Journal, 7(3), 310-318.
- Economides, M. J., & Nolte, K. G. (2000). Reservoir Stimulation (3rd ed.). Wiley.