Capstone: The Frac Hit
Learning objectives
- Explain the frac hit: a depleted parent well lowers the local Shmin, and a child well's fracture bends toward the low-stress zone
- Synthesize the stress path and fracturing: depletion drops Shmin, and fractures grow toward lower stress
- See the frac trajectory bend toward the parent as depletion rises and spacing shrinks
- State the mitigations: spacing, sequencing, and re-pressurizing the parent
The Parent and the Child
The final capstone is drawn from the front line of unconventional development, where wells are drilled close together and fractured in stages. Drill a parent well, produce it a while so it depletes, then drill a child well nearby and fracture it. Two things this course established now collide. The parent's depletion has lowered the local along the stress path, times the pressure drop, carving a low-stress zone around the drained parent. And a hydraulic fracture grows toward lower stress, because it opens against and finds the path of least resistance. So the child's fracture, launched near a depleted parent, bends toward the parent's low-stress zone and can connect to it, a frac hit.
A frac hit is rarely good news. The child's fracturing fluid and pressure surge into the parent, spiking its pressure, sometimes damaging it, and the child's fracture, having spent itself reaching the parent, drains the same rock twice instead of new rock. Move the parent depletion and the well spacing and watch the child's fracture bend: a lightly-depleted parent barely deflects it, but a heavily-depleted parent at close spacing pulls the fracture straight into itself.
Stress Path Meets Fracture
The mechanism is the whole producing-field story in one picture. The parent's stress path set the low- trap; the fracture mechanics of Part 7 make the child's frac seek it. The size of the effect scales the way intuition says: with the depletion, which sets how deep the stress low is, and inversely with the spacing, which sets how far the fracture must reach to feel it. This is why frac hits became a defining problem of infill drilling exactly as fields matured and wells were packed closer, both drivers moving the wrong way at once. The sign of the parent's pressure matters too: a depleted parent (low ) attracts the child frac, while a re-pressurized parent, one injected back up, raises its local and repels the child frac, pushing it away. That is the basis of one mitigation, re-pressurizing or refracing the parent before fracturing the child so the child's fracture is deflected away rather than drawn in.
The Whole Course in One Well
The frac hit closes the course's arc. It needs the stress tensor and effective stress of Parts 1 and 2, the fracture that opens against of Part 7, the stress path of depletion, and the reading of one field's stresses well enough to predict where a fracture will go, the mechanical earth model of Part 8. Manage it with the same levers the model exposes: widen the spacing, sequence the wells so neighbors are fractured before either depletes far, re-pressurize a depleted parent before fracturing its child, and monitor with the 4D and microseismic of Section 10.4. Every one of those decisions is read from a mechanical earth model kept current with the field's pressure. Part 11 now steps back to consolidate the whole course in a review and an exam, and Part 12 assembles it into a working lab.
References
- King, G. E., Rainbolt, M. F., & Swanson, C. (2017). Frac hit induced production losses: evaluating root causes, damage location, and remediation. SPE Annual Technical Conference, SPE 187192.
- Roussel, N. P., & Sharma, M. M. (2011). Optimizing fracture spacing and sequencing in horizontal-well fracturing. SPE Production & Operations, 26(2), 173-184.
- Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.