geomechanics · fracture pressure
Breakdown Pressure (Tensile Fracture Initiation)
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Inputs
psi
psi
psi
psi
Description
Computes the wellbore pressure at which a tensile (hydraulic) fracture initiates at the wellbore wall, using Kirsch's (1898) elastic stress-concentration solution evaluated at the azimuth of minimum hoop stress (θ=0°, aligned with SHmax — the opposite location from Critical Mud Weight's and Hoop Stress (Kirsch)'s θ=90° breakout azimuth), combined with a simple tensile failure criterion. This is the same Kirsch-based, effective-stress approach already used by Critical Mud Weight and Hoop Stress (Kirsch) in this discipline — re-derived directly from that shared physics rather than transcribed from a secondary source — evaluated against tension instead of shear. Genuinely distinct from Fracture Gradient (Eaton 1969)'s FG = Shmin + σt: that is a simplified poroelastic estimate depending only on Shmin and an assumed tensile term, with no SHmax or Pp dependence at all, while this calc uses the full near-wellbore stress concentration and is sensitive to stress anisotropy (SHmax−Shmin) in a way the simpler estimate cannot capture. Do not treat one as a drop-in replacement for the other.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Pb | psi | Breakdown Pressure |
| Shmin | psi | Minimum horizontal stress — typically the Shmin output of the Fracture Gradient (Eaton 1969) calc. |
| SHmax | psi | Maximum horizontal stress — typically the upper bound from the SHmax (Stress Polygon) calc, or an independently known/measured value. |
| Pp | psi | Pore pressure at the depth of interest — typically the output of Drilling's Eaton Pore Pressure calc. |
| T | psi | Rock tensile strength at the wellbore wall, from direct (Brazilian) testing or a locally-calibrated correlation. No calc in this platform currently estimates tensile strength — supply a measured or independently sourced value; a default of 3000 kPa is provided only as an illustrative placeholder, not a literature-derived typical value. |
Assumptions
- Vertical wellbore, with the wellbore axis aligned with Sv (one of the three principal stress directions) — no stress-tensor rotation is needed
- Linear elastic rock behavior up to the point of tensile failure (Kirsch's solution assumes elasticity)
- Impermeable rock / instantaneous breakdown — no near-wellbore pore-pressure change from mud filtrate invasion during the pressurization
Limitations
- Impermeable-rock case only. A permeable formation lets mud filtrate invade and alter the near-wellbore pore pressure during pressurization, which a full poroelastic solution (Detournay-Cheng-style) accounts for and this calc does not — the poroelastic case can give a materially different breakdown pressure, particularly in higher-permeability rock or slow pressurization. Not modeled here; treat this calc's result as the classical instantaneous-breakdown estimate, not a poroelastically-corrected one.
- Tensile strength T must be supplied directly — no calc in this platform currently estimates it from log or core data (a UCS-ratio correlation commonly attributed to Lal 1999 was researched and found likely misattributed in secondary literature, and Lal's own actual sonic-velocity-based correlation could not be verified against the primary paper; both remain unbuilt pending direct source access)
- Vertical-well case only — a deviated or horizontal wellbore requires resolving the full 3D stress tensor into the borehole's local coordinate system (Peska & Zoback 1995), which is not included here
- Does not include thermal stresses from mud/formation temperature differences, which can meaningfully shift breakdown pressure, particularly with a cold mud system (thermal stress relaxation reduces the pressure needed to reach tensile failure)
Use Cases
- → Frac initiation pressure estimate for treatment design: Estimate the wellbore pressure needed to initiate a hydraulic fracture, as a starting point before a treatment's actual observed breakdown pressure is available.
- → Distinguishing initiation from propagation pressure: Compare against Fracture Gradient (Eaton 1969)'s simpler propagation-gradient estimate to understand the difference between the pressure needed to initiate a fracture at the wellbore wall and the pressure needed to keep an existing fracture propagating away from it.
- → Sanity-checking an observed breakdown pressure: Compare a treatment's actual observed breakdown pressure against this prediction to back-check assumed stress or tensile-strength inputs, or to flag a possible poroelastic/permeability effect not modeled here.
Related Calculations
Region Notes
Permian Basin
Wolfcamp intervals with significant stress anisotropy (SHmax notably greater than Shmin) predict a breakdown pressure well below Fracture Gradient (Eaton 1969)'s simpler Shmin-based estimate — a useful cross-check before finalizing a completion design's expected treating pressure.
Global
Cold mud systems (common in deepwater and HPHT wells with active mud cooling) reduce actual breakdown pressure below this calc's prediction via thermal stress relaxation, which is not modeled here — treat this as an upper estimate in those settings.
References
Primary source
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