geomechanics · stress
Principal Stress Summary
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Inputs
psi
psi
psi
psi
psi
Description
Presents the full principal in-situ stress state — vertical, minimum horizontal, and maximum horizontal (upper-bound) stress, plus effective vertical stress — together in one summary, alongside the two stress ratios most commonly used to characterize the stress state (SHmax/Shmin and Shmin/Sv). This is a pure aggregation of Overburden Stress, Effective Stress, Fracture Gradient (Eaton 1969)'s Shmin, and SHmax (Stress Polygon)'s bounds — no new physics, just the four calcs' own outputs re-entered and presented together, plus the Andersonian faulting-regime classification implied by their relative ordering.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Sv | psi | Vertical Stress |
| Shmin | psi | Minimum Horizontal Stress |
| SHmax_max | psi | Maximum Horizontal Stress (Upper Bound) |
| σ' | psi | Effective Vertical Stress |
| SHmax/Shmin | — | Horizontal Stress Ratio (SHmax/Shmin) |
| Shmin/Sv | — | Shmin/Sv Ratio |
| Sv | psi | Total vertical stress — the output of the Overburden (Vertical) Stress calc. |
| σ' | psi | Effective vertical stress — the output of the Effective Stress (Terzaghi/Biot) calc. |
| Shmin | psi | Minimum horizontal stress — the Shmin output of the Fracture Gradient (Eaton 1969) calc. |
| SHmax_min | psi | SHmax lower bound from the SHmax (Stress Polygon) calc — by that method's own definition this is always numerically equal to Shmin, included here only to mirror the stress-polygon range that calc reports. |
| SHmax_max | psi | SHmax upper (frictional-limit) bound — the SHmax_max output of the SHmax (Stress Polygon) calc. |
Assumptions
- Sv, Shmin, and SHmax_min/max are the three principal stresses (vertical wellbore, no tectonic tilt of the principal stress axes)
- All four re-entered values represent the same depth/interval — this calc does not check depth consistency between them
- SHmax is represented only by its stress-polygon bounds, not a single point estimate — the upper bound (SHmax_max) is used for the ratio and regime classification, which is conservative (maximizes anisotropy) relative to the true unknown value
Limitations
- Purely a presentation/aggregation layer — does not recompute or independently verify any of the four source values; errors in the upstream calcs propagate directly
- Uses SHmax's upper bound, not a point estimate, for the ratio and faulting-regime classification — treat the regime classification as indicative, not definitive, without independent SHmax constraint (image-log breakout orientation, extended LOT)
- Does not account for reverse-faulting regimes where Sv could be the minimum principal stress in a way that breaks the Stress Polygon calc's own Shmin-is-minimum assumption — if that assumption already failed upstream, this calc's regime classification inherits the same invalidity
Use Cases
- → One-card stress-state review: Review the complete principal stress state and derived ratios for an interval in one place before feeding individual values into Mohr-Coulomb Failure Criterion, Critical Mud Weight, or Kirsch Hoop Stress.
- → Faulting regime sanity check: Cross-check an assumed tectonic regime (e.g. for the Stress Polygon calc's own Shmin-is-minimum assumption) against the Andersonian classification implied by the re-entered stress magnitudes.
- → Reporting and offset-well comparison: Summarize the stress state and its anisotropy ratios in a single reportable card when comparing multiple wells or intervals in a basin.
Related Calculations
Region Notes
Permian Basin
The Permian Basin's strike-slip to transitional normal/strike-slip regime (also noted in the Stress Polygon calc) is a useful cross-check target when reviewing this calc's implied faulting regime for Delaware/Midland Basin data.
Global
The SHmax/Shmin and Shmin/Sv ratios reported here are commonly compared against regional published values as a first-pass sanity check on individually re-entered stress inputs.
References
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