geomechanics · stress
Overburden (Vertical) Stress
Sv = OBG × TVD
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Inputs
psi/ft
ft
Description
Computes the total vertical (overburden/lithostatic) stress at a given depth from the weight of the overlying rock column. Sv is the foundational stress term used to derive effective stress, pore-pressure trends, and fracture-gradient correlations elsewhere in the Geomechanics and Drilling disciplines.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Sv | psi | Overburden Stress |
| Sv_EMW | lb/gal | Overburden Stress (EMW) |
| OBG | psi/ft | Overburden (lithostatic) pressure gradient, representing the depth-averaged weight of the rock column above the point of interest — typically from an integrated bulk density log or regional model. Same gradient used by the Eaton Pore Pressure calc. |
| TVD | ft | True vertical depth of the point of interest. |
Assumptions
- Vertical (or near-vertical) wellbore, so TVD is the relevant stress-integration axis
- OBG represents a realistic depth-averaged density-weight, not a single point reading
- No significant lateral density anomalies (e.g. salt, reef buildups) across the interval that would invalidate a single-gradient simplification
Limitations
- A single average gradient is a simplification of the true depth-integral — basins with strong density contrasts (shallow unconsolidated sediment, salt, major unconformities) need a depth-banded OBG or direct density-log integration instead
- Does not itself account for structural dip or non-vertical wellbore geometry — TVD, not measured depth, must be used
- OBG accuracy is only as good as its source (regional model vs. an actual integrated density log) — treat the output with the same uncertainty as the input gradient
Use Cases
- → Effective stress input: Use Sv as the total-stress term in the Effective Stress (Terzaghi/Biot) calculation to determine the stress actually carried by the rock matrix.
- → Fracture-gradient prediction: Sv is a required input to the Eaton (1969) fracture-gradient correlation, which estimates minimum horizontal stress from Sv, pore pressure, and Poisson's ratio.
- → Casing and cementing design context: Compare Sv against planned mud weights and cement column weight to sanity-check loading assumptions during well design.
Related Calculations
Region Notes
Permian Basin
Delaware/Midland Basin OBG is commonly ~22.6 kPa/m (1.0 psi/ft) at Wolfcamp depths — the same value already used as the formation default for the Eaton Pore Pressure calc in this basin.
Gulf of Mexico
Shallow, unconsolidated deepwater sediments have a notably lower OBG near the mudline (often 18-20 kPa/m, ~0.8-0.85 psi/ft) that increases with depth as sediments compact — a single basin-wide average OBG is a poor assumption in the first several hundred meters below mudline.
Global
1.0 psi/ft (22.6 kPa/m) is a widely used rule-of-thumb average OBG, but local calibration from an actual integrated density log is always preferable where available.
References
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