Horizontal stress
The Horizontal stress wizard calculates the minimum horizontal stress, Shmin, and the maximum horizontal stress, SHmax, as continuous curves along the well. It reads the vertical stress and pore pressure curves of the well together with the elastic properties of the rock, and each run produces one output curve for the one stress and the one method that are selected. Compressive stress is positive throughout.
It runs at the selected well or, with Batch calculate, at a group of wells, and it shares the well selection, depth range, unit handling and run reporting common to the calculators of the Petrophysics bar.
Launching the wizard
Open the Petrophysics ribbon bar and click Horizontal stress in the GeoMech section; on the button the caption is too wide and is cut in the middle to Horizo...stress. Select the well, limit the interval on the MD range, TVD range, TVDSS range, Markers range or Zones range tab, choose the stress on the Shmin or SHmax tab, pick a method, check the input logs and their units, and then Run.
Shmin and SHmax
The Shmin and SHmax tabs are a mode selector rather than two steps of one run: each tab carries its own method list, its own parameters and its own output curve, and Run produces the curve of the tab that is in front. Run the wizard once on each tab to obtain both curves.
The Input logs group sits above the tabs and is shared, so the curve assignment made for one stress is kept for the other.
Methods
Choosing a method in the Method list rebuilds the input rows and parameters that this method reads, the equation card under them and the default Output curve name, and hides the rest.
Shmin tab
- Uniaxial strain — Shmin = ν/(1−ν) · (Sv − α·Pp) + α·Pp. The tectonically relaxed case, in which the rock is free to compact vertically only. It reads Sv, Pp, Poisson’s ratio and the Biot coefficient, and no Young’s modulus.
- Poroelastic tectonic strain — Shmin = ν/(1−ν) · (Sv − α·Pp) + α·Pp + E/(1−ν²) · (εh + ν·εH). The same model with a horizontal strain term added, so it reads Young’s modulus and the strain pair as well.
- Effective stress ratio — Shmin = kh · (Sv − α·Pp) + α·Pp. A calibrated ratio applied to the effective vertical stress; it reads neither Poisson’s ratio nor Young’s modulus.
SHmax tab
- Poroelastic tectonic strain — SHmax = ν/(1−ν) · (Sv − α·Pp) + α·Pp + E/(1−ν²) · (εH + ν·εh), the same equation with the two strains exchanged, so the strain along the axis being solved for comes first.
- Effective stress ratio — SHmax = kH · (Sv − α·Pp) + α·Pp.
- Uniaxial baseline — SHmax = Shmin, for when equal horizontal stresses are intentionally assumed. It reads an existing minimum horizontal stress curve instead of the stress inputs, and the wizard states under the method that SHmax = Shmin copies the minimum horizontal stress as it is: this method does not model horizontal stress anisotropy.
The equation of the selected method is shown on a card under the parameters, together with the note Evaluated in MPa with compressive stress positive, then converted to the output unit.
Input logs
Only the rows that the selected method reads are shown. Each curve list contains all curves of the selected well and carries its own unit list, and a row is pre-filled where the well has a curve of the matching log type:
- Vertical stress Sv — from a curve typed Overburden Stress or Overburden Pressure. It also sets the sample grid of the run: the other inputs are reinterpolated onto it.
- Pore pressure Pp — from Pore Pressure or Formation Pressure.
- Biot coefficient alpha — a typed value, 1.000 by default, not a curve.
- Poisson’s ratio nu — from Poisson’s Ratio, as a fraction or a percentage. It is read by Uniaxial strain and by both Poroelastic tectonic strain methods.
- Young’s modulus E — from Young’s Modulus, in GPa, MPa, Mpsi or psi. It is read by the Poroelastic tectonic strain methods only.
- Existing Shmin curve — from Minimum Horizontal Stress. It replaces the stress inputs for Uniaxial baseline.
Under Young’s modulus E the wizard reports how the selected modulus was produced: dynamic when the curve was made by the Elastic properties wizard, which measures at sonic frequency and is therefore stiffer than a static modulus, and unknown when the curve carries no run settings and the project cannot tell the two apart. The note is informative only: it never stops the run, and no conversion between static and dynamic is applied.
Supply each curve in the unit it was logged in and let the wizard convert it; the pressure unit lists follow the project measurement system where the curve itself carries no unit.
Parameters
The parameters that the selected method reads are entered under it. Biot coefficient alpha opens at 1.000, the Minimum strain eps_h and Maximum strain eps_H pair at 0.000000, and Effective stress ratio kh (kH on the SHmax tab) at 0.32, the value that corresponds to frictional equilibrium at a friction coefficient of 0.6.
On the SHmax tab the strain pair follows the values entered on the Shmin tab, so both curves describe one stress model; the maximum stress keeps its own strains only from the moment they are edited on that tab.
The Biot coefficient, the strain pair and the effective stress ratio each carry a Constant / Per zone list. Constant applies the typed value over the whole interval. Per zone opens the Parameters per zone table, which holds one row per zone selected on the Zones range tab and one column per per-zone parameter — alpha, eps_h, eps_H and kh or kH:
- A blank cell falls back to the constant on the main form, so a zone that was not filled in is not zeroed.
- Fit all fills every zone with the constants entered on the main form, as a starting point to depart from, and Reset empties the table again.
- Save as table... stores the values as a project table under Statistics / Tables, and Load from table... reads such a table back, matching its rows to the zones by name.
Select the zones first: with no zone selection the wizard reports Select the zones on the Zones range tab first: the table follows that selection.
Quality control
Two rows check the result instead of producing it, and neither of them changes a curve:
- Friction coefficient mu (QC) — 0.60 by default. It sets the frictional strength of pre-existing faults, and after the run the wizard counts the samples that fall outside that bound, where the stress state would already have been relieved by slip.
- Existing SHmax for cross-check on the Shmin tab, and Existing Shmin for cross-check on the SHmax tab — the list offers only curves of the opposite stress type, so a same-type comparison cannot be mistaken for a check. After the run the wizard counts the samples where the maximum horizontal stress is below the minimum one.
A parameter that is out of range is called out while it is being entered: Biot coefficient is outside the usual [0, 1] range., kH is below kh, which puts the maximum horizontal stress under the minimum. and Friction coefficient must be positive; the frictional bound check was skipped.
Output curve
The Output curve field carries the default name of the selected stress and method, for example Minimum Horizontal Stress - Poroelastic tectonic strain or Maximum Horizontal Stress - Effective stress ratio, and can be edited before the run. The unit list beside it offers the pressure units. The saved curve is typed Minimum Horizontal Stress or Maximum Horizontal Stress, which is what the wizard itself looks for when it pre-fills the Existing Shmin curve and cross-check rows on a later run.
Running and saving
The wizard has the standard row of buttons along the bottom of the dialog:
- Save to workflow — adds the wizard with all of its current settings to the project Workflow as a task that can be re-run later.
- Run — calculates the curve with the current settings and shows it in the Input/Output preview beside the calculated inputs.
- Save — stores the calculated curve in the well. A curve that already carries the output name is replaced after a Confirm prompt; if nothing has been calculated yet, the wizard reports Nothing to save.
- Close — closes the wizard.
A task reopened from the Workflow panel restores the saved settings and opens on the tab that matches the saved method, and the button row collapses to Save and Cancel, so the task is edited and stored back instead of being run from there.
Changing the method, a curve, a unit, a parameter or the zone selection discards the previous result, so the preview never shows a curve that no longer matches the settings.
Results
After Run the wizard reports for how many wells the calculation completed and lists them, then how many samples produced no value, separated into Samples with a missing input and Samples rejected by the model. A sample is rejected when:
- Poisson’s ratio leaves the range in which the model is defined, that is 0.5 or above, or −1 or below;
- Young’s modulus is not positive, for the Poroelastic tectonic strain methods;
- an input or a parameter the method needs is not a finite number.
When samples fall outside the frictional bound their number is added to the report together with the friction coefficient that was used. A separate Calculation details message lists what happened per well: a curve that could not be read, a cross-check that was skipped because no curve of the opposite stress was selected, and the count of the samples where SHmax < Shmin. A well whose required input curve is missing is skipped, so no empty output curve is created for it.
See Also