UCS
The UCS wizard calculates uniaxial compressive strength as a continuous curve along a well. Each run applies one empirical or user-defined correlation to the selected well-log curves and produces one UCS output curve.
It runs at the selected well or, with Batch calculate, at a group of wells. It shares the well selection, depth ranges, zone selection, preview, and run reporting described for the calculators on the Petrophysics ribbon tab.
Launching the wizard
Open the Petrophysics ribbon tab and click UCS in the GeoMech group. Select a Well, or select Batch calculate and a well group. Limit the interval on the MD range, TVD range, TVDSS range, Markers range, or Zones range tab, choose a method, check its inputs and units, and click Run.
The example below uses the Horsrud porosity method with NPHI recorded in percent. After the run, the input and calculated output are plotted side by side in the Input/Output panel.
Methods
The Method list has five non-selectable group headings: Sonic, Elastic modulus, Porosity, Combined logs, and User-defined. Selecting a method leaves only its required inputs visible, changes the equation image and legend, fills the default output name, and loads its default coefficients.
The equation image uses A, B, and, where needed, C because the values remain editable in Method coefficients. Use Reset to defaults to restore the values listed below.
Sonic
- Horsrud (2001) - Shale — reads Compressional slowness (DTC) and evaluates UCS = 0.77 × (304.8 / Δt)^2.93, with Δt in us/ft and UCS in MPa.
- McNally (1987) - Sandstone — reads Compressional slowness (DTC) and evaluates UCS = 1200 × exp(−0.036 × Δt), with Δt in us/ft and UCS in MPa.
Elastic modulus
- Bradford et al. (1998) — reads static Young's modulus E and evaluates UCS = 2.28 + 4.1089 × E.
- Limestone — reads static Young's modulus E and evaluates UCS = 13.8 × E^0.51.
- Dolomite — reads static Young's modulus E and evaluates UCS = 25.1 × E^0.34.
For these three methods E is converted to GPa before the equation is evaluated, and UCS is calculated in MPa before output conversion.
Porosity
- Horsrud (2001) - Shale — reads porosity and evaluates UCS = A × φ^B. For v/v, A = 2.922 and B = −0.96; for %, A = 243.6 and B = −0.96. The equation card uses φ for both choices and its legend identifies whether porosity is a fraction or a percentage. Switching the unit changes an untouched default pair to the corresponding defaults.
Combined logs
- Moos et al. (1999) - Density + Vp — reads density ρ and P-wave velocity Vp and evaluates UCS = 1.745 × 10^−9 × ρ × Vp² − 21, with ρ in kg/m3, Vp in m/s, and UCS in MPa.
User-defined
The User-defined correlation method provides these formulas:
- UCS = A + B × X
- UCS = A × X^B
- UCS = A × exp(B × X)
- UCS = A + B × X + C × X²
- UCS = A + B × X + C × Y
Select the X curve; the two-input formula also shows a Y curve. The curve rows report their recorded units, and Coefficients produce specifies whether the fitted coefficients produce MPa or psi. The coefficients are unit-dependent and must match the selected curves and their displayed units.
Inputs and units
Each visible curve list contains every curve of the selected well. Where a recognized log type or mnemonic exists, the wizard preselects a suitable curve and follows its recorded unit. The grey expected label identifies the correlation's native unit; the selected input is converted before calculation.
- Compressional slowness (DTC) — us/ft, us/m, m/s, or ft/s.
- Static Young's modulus — GPa, MPa, Mpsi, or psi.
- Porosity — v/v or %.
- Density — kg/m3 or g/cc.
- P-wave velocity — m/s, ft/s, km/s, us/ft, or us/m. A sonic slowness curve can therefore be used directly and is inverted during conversion.
- X curve and Y curve — use the units recorded on the selected user curves without automatic conversion.
The first input required by the selected method supplies the output sample grid. For a method with another input, that curve is reinterpolated onto the same measured-depth samples. Unit defaults otherwise follow the project measurement system.
Dynamic Young's modulus
The published elastic-modulus correlations require static Young's modulus. When the selected curve is recognized as dynamic, including a Young's modulus produced by the Elastic properties wizard, the UCS wizard displays: This curve looks like a dynamic Young's modulus. Select a static curve or define the dynamic-to-static conversion below.
Either select a static curve or select Apply dynamic-to-static conversion and enter the relation E_static [GPa] = A + B * E_dynamic [GPa]. The fields open with A = 0 and B = 1; replace them with the conversion appropriate to the data. The converted modulus is passed to the selected correlation. Run is stopped with a warning while a recognized dynamic curve is selected without an enabled conversion.
Output, run, and save
The Output curve field opens with UCS - <method title>, for example UCS - Horsrud (2001) - Porosity, Shale, and can be edited. Its unit list offers MPa and psi.
The bottom button row contains:
- Save to workflow — stores the selected wells, interval or zones, method, input curves and units, coefficients, dynamic conversion, and output settings as a project Workflow task.
- Run — calculates with the current settings and fills the Output track in the preview.
- Save — stores the calculated curve under the well's Logs structure. An existing curve with the same name is replaced only after the standard confirmation.
- Close — closes the wizard.
A saved curve uses the UCS mnemonic and the type Uniaxial Compressive Strength. It is immediately available to other applicable calculations without restarting g-Space or manually remapping the mnemonic. Right-click the saved curve in the Data manager and select Show UCS settings to reopen the method, inputs, units, coefficients, conversion, output, and calculated interval that produced it.
Reopening a UCS task from the Workflow panel restores its saved setup. In task-editing mode the button row becomes Save and Cancel.
Results
After Run, the wizard lists the wells that completed and counts samples that produced no UCS value by cause:
- a required input is null;
- an input is outside the selected correlation's domain;
- the calculated UCS is not positive, which indicates that the correlation is being used outside the rock conditions it represents;
- the result is not a finite number.
A well with a missing or invalid required curve is listed separately under wells skipped or partially calculated, and no empty output curve is created for that well.
See Also