Failure Criterion

With the principal stresses and rock properties defined for your stability analysis, the failure criterion can then be specified on the Failure Criterion form (advanced stress & failure > Setup > Failure Criterion or wellbore stability > Setup > Failure Criterion). Two approaches to using a failure criterion can be used: 'Constant', in which the selected criterion is applied throughout the calculation, and 'Variable', in which you can specify additional contingencies based on rock type or completeness of the data.

When using a constant failure criterion, the following calculations are available (see the Failure Criterion appendix for calculations): Mohr-Coulomb, Tresca, Modified Wiebols-Cook, Circumscribed Drucker-Prager, Inscribed Drucker-Prager, Hoek-Brown, Modified Lade (the default selection), and Stassi D’Alia. The Mohr-Coulomb, Drucker-Prager Inscribed, Circumscribed Drucker-Prager, and Lade criteria depend on the uniaxial rock strength and internal friction coefficient selections. The Stassi-D’Alia criterion depends on the tensile strength in addition to the uniaxial rock strength and internal friction coefficient. The Tresca criterion depends on the uniaxial rock strength. Finally, the Hoek-Brown criterion is a function of two empirical constants, Is and Im. Is and Im can be specified individually, if Hoek-Brown is selected, using the drop-down boxes at the bottom of the form.

New solutions default to the Constant option. The Variable option allows you to specify two additional failure criterion options:

• Use rock types  Applies the failure criterion defined in the Rock Dependent Properties view for each rock type in the project.
• Use log  You can select an alternate failure criterion log.

If the log data is not sufficient to perform the calculation, you can select a backup option from the eight failure criterion calculations that are described above.

You can include different failure criteria for specific lithologies if you have defined these criteria in the Rock Types view.

To select the failure criterion

1. Choose between using a Constant or Variable failure criterion. Based on your selection, the following selections then need to be made:
   1. Constant  Select the failure criterion that you want applied throughout the calculation. You can select multiple failure criteria and designate one of them as the Active criterion. The Active criterion is shown in bold and is the one used to predict the mud weight.
   2. Variable  Select between applying the failure criterion per rock type with the Use rock types option or using a log as the failure criterion, with the Use log option. When using a log, the log will then need to be specified in the adjacent drop-down list.
2. When using the Constant approach, click the checkbox next to a criterion to select or de-select it. Right-click to access more selection options, or use Set as active to designate a criteria as the one used to predict mud weight.
3. When using the Variable approach, if insufficient or no data is available to perform the calculation, a fallback failure criterion needs to be selected.
4. Optionally, if you are using the Constant approach and have selected Hoek-Brown, you can specify a log or constant value for the Is and Im parameters.
5. Click OK to proceed to the optional Advanced Mode form which allows you to override certain calculation parameters with user-defined constants. If you do not wish to use Advanced Mode, you can skip it and click the Validate Input Data icon, which opens the WBS Preparation view for visual analysis of your data.