Setting rock dependent constants

On the Rock Dependent Constants form (Wellbore Stability > Setup > Settings > Rock Dependent Constants) you can specify some of the rock mechanical properties for the rock types defined in your project.

Rock mechanical properties

Biot Coefficient  Parameter used in poroelasticity, which describes for a porous and fluid-saturated rock how much of a change in confining pressure is transferred to the pore pressure under conditions when fluid cannot escape from the pores. The Biot coefficient is given by 1 – Cs/Cm, in which Cm is the compressibility of the rock and Cs is the compressibility of the solid material/crystals that make up the rock. Since Cs <= Cm, the value lies between 0 and 1. For highly porous media Cs is much smaller than Cm and the coefficient comes close to 1, which in most cases is taken as the standard value.

  • When not using the specified coefficient of the rock types in the case, you can use a default value by checking the use constant checkbox and specifying a value.
  • Currently the Biot Coefficient is used to calculate effective stresses; otherwise, it is an elastic material model.

Failure Criterion  Borehole failure calculations are governed by a failure criterion, with the Modified Lade criterion as the program’s default selection. See Failure criteria for details on the failure criterion calculations referenced below.

When using a constant failure criterion, there are eight calculations that you can select:

  • Mohr-Coulomb
  • Tresca
  • Modified Wiebols-Cook
  • Circumscribed Drucker-Prager
  • Inscribed Drucker-Prager
  • Hoek-Brown
  • Modified Lade (the default selection)
  • Stassi D’Alia

The Mohr-Coulomb, Drucker-Prager Inscribed, Circumscribed Drucker-Prager, Wiebols-Cook, 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, which depend on properties of the rock and the extent to which the rock is broken before being subjected to failure. The value for either constant can be globally defined in the WBS Preparation workflow panel, or on a per-lithology basis in the Rock Dependent Properties view.

You can include different failure criteria for specific lithologies in the rock dependent constants option.