What Does Rock Failure Criterion Mean?
The rock failure criterion is crucial in predicting the failure of rocks or rock masses. The criteria includes methods of predicting how and when a rock will fail when acted upon by external forces.
During the drilling of a wellbore, the use of optimal mud weight, location of casing seats, and drilling parameters that minimize swab and surge are important parameters that ensure that the newly created borehole does not collapse in on itself.
To calculate the precise parameters, engineers turn to the rock failure criterion for adequate calculations. Several failure criteria have been developed over the past years such as the Mogi-Coulomb criterion and the Mohr-Coulomb criterion.
Trenchlesspedia Explains Rock Failure Criterion
Mogi-Coulomb rock failure criterion is used in wellbore stability analysis understanding rock strength under true triaxial condition. It is found that the intermediate principal stress influences the ultimate strength of a rock.
When ignoring the intermediate stress and utilize solely the Mohr-Coulomb criterion greatly underestimate the overall rock strength. Conversely, some believe the Mogi-Coulomb criterion greatly overestimates overall rock strength.
Mohr-Coulomb rock failure criterion is the most commonly used shear failure criteria proposed for rocks. In the Mohr-Coulomb criterion, only the maximum (σ1) and minimum (σ3) principal stresses are considered and it is assumed that the intermediate stress (σ2) has no influence on the strength of a rock.
This criterion ignores the strengthening effect of the intermediate stress σ2 and hence is considered to be too conservative in estimating critical mud weight essential to maintain the stability of the wellbore.
As rock failure is one of the uncontrollable instability factors that often lead to the collapse of a wellbore, the use of the Mohr-Coulomb failure criterion is frequently employed to understand the risks of collapse.