Definition - What does Mohr-Coulomb Criterion mean?
Mohr-Coulomb 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.
Trenchlesspedia explains Mohr-Coulomb Criterion
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.
The Mohr-Coulomb criterion is expressed as:
σ1 = C0 + q σ3
q= the slope of the line as it relates to σ1 and σ3.
σ3 is the minimum principal stress of the rock formation.
σ1 is the maximum principal stress of the rock formation.
The Mohr-Coulomb criterion does not take into account the polyaxial stress state, which accounts for intermediate principal stress. The intermediate stress state has a pronounced effect on rock strength.