Assessing the Minimum Safe Drilling Mud Weight to Avoid Damage Zone by Frictional-Strengthening Cohesion-Weakening Models

In order to evaluate the potential of wellbore stability and required drilling mud weight, a realistic advanced elastoplastic model representing accurately the post-peak rock behavior must be implemented to calculate the stresses and strains surrounding a wellbore. In this paper, the effect of drilling mud weight on the normalized yield zone area (NYZA) around a carbonate oil wellbore placed in the southwest of IRAN is numerically evaluated based on the various advanced constitutive models. A kind of the frictional-strengthening cohesion-weakening model that estimates well the dimension and shape of the damage zone around the studied wellbore is selected to propose a relation for the NYZA versus mud weight diagrams. The general form of this mathematical equation for the NYZA-Mud Weight diagrams at different depths of the investigated wellbore obeys the negative exponential relation, and the coefficients of this relation were determined based on fitting. The evaluation of the results shows that there is acceptable accuracy between the predicted values and the values of NYZA measured by the numerical model. Furthermore, to assess the safe mud weight based on this advanced model considering the actual post-peak rock behavior, all criteria to distinguish between minor and major breakouts are reviewed in this paper, and the minimum safe mud weights are determined on the basis of the various criteria for the studied wellbore. The minimum safe mud weight based on the NYZA = 1 criterion is estimated very low. This is while the minimum safe mud weight for the studied wellbore according to the NYZA = 0.08 criterion is too high and unreasonable. On the other hand, the minimum mud weight is more realistically estimated based on the 90° criterion in comparison with the NYZA = 1 and NYZA = 0.08 criteria.