Major factors influencing critical well bore pressures are summarized and used in a study in which well collapse pressure is calculated using the Mohr-Coulomb, the Modified Lade, and two versions of the Drucker-Prager criterion. Both linear and nonlinear rock models are considered. Compared to the other two failure criteria, the Inscribed Drucker-Prager and the Mohr-Coulomb are conservative as they predict higher well collapse pressure than the other two criteria, i.e. the Modified Lade and the Circumscribed Drucker- Prager. On the other hand, the latter provides very optimistic predictions that are significantly below predictions given by the other three criteria. Thus, the outer Drucker-Prager seems to be significantly underestimating well collapse pressure. In addition, the collapse pressure obtained using the modified Lade criterion is lower than the corresponding pressure obtained from the Mohr-Coulomb criterion, which – in turn – is below the critical well pressure obtained from the Inscribed Drucker-Prager criterion. Linear elastic collapse pressures are above the corresponding critical well pressures obtained when nonlinear material properties are taken into account. The effect of wellbore inclination on stability is also discussed.
Critical well bore pressures are commonly used in petroleum engineering design and monitoring of drilling and production operations to minimize the risk of excessive costs incurred as a result of wellbore instability incidents. Wellbore instabilities receive special attention due to their huge economic impact. The related downtime associated with well construction is about 10–15% of total well cost or about 50% of total nonproductive time . Among many factors that typically enter wellbore stability studies such as in-situ state of stress, pore pressure regime, rock mechanical and constitutive properties, the wellbore direction and inclination, and – sometimes – thermal effects and chemical factors, the accuracy of critical wellbore pressure predictions will depend strongly on the correctness of the failure criterion used and the possibilities are many. Well drilling disturbs the natural stress state in the rock, causes stress redistributions and produces stress concentrations at or near the borehole wall. This may potentially lead to different types of hole problems such as stuck pipe, borehole collapse, etc. Mechanical wellbore stability is discussed in terms of critical wellbore pressures that are related to rock stresses around a wellbore: sz (axial), sr (radial), and sq (hoop), Fig. 1. It is defined by specifying critical well pressures or the mud weight window, i.e. the lower and upper limit of drilling mud weight that can be used for safe drilling of a new well. The lower limit is associated with shear failure (borehole breakouts, Fig. 1, are also evidence of shear failure) and calculated using the rock failure criteria. The upper limit corresponds to rock fracturing when tensile rock failure occurs, i.e. the tangential stress at the wellbore wall exceeds the tensile strength To of the rock, sq >To. Well fracturing pressure is calculated from this criterion. Exceeding the upper wellbore pressure may lead to lost circulation but sometimes is done intentionally as in well fracturing for stimulation or in-situ stress measurements .