Borehole stability analysis: Considering the upper limit of shear failure criteria to determine the safe borehole pressure window

The issue of borehole stability has attracted great attention worldwide but serious downhole problems still exist. The influences of various factors on borehole stability have been extensively studied in the literature. It is usually believed the lower limit of safe borehole pressure window is determined by collapse failure while the upper limit is determined by fracture failure of rocks. However, it is proved in this paper the upper limit can also be decided by collapse failure. A new model of determining the safe borehole pressure window is built and the new upper limit based on shear failure criteria is proposed, therefore, a more precise safe borehole pressure window can be determined. The effects of borehole orientation, structural weak planes, in-situ stress and pore pressure are then thoroughly analyzed. The results show that the two limits defined by shear failure criteria are resulted from principal stress alteration and usually featured by change of failure mode. The upper limit of shear failure criteria is quite critical in determining safe mud pressure under various conditions. The increase of mud pressure within boreholes may not alleviate collapse failures. The Mohr-Coulomb criterion confines a narrower window while the Drucker-Prager criterion predict a wider one. For determining the safe borehole pressure window, a uniform insitu stress and a low pore pressure will make it more necessary to consider the upper limit of shear failure criteria. The model built and conclusions drawn can provide new guidance for mud weight selection and well trajectory design in drilling practice.