Borehole Stability Analysis Based On The Unified Strength Theory

As one of common engineering technologies, drilling is widely applied in different kinds of engineering fields, such as geotechnical engineering, exploration engineering and reservoir development. During the process of drilling, the borehole stability problem caused by stress concentration is focused on by engineers especially. Instability of borehole may cause wasting of resources and environmental pollution, so it is necessary to do further research on the subject.A main aspect of wellbore stability is the selection of an appropriate rock failure criterion. The Mohr-Coulomb criterion is most commonly used, but it involves only the maximum and minimum stress and ignores the influence of the intermediate stress on material strength. However, much evidence has suggested that the intermediate stress has strengthening effect on material strength. As a result, adopting the Mohr-Coulomb criterion in borehole stability analysis is conservative relatively.The unified strength theory, which considers effects of all the three principal stress, surmounts this defect. The UST can reflect the influence of the intermediate stress on strength through parameter b, which can be obtained from experiment, therefore it will be more reasonable and universal than the Mohr-Coulomb criterion to be used in borehole stability analysis.Based on the above background, the key researches are as follows:(1) A new3D analytical model was developed to estimate the mud pressure required to avoid shear failure. Based on the unified strength theory, closed-form solutions for vertical borehole and calculation process for deviated borehole were achieved. These solutions have been applied to field case and compared with traditional results, and it seems that they consistent with the field data well.(2) Parametric analysis was conducted for the unified solutions. The influences of deviated angle, azimuth angle and the intermediate principal stress parameter on mud pressure were analyzed.(3) The tensor expression of the unified elasto-plastic constitutive relationship was derived using backward Euler method, and the UMAT subroutine was developed and verified. Based on the UMAT developed, a simple borehole model was simulated. The distribution of the principal stress and plastic region were analyzed.