Statistical Model of Borehole Collapse Pressure in Brittle Formation Considering Rock Strength Size Effect

During the drilling process, when drilling with different boreholes unlocks the formation, the cracks, joints and other flaws exposed on the borehole wall are different. Experiment shows that generally the smaller the borehole, the more stable the borehole wall is. However, this intuitive understanding is lacking of theoretical support. Since the flaws of the wall such as cracks and joints cannot be accurately described, the influence of wellbore size on well stability is rarely used in the calculation of collapse pressure. To solve this problem, a brittle formation borehole stability model based on Weibull's statistical strength-size-effect theory was established, and the main influencing factors such as inhomogeneity and borehole size were discussed. The results showed that with the uniformity of the surrounding rock increased, the impact of borehole size on the stability of the borehole wall is reduced; compared to the reference 6˝ borehole, in which the collapse pressure increases with the borehole size increases. For the actual construction, while meeting the requirements of other projects, use small boreholes as much as possible for the formation that is easy to collapse.