Numerical Investigation of Borehole Breakout and Rock Spalling Based on Strain Energy Criteria

Borehole breakout is a natural phenomenon resulting from in-situ stress redistribution around a borehole, and its orientation and geometries are critical indicators for in-situ stress direction and magnitudes. The aim of the study is to better understand the borehole breakout formation mechanism and reproduce breakout geometries using numerical simulations. A 2D bonded-particle model (BPM) is developed based on Tenino sandstone using particle flow code (PFC), and simulations are conducted by applying different maximum horizontal stress values with the constant minimum horizontal stresses to a synthetic rock sample. V-shaped failure patterns are observed, and the obtained breakout geometries are compared with the published true-triaxial experimental results on the same rock properties and loading conditions. The modelled breakout widths are generally larger than the experimental results due to the lack of vertical stress in two-dimensional BPM simulation. For breakout depth, it is found that some broken particles remain within the breakout zone, constraining the damage zone development and thus withstanding more internal loads. Consequently, the breakout propagation process is suppressed, and the simulated breakout depths are significantly lower than the experimental values. To overcome the limitations of the breakout simulation, a particle removal algorithm using the strain energy release criteria is developed and implemented into the model. With the embedding of the algorithm, the breakout propagation process in the vicinity of the borehole shows a good agreement with the experimental breakout test results, indicating effective removal of failed rock is critical for accurately simulating borehole breakout. Model results from this study can provide new insights into the formation mechanism of V-shaped borehole breakout, as well as strain energy evolution during breakout initiation and propagation.