The stability of breakouts was studied in an effort to explain why breakouts in hollow cylinders eventually stabilize and higher external stresses are needed for further breakout propagation. This occurs despite the fact that the stress concentration in the breakouts is larger as the breakout depth increases. Experiments on hollow cylinder specimens with various hole geometries showed that the hole failure stress is affected by the hole geometry. In particular, holes with elliptical breakouts showed a significant increase in hole failure stress. Hole failure is considered the development of shear bands and fractures which alter the shape of the hole. The experiments were simulated numerically with a finite element program using a Cosserat elastoplastic model which can predict the breakout development and perform stable post-failure calculations. The results show that holes with elliptical breakout are more stable than circular holes and that the critical stress for hole failure increases with breakout depth. Despite the hole failure, the breakouts grow stably and the specimens can still support higher stresses.
1. INTRODUCTION
Boreholes and other underground cavities created into the rock may fail in compression under the in situ stress field. Failure takes place adjacent to the cavity surface due to the stress concentration around the cavity, which is generally significantly higher than the in situ stress. Under the stress concentration, portions of the cavity may fracture or spall resulting often in an elongation of cavity cross section in the direction of the minimum principal compressive stress orthogonal to the cavity axis, which are called breakouts. In boreholes used for production of hydrocarbons, cavity failure is accompanied with undesirable production of solids. Hollow cylinder laboratory experiments of weak reservoir sandstones or outcrop analogues have shown that under isotropic external compression cavity failure occurs through the development of two breakouts (Figure 1) or through spiraling shear bands all around the cavity (Figure 2). Breakouts are observed in sandstones that exhibit significant dilatancy in shear while shear spirals in sandstones that exhibit compaction or moderate dilatancy in shear. The experimental results showed also that after initial hole failure (Figure 1a and Figure 2a) the external stress has to increase for further breakout or shear spiral development (Figure 1b and Figure 2b). Thus the breakout geometry is stable at a given external stress and higher stress is needed for further breakout development. The stability of the breakouts is in contrast to the higher stress concentration that develops at the breakout tip as the breakout depth increases. Zheng et al. [1] analyzed numerically the breakout development and suggested that the reason for the increased breakout stability is the fact that at some distance from the breakout tip, the deviatoric stress decreases with increasing breakout depth while the hydrostatic stress increases. Zheng and Khodaverdian [2] performed hollow cylinder experiments with various hole shapes to prove that the circular hole is not the more stable geometry.
The present work builds on previous work by the authors and others on the subject of cavity stability and attempts to provide a method of theoretical analysis of the experimental observations.
