Borehole breakout has always been a challenging problem in drilling, especially in fractured and weak formations. Natural fractures are an important factor that affects wellbore breakout, but the traditionally used bedding plane models are not sufficient to capture borehole breakout in highly fractured formations. This paper proposes a coupled geomechanics and fluid flow model for evaluating borehole breakout in a naturally fracture network, using a discrete element method. The model considers anisotropic drilling fluid invasion within natural fractures. The effect of drilling fluid pressure, fracture density, in-situ stress, and seepage time on borehole breakout was investigated. The simulation results show that the natural fracture density does not directly control the breakout area around the wellbore. Intersection of fractures and borehole, increase in fluid seepage, and large in-situ stress ratio aggravate borehole instability. Increasing wellbore pressure without considering fluid seepage poses positive effects on preventing breakouts. The proposed numerical simulation approach provides a useful tool to understand and assess borehole breakout for drilling design in fractured formations.
Borehole breakout is a concerning problem of exploration and development of oil and natural gas. It is one of the most important problems in borehole stability issues, which is a type of stress-induced rock failure depending on the state of stress around the wellbore (Haimson, 2007). Generally, the borehole breakout phenomenon is caused by near-wellbore stress concentration. The breakout orientation is usually aligned with the minimum horizontal stress direction. The stress magnitude and orientation dominate the size and direction of breakout (Sahara, 2014). However, with the drilling formation becoming more complex, breakout orientation and size anomalies are constantly observed in the vicinity of the borehole. Extensive experimental and numerical studies have been conducted to investigate the mechanism of breakout and the controlling factors that affect the borehole breakout.
Meier (2013) found that the critical pressure for the initiation of borehole breakout decreases when the borehole diameter is below 10 mm through experiment tests. Granites, limestones, and sandstones were used for vertical borehole drilling experiments by Haimson (2007) to explore the micromechanics of borehole instability. Yin (2018) used multi-functional triaxial geophysical apparatus to analyze the effects of intermediate principal stress for yielded plastic zone around the borehole. Haimson (2004) studied the shape and micromechanics of vertical drilling borehole of two high-porosity sandstone under far-field principal stress, and concluded that breakout size is correlated with the far-field principal stress and consequently can be used as indicators of the far-filed stress.
