Breakout Orientation Perturbation Modeling in Fractured Crystalline Rock

Abstract

The key component of a comprehensive geomechanical model is knowledge of the current state of stress. Breakouts are commonly used as principal indicator of stress direction. However, variation of breakout orientation with depth, especially in the vicinity of fracture zones, is frequently observed. Numerical modeling which taking into account the elastic property changes as a result of fracturing and fracture filling is developed to better quantify the breakout orientation heterogeneity observed in this study. Two different mechanisms for the breakout rotation are proposed. Anomalies of breakout orientation in the vicinity of fracture zones reflect the large-scale stress heterogeneity which might be caused by the previous slip acting on the fault plane. While the local breakout orientation anomalies around minor fractures might be the effect of the material heterogeneities around borehole due the intersection between the borehole with the fracture. The results of this study provide a better understanding of stress-induced borehole elongations in fractured rocks. Borehole breakout heterogeneities do not seem to be related merely to the principal stress heterogeneity, but also to the effect of mechanical properties heterogeneities, i.e. weak zones with different elastic moduli, rock strength and fracture patterns. Consequently, care has to be taken when inferring the principal stress orientation from borehole breakout data observed in fractured rock.