Analysis of Stress Heterogeneities in Fractured Crystalline Reservoir

Abstract

The present-day state of stress has a key influence on fluid flow through fractured geo-reservoirs. Breakouts are commonly used as principal indicators of stress direction. However, variation of breakout orientation with depth, especially in the vicinity of fracture zones, is frequently observed. This study describes a systematic analysis of breakout occurrence, variation of breakout orientation and its modeling. Numerical modeling which, taking into account the elastic property changes as a result of fracturing and fracture filling, is developed to better quantify the very local-scale breakout orientation heterogeneity observed in this study. Two different mechanisms for the breakout rotation are proposed. Whereas 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, the local breakout orientation anomalies around minor fractures might be the effect of the material heterogeneities around boreholes due to 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 to the principal stress heterogeneity only, but also to the effect of mechanical heterogeneities like 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.