Stress Heterogeneity and Natural Fractures in the Basel EGS Granite Reservoir Inferred from an Acoustic Televiewer Log in the Basel-1 Well

Abstract

Knowledge of the state of stress and the natural fracture system in the subsurface are pre-requisite pieces of information to model and understand the behaviour of Engineered/Enhanced Geothermal Systems during stimulation and subsequent operation. This information is also critical to understanding induced seismicity. An ultrasonic borehole image (UBI) log acquired prior to the stimulation of the BS1 well at Basel provides high quality information for characterizing the in-situ stress field and fracture system in the host granite of the reservoir. We present evidence derived from borehole breakouts and drilling induced fractures visible on the UBI log that the stress profile within the BS1 well prior to stimulation displayed a significant level of complexity. We use a new methodology based upon critical stress theory to estimate the minimum and maximum horizontal stress magnitudes. The stress orientation and stress magnitudes fluctuate over the metre to hundreds of metre scales, with fluctuations demonstrably correlated to the occurrence of natural fractures visible on the UBI log. The complexity of the stress field in BS1 contrasts with approximation, commonly used in numerical models, that the pre-stimulation state of stress in the subsurface has a constant orientation and magnitudes that vary in a linear manner with respect to depth. These approximations appear valid only in the long-wavelength limit. The findings suggest that the numerical simulation of the EGS reservoir stimulation process using physical models to estimate seismic hazard and permeability creation should include consideration of the complexity of the initial stress conditions.