ABSTRACT:
Elastic wave velocities, water permeability k and fracture toughness KIC under mode I were measured in thermally pre-cracked Westerly granite. A series of chevron cracked notched Brazilian disc and a series of cylindrical specimens were thermally damaged slowly up to 850°C. A dramatic 80% decrease of KIC was observed with thermal treatment, from 1.4 to 0.2 MPa.m0.5. Microcrack density increased from ~0.2 for RT specimens to ~10 for specimens heat-treated at 850o C. The decrease of 50% in KIC for ?>1, 80% for ?>5 was successfully predicted using non-interactive effective medium theory. An exponential decrease of k and increase of elastic wave velocities with increasing effective stress is observed and hysteresis occurs during effective stress decrease. Vp measurements are a good geophysical indicator of the damage and hence the deterioration of KIC and enhancement of permeability with microcracking, and could be used as a nondestructive technique to monitor and predict the evolution of these parameters in the laboratory and field.
1 INTRODUCTION
Mechanical and transport properties of crustal rocks are profoundly influenced by cracks and pore spaces. The existence of embedded microcrack fabrics in rocks significantly influences the fracture toughness (KIC) (Nasseri et al. 2005, 2006), the elastic wave velocities and the permeability of rocks (Benson et al. 2006). Need for understanding of a coupled hydro-thermo-mechanical effect on fracture toughness and permeability in rocks is increasingly in demand. Seismic and velocity field data and its relation to such coupled phenomena is important and has a role in improving oil production, a better understanding of the stability of underground waste depositories, volcanic eruptions and earthquake processes (Rutqvis et al. 2005, Engvik et al. 2005). By performing controlled thermal cracking experiments, we investigate quantitatively the influence of thermal damage on KIC and permeability.
