Experimental Studies Of Damage And Physical Properties Evolution On Brittle Rock Samples

Crustal rocks contain variable amount of cracks of diverse size, shapes and orientations de-pending on tectonic stresses and geological settings. These rocks are strongly heterogeneous, ani-sotropic and complicated. Experimental results show that the dominant influence on effective elastic properties of rocks come from cracks, not pores, although cracks contribute to porosity in a negligible way, Increasing the crack damage will significantly affect the mechanical and physical properties of rocks. Fluids are pervasive and ubiquitous in earth crust and most of the rocks are fluid saturated. The fluids also exert significant influence on the rock properties through mechani-cal and chemical effects. Thus, it is a long-term goal to understand the crack damage and physical properties evolution of brittle rock samples.On the basis of the previous research, two kinds of rock sample are considered in this study according to effective elastic medium theory (Non Interactive Assumption, NIA). One is isotropic medium, corresponding to hydrostatic condition in laboratory. The other is transversely isotropic medium, consistent with the deviatoric stress (uniaxial and triaxial condition). Experimental stu-dies of damage and physical properties evolution on brittle rock samples are investigated by the technology of strain-stress, elastic velocities, permeability, crack density evolution, AE location and moment tensor.1On microscopic scale, rock damage is characterized by the dislocation and destruction of binding bond, the original micro-crack growth, extension, nucleation and lead to the final macros-copic fracture zone. On macroscopic scale, however, it is characterized by the reduction of the mechanical properties, such as the stress-strain properties, the elastic wave velocity. With the effective medium theory, the macroscopic elastic properties can be combined with the micro crack density. The crack density inversion show that the vertical cracks induced by stress mainly propa-gate after rock initial dilatancy. When the stress increases to the rock dominant dilatancy, the cracks inside rock sample will significantly extend to form a macroscopic failure plane. Before initial dilatancy, if the rock has a lot of initial random crack damage, these cracks will close first However, for the rock sample with less initial crack damage, it won't experience the crack closure phase. These results are consistent with the five phases of stress-strain curve:crack closure, elastic phase, stable crack extension, unstable crack extension and after peak stress. AE location and moment tensor analysis revealed useful in order to image the rupture processes (crack initiation, nucleation and propagation). AE mechanism analysis shows that first the cracks close. With the stress larger than rock initial dilatancy, the tensile stress at the tip of cracks induced by far-field stress make the cracks slide and most AE are tensile cracks. When stress is larger than the stress of rock dominant dilatancy, cracks propagate to introduce wing cracks in a curve way. Then these wing cracks interact with each other to form the macroscopic failure. Most of these AE are shear cracks.2. Initial crack damage has significant influence on physical and mechanical properties of rocks. Comparing to the rock with less initial damage (undamaged rock sample), the rock with larger initial damage (damaged sample) has some characterization as:the P velocity decreases about1000m/s-3000m/s, and the permeability decreases4-5orders, and the initial damage reduces the stress level of rock dilantancy significantly, although has no influence on brittle strength. With differential stress increasing, elastic anisotropy appeared earlier in damaged specimen than in undamaged one. The terminal failure plane orientation has been chosen as early as initial dilatancy for damaged specimen, and dominant dilatancy for undamaged ones. Anisotropy at failure is ap-proximately the same for both types of specimen. AE results show that, the larger initial damage, the easier these cracks interact with each other, thus lead to the shear localization early. For unda-maged samples, cracks initially grow and distribute randomly inside the sample.3The fluid exerts significant effects on physical and mechanical properties with the increasing damage. The existence of fluid, especial uncompressible fluid like water, will stiffen the cracks, thus make the crack contribution to compliance less. Then P velocity will increase as high as2600m/s. The experimental results show that, the lower crack aspect ratio, the larger difference between the dry moduli and saturated moduli, which means the shape of damage is important for the elastic properties. Thin cracks have larger fluid effects than pores. The fluids also influence the rock properties through mechanical and chemical effects such as subcritical crack growth. Time-dependent creep results show that saturated damaged sample are more sensitive to stress, and the cracks extend easily. Crack density results show that failure was reached for vertical crack densities of0.35for dry specimen, probably0.5for water saturated specimen.