Fracture process zone in granite

Abstract

Abstract. In uniaxial compression tests performed on Aue granite cores (diameter 50 mm, length 100 mm), a steel loading plate was used to induce the formation of a discrete shear fracture. A zone of distributed microcracks surrounds the tip of the propagating fracture. This process zone is imaged by locating acoustic emission events using 12 piezoceramic sensors attached to the samples. Propagation velocity of the process zone is varied by using the rate of acoustic emissions to control the applied axial force. The resulting velocities range from 2 mm/s in displacement-controlled tests to 2 lam/s in tests controlled by acoustic emission rate. Wave velocities and amplitudes are monitored during fault formation. P waves transmitted through the approaching process zone show a drop in amplitude of 26 dB, and ultrasonic velocities are reduced by 10%. The width of the process zone is -9 times the grain diameter inferred from acoustic data but is only 2 times the grain size from optical crack inspection. The process zone of fast propagating fractures is wider than for slow ones. The density of microcracks and acoustic emissions increases approaching the main fracture. Shear displacement scales linearly with fracture length. Fault plane solutions from acoustic events show similar orientation of nodal planes on both sides of the shear fracture. The ratio of the process zone width to the fault length in Aue granite ranges from 0.01 to 0.1 inferred from crack data and acoustic emissions, respectively. The fracture surface energy is estimated from microstructure analysis to be -2 J. A lower bound estimate for the energy dissipated by acoustic events is 0.1 J.

Triggering Processes in Rock Fracture

“…The role of the large-scale imperfections can be further established by considering the distance of the triggered events from a given imperfection. Figure 3(a) shows that triggering cascades tend to propagate away from the imperfection, consistent with process zone models [51][52][53]. Figure 3(b) shows that the topological structure of triggering cascades is characterized by an increase of hdi f with increasing size and that hdi f > 5 for the largest triggering cascades.…”

Analysis of induced seismicity in geothermal reservoirs – An overview

“…Because the distribution of the seismic cloud in the reservoir strongly depends on the type and geometry of the seismic network, velocity model and location method used, its size cannot be related one-to-one to the actual fracture location and mechanisms operating in situ, as evidenced by the difference in the extension of the acoustic (seismic) cloud and the actual fracture pattern observed in granite cores in the laboratory (Zang et al, 2000). Although basic aspects of generating fluid-induced seismicity are understood, there are, nonetheless, gaps in our understanding of the key reservoir parameters that control the rate and magnitudes of the seismic events.…”

“…Also from laboratory testing it is known that acoustic emission activity and source mechanisms are very different for e.g., sandstones (Lockner and Byerlee, 1972;Zang et al, 1996) as compared to granite (Lockner and Byerlee, 1992;Zang et al, 2000). This is caused by different fracture mechanisms operating during fluid injection into different rock types as discussed in Section 5.…”

Magma-driven hydraulic fracturing and infiltration of fluids into the damaged host rock, an example from Dronning Maud Land, Antarctica

“…An experimental study based on acoustic emissions from propagating hydraulic mode-I fractures in Weber Sandstone was carried out by Lockner and Byerlee (1977). An ongoing laboratory scale study on tensile fracturing of granite using acoustic emissions (G. Dresen, personal communication, 2003) shows a process zone similar in extent to that observed by Zang et al (2000) for shear fractures, within the limits of spatial resolution.…”

“…In laboratory experiments, the formation of microcracks in the process zone can be monitored by analysis of acoustic emissions (e.g. Lockner, 1995;Zang et al, 2000) and afterwards compared with the microstructure of the deformed sample (e.g. Swanson, 1987;Janssen et al, 2001).…”