M.H.B. Nasseri

Failure mechanism in schistose rocks

Abstract Failure pattern of four anisotropic schistose rocks have been critically examined in macro and micro scale. Specimens tested at different orientation (β) varid from 0 to 90° with respect to major principal stress direction (σ 1 ) in unconfined and confined state upto 100 MPa of σ 3 . Quartzitic and Chlorite schists due to presence of continuous weakness planes exhibit “axial cleavage monofracture and multifracture” pattern in unconfined state which is typicall of hard fine grained rocks. Where as Quartz mica and Biotite schists due to absence of persistent continuous foliation plane and being coarse grained exhibit “multi shear cataclasis failure pattern”. The former two rocks undergo violent brittle failure changing to monfracture type at all orientations and “Saw—tooth structure is observed in SEM. The two latter rocks undergo shear failure at all orientations and confining pressures. Their fracture system make an oblique angle with σ 1 at all σ 3 in macro scale. Micro-fracturing pattern for Quartz mica schist is of grain boundary crack pattern and SEM shows high density cracks for Biotite schist due to higher content of Biotite.

Hydraulic fracture energy budget: Insights from the laboratory

In this paper we present results from a series of laboratory hydraulic fracture experiments designed to investigate various components of the energy budget. The experiments involved a cylindrical sample of Westerly granite being deformed under various triaxial stress states and fractured with distilled water, which was injected at a range of constant rates. Acoustic emission sensors were absolutely calibrated, and the radiated seismic energy was estimated. The seismic energy was found to range from 7.02E−8% to 1.24E−4% of the injection energy which is consistent with a range of values for induced seismicity from field-scale hydraulic fracture operations. The deformation energy (crack opening) of the sample during hydraulic fracture propagation was measured using displacement sensors and ranged from 18% to 94% of the injection energy. Our results support the conclusion that aseismic deformation is a significant term in the hydraulic fracture energy budget.

ISRM Suggested Method for Laboratory Acoustic Emission Monitoring

Acoustic emission (AE) is defined as high-frequency elastic waves emitted from defects such as small cracks (microcracks) within a material when stressed, typically in the laboratory. AE is a similar phenomenon to microseismicity (MS), as MS is induced by fracture of rock at an engineering scale (e.g., rockbursts in mines), that is, in the field. Thus, seismic monitoring can be applied to a wide variety of rock engineering problems, and AE is a powerful method to investigate processes of rock fracture by detecting microcracks prior to macroscopic failure and by tracking crack propagation. A basic approach involves using a single channel of data acquisition, such as with a digital oscilloscope, and analyzing the number and rate of AE events. Perhaps the most valuable information from AE is the source location, which requires recording the waveform at several sensors and determining arrival times at each. Thus, investing in a multichannel data acquisition system provides the means to monitor dynamics of the fracturing process. The purpose of this suggested method is to describe the experimental setup and devices used to monitor AE in laboratory testing of rock. The instrumentation includes the AE sensor, preamplifier, frequency (noise) filter, main amplifier, AE rate counter, and A/D (analog-to-digital) recorder, to provide fundamental knowledge on material and specimen behavior in laboratory experiments. When considering in situ seismic monitoring, the reader is referred to the relevant ISRM suggested method specifically addressing that topic (Xiao et al. 2016).