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Abstract
Analysis of Acoustic Emission (AE) induced during brittle and porous rock fracturing at variety of loading conditions has been performed. On the base of advanced analysis of AE parameters, ultrasonic velocities and mechanical data we found that regardless of applied loading conditions the process of rock fracture can be separated into two main stages: (A) accumulation of non-correlated cracks localized almost randomly in the whole volume of uniformly stressed rock. (B) Final stage of sample fracturing could be characterized by appearance of AE nucleation site followed by initiation and propagation of the macroscopic fault. Contribution of tensile sources is reduced significantly, shear type and pore collapse type events dominate during propagation of a fracture process zone through the sample regardless of applied loading conditions. In the case of porous rock, nucleation of compaction bands could be clearly identified by the appearance of AE clusters inside the samples. Microstructural analysis of fractured samples shows excellent agreement between location of AE hypocenters and faults or the positions of compaction bands, confirming that advanced AE analysis is a powerful tool for the process of rock fracture investigation.
Highlights
There are three basic crack modes identified: Mode I, Mode II and Mode III
In case of deformed granite sample (Figure 4a) we observed existence of the dense system of almost parallel microcracks having sizes close to the grain size. These possibly result in stress induced anisotropy of elastic wave velocities (Figure 2c) and in existence of high percentage of tensile sources during initial stage of loading (Figure 2e)
We suggest that during later stages of loading main macroscopic fault could crosscut the system of these echelon cracks, resulting in substantial increase of shear-type events, largely at the expense of tensile events (Figure 2e)
Summary
There are three basic crack modes identified: Mode I (extension, opening), Mode II (in-plane shear) and Mode III (out of plane shear). Dilatancy is generally observed as a precursor to brittle faulting and to the development of shear localisation, recent field [10] and laboratory [8,9, 11,12,13] observations have focused attention on the formation of localised compaction bands in porous sandstones. This suggests that the presence of compaction bands may affect fluid circulation in the crust, extraction of oil and gas from reservoir rocks, groundwater circulation in aquifers, as well as the sequestration of carbon dioxide
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