53rd U.S. Rock Mechanics/Geomechanics Symposium,
New York City, New York
2019. American Rock Mechanics Association
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ABSTRACT: We experimentally investigate the effect of the pore fluid on AE in four sandstone plugs cored from a homogeneous slab. These plugs were fully saturated with brine, mineral oil, and hexane, while one of them remained dry. AE level was highest in the hexane-saturated sample, smaller in the oil-saturated sample, and even smaller in the brine-saturated sample. It was even smaller in the dry sample. It appears that in the liquid-saturated plugs, the AE level is monotonically related to the acoustic impedance of the pore fluid – the smaller the impedance the higher the AE. It does not appear to be related to the viscosity of the pore fluid.
Acoustic emissions (AE) are commonly observed in natural rock during its deformation and failure. This effect can serve as a precursor to failure, as well as an indicator of fracture initiation, growth, and closure. AE is described as the generation of elastic waves by energy release from within the material (Dunegan, 1977). This release of energy can be linked to the internal mechanisms, such as slippage and/or brittle failure of grains, pore collapse, and initiation of microcracks due to applied stress. Lockner (1993) points out that AE produced by rapid crack growth is a ubiquitous phenomenon associated with brittle failure.
Jin et al. (2017) performed an AE study on Berea sandstone, Marcellus, and Eagle Ford shales under fixed confining pressure and increasing axial stress. The shale samples were obtained in both horizontal and vertical orientation. AE response was dissimilar for different orientations reflecting how bedding planes can play a role in AE whereas the homogeneous Berea sandstone sample showed increase in the AE level when approaching the yield point. The difference in the AE response of these samples was related to the type of grains, mineralogy and the orientation. Jia et al. (2018) studied the AE response from a shale sample and found that it behaves differently from granite sample investigated previously by the same authors. The AE from the former only spiked at near failure while the latter showed a linear response while undergoing dilation.
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