In this paper, the role of micro-scale properties and behavior on the detection of crack initiation, propagation and geochemical alteration is examined through three topics: (1) identification of a geophysical precursor for a system transitioning from meta-stability to unstable behavior with specific focus on crack nucleation, propagation and coalescence; (2) demonstration of acoustic emissions from geochemically-induced fractures; and (3) understanding the role of depositional layers and mineral fabric on tensile crack formation. The results from these studies advance current understanding of which microscopic properties of evolving fracture systems are most useful for predicting macroscopic behavior and the best imaging modalities to use to identify the seismic signatures of time evolving fracture properties.
The recent growth in shale gas extraction, geothermal energy development and storage of anthropogenic gases and fluids has led to increased human interaction with the Earth's subsurface. A management challenge for these subsurface sites is to optimize extraction/storage approaches to yield maximum potential while minimizing risks. Fractures are one of the dominant factors that influence the success or failure of these management tasks because all subsurface activities perturb fluid pressures and stresses in rock, causing mechanical discontinuities to open, close, initiate, coalesce and/or propagate, while natural and engineered fluids can result in geochemical alterations that lead to crack growth. With the goal of sustaining production/isolation throughout the life-cycle of a subsurface site, it is necessary to detect and image fracture systems to monitor alterations as well as to link geophysical measurements to mechanical and hydraulic integrity of the subsurface rock.
Failure in rock is a progression of energy transfers from the smallest scales (lattice or microstructure) to potentially the full scale of a system under consideration. At the smallest scale, the mineral composition, distribution, orientation and bonding among minerals are known to affect the engineering properties of a rock in addition to the presence of structural features such as micro-cracks, layers and other sources of porosity. For example, Brace (1965) demonstrated experimentally that the anisotropy in the intrinsic elastic property of linear compressibility is effected by mineral crystal orientation based on measurements on rock with oriented mica, calcite and quartz at pressures of 0.2 to 0.9 GPa to remove the effects of micro-cracks. Agliardi et al. (2014) found that failure modes and uniaxial compressive stress depended on the orientation of loading relative to foliation in gneissic rock but with significant scattered in the values of UCS. While Chandler et al. demonstrated for shale the effect of the orientation of fine scale layering relative to loading direction on fracture toughness. They found that layer orientation alone was not sufficient to explain the observed anisotropy in fracture toughness.
