Abstract

Microseismic monitoring of hydraulic fractures typically produce spatial and temporal plots of hypocenters without any estimates of uncertainty leading the engineer to believe the hypocenter locations are absolute. The hypocenter location problem becomes more complex in anisotropic shale reservoirs. Hypocenter locations are determined from the arrival times of P-wave and/ S-waves and a known velocity model. The difference in the velocity structure and the complex fracture network make accurate fracture mapping difficult. The fracture properties are a function of the magnitudes and directions of applied stresses. We report on a series of laboratory nanoseismic studies during controlled hydraulic fracturing of limestone, sandstone and a strongly foliated metamorphic rock, pyrophyllite. In addition to hypocenter locations, we report on focal mechanisms, frequency analysis, source dimensions and spectral characteristics of the emitted event. Hydraulic fracturing is dominated by shear failure; fracture alignment is controlled as predicted by applied stresses in isotropic materials. However, hydraulic fracture propagation in anisotropic materials is altered by the magnitude of anisotropy but is predictable when the anisotropic elastic constants are known.

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