Acoustic emission and SEM analyses of hydraulic fractures under triaxial stress conditions

Abstract

Microseismic monitoring has become an essential tool in understanding the fracture growth and diagnostics during hydraulic fracturing operations. Although fractures are considered as planar features in modeling, field and lab studies indicate that most of the fractures are non-planar complex features. These departures affect energy dissipation during fracture propagation and affect proppant dispersement. Hypocenter locations and fault plane solutions of microseismicity are used to determine the orientation and geometry of fractures. The variation in velocity structure and the complex fracture geometry results in the uncertainty associated with hypocenter location. Laboratory acoustic emission measurements during hydraulic fracturing provide controlled conditions, better azimuthal coverage and understanding of the velocity model. We report the results of controlled laboratory hydraulic fracturing experiments instrumented with piezoelectric acoustic emission sensors. The rock samples studied are a tight sandstone and pyrophyllite. Sandstone is considered to be isotropic while pyrophyllite is a strongly foliated metamorphic rock having strong elastic anisotropy (∼25%) and permeability in the nanodarcy range, both very similar to shales. The samples are loaded triaxially to replicate the insitu stress conditions. The uncertainty in hypocenter locations, frequency analysis, source mechanisms and the effects of stress on fracture propagation will be discussed. SEM observations of the fractures are correlated with the mapped microseismic events. Fracture initiation in anisotropic materials is affected by the magnitude of anisotropy and can be predicted if the elastic constants are known. Fractures are nonplanar and shear failure is observed via focal mechanisms to be the dominant fracture mechanism.