During the past years a better understanding of the fracturing processes to enhance productivity in hydrocarbon and geothermal reservoirs has become more important for the industry. Within this context hydraulic fracturing type experiments on 150 mm edge length cube shaped blocks with a 13 mm diameter central borehole under different true-triaxial confining stresses up to 20 MPa have been conducted in this study. Deformations and Acoustic Emission (AE) were monitored throughout the experiments. Rock types that have been tested comprise rhyolites, sandstones and slates with low porosity and permeability. Fracturing has been conducted with water as a fracturing fluid and via pressurization of an elastic polymer sleeve to eliminate poroelastic effects. AE data evaluation revealed several characteristics of fracture initiation and propagation as ’trapping’ and ’deflection’ of the induced fractures at pre-existing fractures, branching and deviation of the induced fracture away from the expected orientation, differences in source types as well as fracture propagation parallel to the borehole. Also the activation of pre-existing fractures prior to fracture initiation and different kinds of fracture initiation patterns were observable.
Hydraulic fracturing is applied in a broad range of applications as in hydrocarbon and geothermal reservoir productivity enhancement, stress measurements or stress relief in e.g. tunneling. Within this range of applications companies are confronted with changing lithological material properties, stress magnitudes, fracture dimensions and public vulnerability in the case of induced seismicity. The process of hydraulic fracturing is well understood for homogeneous and isotropic media . Most rocks exhibit a certain degree of heterogeneity and anisotropy caused by bedding, cleavage or pre-existing discontinuities as joints or faults. For field hydraulic fracturing operations the dimensions of the induced fractures can be in similar dimensional scale to the before mentioned attributes. Therefore, fracture propagation direction, fracture geometry and injection pressures may be strongly affected by anisotropy .
Since the late 1970`s hydraulic fracturing experiments are conducted in the laboratory in combination with AE monitoring [3, 4, 5]. Throughout the decades AE monitoring of laboratory hydraulic fracturing experiments became more and more sophisticated by incorporating methods known from disciplines like seismology, seismics and non-destructive testing. These methods comprise precise arrivaltime detection algorithms [6, 7], different localization approaches allowing for anisotropic velocity fields [8, 9, 10], collapsing techniques [11, 12, 13], source type evaluation methods , couplants  and AE-tomography approaches [16, 17]. Furthermore, the availability of affordable monitoring hardware gave rise to the usage of the AE monitoring technique for laboratory experiments in geosciences.