Analysis of fracture propagation behavior using hydraulically induced acoustic emissions in the Bernburg salt mine, Germany

Hydraulic fracturing experiments were carried out at the Bernburg salt mine in Germany. The induced acoustic emission (AE) events were detected by accelerometers surrounding the sources, and the observed AE events had a frequency component of over 40 kHz. Many similar waveforms were identified in the detected events, and their source locations were determined with an error of less than 6 cm. The principal direction of the source distribution changed with depth according to the rotation of the principal stress direction, and the shape of the fracture was determined to be elliptical. The source locations were distributed as an ellipse with a radius of about 1 m in both fracturing and re-fracturing tests at six different levels. A theoretical examination of fracture growth showed that macroscopic fractures extend with an aspect ratio which is a function of normal and shear stresses, and suggests that shear stress leads to the formation of elliptical fractures and that the elliptical shape of macroscopic fractures is controlled by the ratio of the shear stress to the normal stress.