Effect of Injection Rate on Hydraulic Fracturing of Opalinus Clay Shale

For hydraulic fracturing operations in the field, the pumping rate is generally controlled but it is not fully understood how this affects the resulting fracture processes. This paper presents the results of an experimental study using a laboratory hydraulic fracturing setup, in which the injection rate was varied across four experiments, and the fracturing process was observed visually as well as with acoustic emission sensors. Prismatic 2 × 4 × 1 in. specimens of Opalinus clay shale, with a pre-cut 0.33 in. width flaw, were subjected to hydraulic pressure under constant biaxial far-field loading conditions. The pressure was measured internally inside the flaw as well as at the pressure volume actuator. Both a high speed- and a high resolution camera were used to visually record changes on the face of the specimen (i.e. fracturing). Acoustic emissions were recorded with an array of eight piezoceramic sensors embedded in specialized platens. The results of the experiments were then used in an analysis of the peak pressures and fracture propagation speed related to the injection rate. Higher peak pressures, fracturing speeds and fracturing accelerations were observed with higher injection rates. Additionally, the spectral analysis of the largest AE events showed that the highest injection rates resulted in higher power at lower frequencies. The highest injection rate was also associated with greater AE activity in general.