51st U.S. Rock Mechanics/Geomechanics Symposium,
San Francisco, California, USA
2017. American Rock Mechanics Association
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ABSTRACT: 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.
1. INTRODUCTION AND BACKGROUND
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. Recently, there has been an increase in the number of experimental studies being conducted on hydraulic fracturing in the laboratory. Experiments on natural rock typically use external displacement measurements, acoustic emission observation and post-failure inspection often combined with resin to capture the fracturing behavior during hydraulic fracture (Casas et al., 2006; Lecampion et al., 2015; Stanchitis et al., 2015a; Stanchitis et al., 2015b). Other researchers have used transparent model material, such as PMMA, to visualize the fractures (Rubin, 1984; Bunger et al., 2005; Lecampion et al., 2015). Visualizing the fractures in a natural rock, in real-time while applying confining pressures, has been difficult to achieve in the laboratory.
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