Effect of Grain Size in Granitic Rock on Crack Extension in Hydraulic Fracturing

ABSTRACT: We monitored AE (acoustic emission) during hydraulic fracturing experiments by employing four 20cm-cubic granitic rock specimens of different grain size. Under two horizontal confining pressures, 6 and 12 MPa, from a vertical borehole, a horizontal crack was induced along a rift plane in the larger grain size specimens, while a vertical crack was induced normal to a rift plane in the smaller grain size specimen. We also found that the numbers of located AE sources increased with increasing grain size. In addition, magnitudes of breakdown pressure decreased with increasing grain size. These results indicate that rock textures such as grain size considerably affects the geometry, surface roughness and micro-fracture mechanism of hydraulically induced cracks.