Laboratory hydraulic fracturing test on large-scale pre-cracked granite specimens

Abstract

Hydraulic fracturing could be a useful method for controlling the propagation path of cracks near an injection borehole by means of the perforating technique, pre-manufactured cracks, or the natural defects of the rock mass. The crack propagation path is not commonly along a fixed direction. Rather, it is likely to deflect gradually toward the direction of maximum horizontal stress. In order to study the behaviours of crack deflection in the hydraulic fracturing process, granite specimens having a through hole with pre-manufactured cracks on both sides, with a size of 1000 × 1000 × 1000 mm, were used in fracturing tests. The acoustic emission events during the whole fracturing process were recorded by an acoustic emission measurement system. The value of the horizontal stress difference during the tests clearly impacted the crack propagation path during the fracturing process. (1) New cracks were initiated from the tip of the pre-manufactured crack under the different loading conditions used in the tests, and the pre-manufactured crack was able to supply an initiation point for crack propagation. When the horizontal stress difference approached the average tensile strength value of the specimens, the crack propagation path on the surface of the specimen became irregular. (2) The work done during the hydraulic fracturing process was used mostly to overcome the restriction of the horizontal stress. A greater value of horizontal stress difference resulted in greater curvature of the crack propagation path. Additionally, the value of the horizontal stress difference was in proportion with the value of the energy for crack initiation. (3) The main failure model of the cracks generated during the fracturing process was tensile failure. Shear failure, with the high energy release, occurred mostly at the moment of crack initiation, when crack deflection activities were observed. The cracks generated by shear failure during the fracturing process were usually irregular.