ABSTRACT:

Laboratory hydraulic fracturing visualization experiments on transparent rock analogues containing preexisting fractures are presented. Soda-lime glass cubes (10 cm×10 cm×10 cm) containing pre-existing, partially open fracture networks are subjected to true-triaxal stress state while viscous fluid is injected through a small (~3 mm diameter) analogue borehole to induce fractures in the sample. The preexisting fractures consist of either laser-etched microcracks or thermal-shock-induced fractures which are partially healed by re-heating of the glass. Initiation and propagation of the hydraulic fractures are monitored and recorded visually via webcams and acoustically using acoustic emissions. A series of experiments conducted with different fluid injection rates of the fracturing fluid show that the injection rate can have a significant impact on the interactions between hydraulic fractures and pre-existing fractures—fast injection results in less interactions—, affecting the overall geometry of the resulting hydraulic fracture network. Experiments involving time-varying (frequency-modulated) injection rate are also conducted, yielding different complexity and characteristics of the hydraulic fracture network.

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