51st U.S. Rock Mechanics/Geomechanics Symposium,
San Francisco, California, USA
2017. American Rock Mechanics Association
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90 since 2007
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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.
Propagation of fluid-injection-induced fractures within rock can be affected by weak interfaces, such as bedding and foliation planes, and open and healed fractures. How they interact with each other results in different fracture network geometry (e.g., Fisher et al, 2002), determining the success of oil and gas and geothermal reservoir stimulation.
Over the past decades, only a limited number of field observations (e.g., Warpinski & Trufel, 1987) have been made to understand the geometry of injection-induced fractures in real rock. In recent years, laboratory experiments involving large, meter-size rock samples have been conducted (e.g., Suárez-Rivera et al., 2013) to understand the fracturing behavior of heterogeneous shales. Usually, fracture growth is monitored via microearthquakes (in the field) and acoustic emissions (in the laboratory). However, the location of the fracturing events do not necessarily agree with the propagation of hydrologically connected (i.e., stimulating) fractures from the wells. Therefore, a better understanding of the relationships between the evolution of the fracture network and accompanying seismic signals is needed, through concurrent optical and seismic visualization.
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