Hydraulic fracturing is a key technology that can increase the permeability of a hot dry rock reservoir to an economically viable level for effective heat extraction. It is crucial to create fracture networks in the subsurface for sustained heat production. Previous studies have shown that hydraulic fractures can grow in planar structures or branch into multiple strands. However, it is unclear what conditions and injection design can lead to complex fracture patterns. To address this, we conducted laboratory hydraulic fracturing experiments using analog-rock samples that were constructed with controlled heterogeneity for repeatable experiments. We employed different combinations of injection rates and fracturing fluid viscosities to fracture the heterogeneous samples. The results indicate that: a) Slow injection of low-viscosity fluids results in diffusion-dominated fluid flow; 2) Injection of medium-viscosity fluids at moderate rate leads to complex fracture networks via crack branching; 3) Fast injection of high-viscosity fluid causes planar hydraulic fracture pattern. Those experimental results are repeatable, which suggests that manipulating injection rates and fluid viscosities allows us to control hydraulic fracture patterns in rock formations with pre-existing and permeable weak layers. For application in the field, our results can help to optimize injection fluid properties to create complex hydraulic fracture networks for sustained heat production from geothermal reservoirs.

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