Fracture Initiation and Propagation in the Hot Dry Rock Subject to the Cyclic Injection Hydraulic Fracturing Treatment

Cyclic injection hydraulic fracturing is a promising way for the geothermal energy exploitation by reactivating the fractures in geothermal reservoir. However, fracture initiation and growth induced by cyclic injection schemes have been inadequately studied for hot dry rock (HDR), and the cyclic injection fracturing optimized often by experience. For this reason, the initiation and propagation of hydraulic fractures in the HDR under different cyclic injection methods were determined by experiment research for hydraulic fracturing. The results show that the cyclic frequency and injection rate play different roles in the stimulation of HDR. The cyclic injection with low frequency-low pressure can create more branched fractures, forming a short but complex hydraulic fracture network. However, when high flow-high frequency injection method is subjected, the branch fractures formed are significantly reduced, but each branch fracture can be fully expanded. To fully exploit the advantages of different injection methods, a numerical model that contains a fracture network was established with PFC software, and an alternating cyclic injection scheme with synergistic control of the cyclic frequency and injection rate was proposed. The comparison results indicated that the alternating cyclic injection method can effectively improve the fracturing effect in the HDR. The stimulation area of the alternating cyclic injection method is about 2.3 times and 2.7 times that of the low flow-low frequency and high flow-high frequency injection methods, respectively. The method presented here can be adopted to optimize the fracture growth regime and provide a scientific basis for EGS hydraulic fracturing design.