Investigation of the effect of different injection schemes on fracture network patterns in hot dry rocks - A numerical case study of the FORGE EGS site in Utah

Abstract Multi-stage stimulation using alternative injection has been successfully applied in low mobility hydrocarbon production. However, fracture initiation and growth induced by different injection schemes have been inadequately studied for hot dry rock (HDR) geothermal reservoirs. Here, the impact of injection schemes on hydraulic fracture (HF) propagation regimes was determined with PFC2D software. The results show that the propagation of natural fractures (NFs) created by cyclic injection are dominated by the mode of shear activation and direct penetration. However, cyclic injection with frequent starting and stopping can produce non-uniform stress and fatigue, resulting in crack initiation and more branched fractures growth. The stepped injection can activate NFs effectively, and the HF propagation are featured by a style of turning at the tip of NFs. However, the stepped injection often produces a single main fracture with few branches. Different injection methods can lead to different propagation regimes and ultimately result in variation of the fracture network. A numerical model of the FORGE site that contains relevant geological structure and a fracture network was established with 3DEC software, and the impact of NFs on the HF network formation was investigated systematically. Compared with cyclic injection, the fracture network formed by stepped injection is more susceptible to the distribution of the NFs. The value of stepped injection is about 1.31 times in surface area and 1.17 times in aperture than the cyclic injection. Cyclic injection is conducive to creating fractures in the matrix, while stepped injection is more inclined to activate the preexisting NFs. The method presented here can be adopted to other geologic settings to optimize the fracture growth regime and provide a scientific basis for Enhanced Geothermal System (EGS) multi-stage fracturing design.