In 2017 and 2019, injection testing was carried out in three zones in Well 58-32, drilled into granitic rock at the Frontier Observatory for Research in Geothermal Energy (FORGE) site near Milford Utah. Some of the injection tests were simulated numerically with a distinct element method (DEM) based code - XSiteTM, which is a fully coupled hydro-mechanical model with explicit representations of discrete fracture network (DFN). The 3D DFN contains more than 1000 natural fractures. The objective of these back analyses is to calibrate the model with respect to unknown and uncertain in situ reservoir parameters and to match the recorded pressure histories. Of particular interest are the increasing trends in the injection pressure histories and their relative magnitudes in Cycles 4 and 5, conducted in a perforated section of the cased well. The pressures measured during Cycle 5 exceeded those recorded during an identical injection stage (Cycle 4) pumped earlier in the same perforation cluster. The numerical analyses show that the interaction of the hydraulic fracture and 3D DFN, and response of DFN to fluid flow and dissipation are vital to understanding the injection pressure performance. The location, size, and properties of natural fractures significantly affect the injection pressure. The numerical study shows that the increasing pressure trends are due to fluid diversion into the DFN as localized leakoff and deformation (including slip and dilation) of the DFN. The higher pressure in Cycle 5 is mainly the result of irreversible deformation by fluid injection in the previous test (Cycle 4). The numerical investigation enhances our understanding of the FORGE reservoir. It sheds light on what might be called self-shadowing, where one injection cycle impacts the injection performance of a subsequent stage pumped at the same location.

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