The paper presents 3D modeling and analysis of a series of hydraulic stimulations conducted at the DOE Utah FORGE EGS site. We have simulated Stage-1 and Stage-3 field stimulations and examined the pressure history, produced fracture geometry, and potential complexity. We use an advanced hydraulic fracture model that takes hydraulic and natural fracture propagation and interactions into account. The rock and fracture deformation are modeled by the displacement discontinuity method (DDM), fracture fluid flow is simulated using the finite element method, and the fracture propagation is implemented in the framework of the linear elastic fracture mechanics. The results demonstrate that asymmetric height growth is to be expected in the stress regimes and lithology commonly encountered in EGS and that height growth can be hindered by the rock mass discontinuities such as preexisting fractures, rock anisotropy, and lithological boundaries. In both the Stage-1 stimulation (i.e., activation of natural fractures and subsequent wings formation) and the Stage-3 stimulation (i.e., hydraulic fracture propagation in the presence of natural fractures) in Utah FORGE, the data seems to suggest upward growth and interaction with natural fractures, leading to a moderately complex SRV consisting of a HF/NF network.
Low permeability hot rocks are a significant source of geothermal energy that can be harnessed utilizing the enhanced or engineered geothermal systems (EGS) concept, which recovers energy by injecting cold water via fracture networks that can be either naturally occurring or artificially created. Therefore, by acting as the primary conduit for fluid flow and heat exchange, cracks and fractures play a crucial role in the extraction of geothermal energy. Hydraulic fracturing technology which is extensively used in unconventional petroleum reservoirs has gained popularity in EGS or what we have termed "unconventional geothermal reservoirs" (Ghassemi, 2021). However, the geometry of hydraulic fractures in EGS which is crucial for well stimulation is still not adequately understood (Ghassemi and Kumar, 2023). Hydraulic fracturing in the presence of natural fractures is a complex problem that needs to be carefully examined for better stimulation results (Kamali et al., 2022). The use of low-viscosity fracturing fluids such as water in geothermal reservoir stimulation can cause very severe proppant settlement near the injection point, resulting in a small, propped fracture network. Additionally, proppant transport and deposition also affect heat transfer in the fracture network (Liu et al., 2022).
