Hydraulic fracturing of naturally fractured hot dry rock based on a coupled thermo-hydro-mechanical model

Abstract

The successful implementation of enhanced geothermal systems (EGS) depends on the complexity and transmissivity of the fracture networks induced by hydraulic stimulation. The reactivation of pre-existing natural fractures by hydraulic fractures plays a pivotal role in forming complex fracture networks in hot dry rock (HDR). Thermo-hydro-mechanical coupled models, employing the combined finite-discrete element method, are proposed herein to elucidate the interaction mechanism between hydraulic fractures and pre-existing natural fractures in HDR. Three representative types of natural fractures were incorporated into the model, which was validated through hydraulic fracturing experiments. The impact of geological and engineering factors on fracture interaction modes was systematically investigated, with an emphasis on the geometry of fracture networks. The results demonstrated that increased initial rock temperatures would induce greater thermal stress, which favors the opening of the pre-existing fracture, facilitating the hydraulic fracture to deflect into the natural fracture. Correlation analysis demonstrated that the horizontal stress contrast is the dominant factor affecting the complexity of fracture networks, and the HDR reservoirs characterized by weakly sealed natural fractures exhibited greater sensitivity compared to those with strongly sealed natural fractures when the horizontal stress contrast is less than 8 MPa. Additionally, the impact of reservoir temperature diminished with increasing bonding strength of natural fractures, and its effect on the fracture geometry became negligible when rock temperature was below 200 °C. Moreover, the impact of flow rate on the morphology of fracture networks was more pronounced than that of fluid viscosity. As a result, prioritizing the adjustment of injection flow rate over fracturing fluid viscosity is recommended for optimizing the stimulation of HDR. The key findings are expected to offer in-depth guidance for EGS stimulation treatment.