Thermal-shock-induced activation of cemented natural fractures and reinitiation of hydraulic fractures in deep shale

Injecting fracturing fluid that is cooler than the surrounding deep shale formation creates a thermal contrast, yet its effect on interactions between a hydraulic fracture (HF) and initially intact cemented natural fractures (NFs) remains insufficiently quantified. A thermo-hydro-mechanical (THM) phase-field framework examines NF activation, HF capture and reinitiation, and competition among fracture paths. Cemented NFs are represented as initially load-bearing weak planes with reduced fracture resistance, allowing damage and hydraulic communication to emerge from the coupled solution. Following benchmark verification and resolution assessment, paired hydro-mechanical (HM) and THM simulations under prescribed pressure are conducted for single, orthogonal, and oblique NF configurations. In the single-NF case, cooling initiates NF activation before HF contact and activates its full length, whereas the NF remains inactive under HM conditions. Cooling shifts S xx toward tension by 16.1 MPa; the induced stress S i reaches 9.4 MPa and remains positive over 64 mm. In multiple-NF systems, cooling partly counteracts compressive shielding and selectively promotes activation and HF reinitiation. For the oblique configuration, activated length increases from 205 to 364 mm, while its normalized value rises from 0.315 to 0.560 (77.6%). Thus, cooling redistributes stress and reorganizes HF-NF interaction paths, affecting fracture connectivity during such stimulation.