Characterization of fracture extension and damage evolution in hot dry rock by cycle hydraulic fracturing (CHF): application of CHF in enhanced geothermal systems

To address the research gap regarding fracture propagation and damage evolution in hot dry rock (HDR) subjected to cyclic hydraulic fracturing (CHF) under varying cycle pressures, this study conducted real-time high-temperature true triaxial CHF tests. In addition, a fully coupled thermo-hydro-mechanical-damage (THMD) numerical model for CHF was established for the first time. The results show that cyclic fracturing is an effective method to reduce the breakdown pressure (BP) of HDR with the formation of complex fracture networks. Influenced by the geostress, the hydraulic cracks in HDR under CHF are difficult to extend along the direction of non-maximum principal stress. Compared to traditional hydraulic fracturing specimen (T1), BP reductions for CHF specimens (C1, C2, and C3) were 1.6 MPa, 3.8 MPa, and 5.5 MPa, respectively. RA-AF analysis revealed that tensile destruction dominates granite rupture under both traditional hydraulic fracturing (THF) and CHF, with CHF leading to a higher proportion of shear cracks. Specifically, shear crack percentages increased by 4.37 %, 7.56 %, and 13.07 % for C1, C2, and C3 compared to T1. Higher cycle pressures increase the number of hydraulic crack branches, damage units, peak seepage pressure, and cryogenic fluid flow range. The cracking mechanism in HDR under CHF involves a complex interplay of reduced tensile strength, thermal stress from cold shrinkage, fluid pressure, and cumulative cyclic loading damage.