Hot Dry Rock geothermal exploitation is an important energy exploitation mode, and the subcritical propagation of hydraulic fracturing will significantly reduce breakdown pressure and improve recovery efficiency. Previously, double-twist test was used to measure subcritical crack extension for predicting the material life, which is not suitable to the hydraulic fracture extension. In this paper, a new step loading experiment method is proposed, and the subcritical fracture propagation behavior caused by fluid-solid coupling was studied by changing the step loading time. In the experiment, the true tri-axial hydraulic fracturing equipment was adopted, and guanidine gel solution was 1% in concentration for the fracturing fluid, and 70% of the fracture pressure was taken as the initial step pressure. During the experiment, acoustic emission was used to detect damage accumulation and fracture propagation. The effects of stress difference and pressurization time on subcritical crack propagation were analyzed. The research shows that (1) The step loading method can effectively reduce the breakdown pressure in both horizontal and vertical wells. When the step loading time interval is 10 min, the breakdown pressure under horizontal well conditions is reduced from 42.3 MPa to 41.1 MPa with a reduction of 2.5%. Under vertical well conditions, the breakdown pressure is reduced from 21.1MPa to 20MPa with a reduction is 4%. (2) Branching fractures are extensively induced after step loading according the fracture morphology. (3) RA and AF calculated from acoustic emission data show that shear cracks prevail in the step loading. It is vital to study the subcritical crack propagation, especially in enhanced geothermal system, which is of great significance to improve the economic and effective development of geothermal mining.
In recent years, the fracture mechanics is used to study the propagation of rock cracks. Experiments have shown that the crack will initiate when the stress intensity factor at the crack tip is higher than the fracture toughness. When the stress intensity factor at the crack tip is much lower than the fracture toughness, the crack can still propagate with a very slow speed, resulting from fatigue or stress corrosion. This stable, quasi-static crack propagation is called subcritical extension. This fact has been mentioned in the past (Smith, 2015).