Experimental investigation on hydraulic fracturing under self-excited pulse loading

Deep coal seams are commonly characterized by low permeability, high gas pressure, and high crustal stress. Accordingly, hydraulic fracturing is an effective and popular technology widely adopted for reservoir reformation. Moreover, pulse hydraulic fracturing (PHF) shows more tremendous advantages in generating a uniform and good-conductivity fracture network than traditional hydraulic fracturing (THF). In previous research, pulse loading was mostly supplied by a pulsating pump that is inconvenient, costly, and insecure. The experiment concerning putting a self-excited pulse oscillator (SPO) into the hydraulic pipeline system to offer pulse loading has not been proposed. Therefore, this paper applied pulse loading to fracturing similar coal specimens through a self-developed SPO. The differences in fracturing failure morphology, pressure–time curve, and acoustic emission information between PHF and THF were comprehensively compared. In addition, the effects of self-excited pulse loading on coal fracturing were analyzed and discussed, and the main conclusions can be concluded: The fracturing failure specimens under pulse loading show a more complex crack propagation pattern. PHF maintains a longer duration at the pressure stabilization stage than THF and owns a minor initiation pressure, indicating that more micro-cracks can be generated due to fatigue damage. The self-excited pulse loading induced by SPO shares similar effects on fracturing with the pulse loading provided by a pulsating pump. The greater the total pressure at the entrance of oscillator, the greater the pulse effect is. Furthermore, a more uniform cavitation effect and stable pulse loading can be obtained. In all, the research results can provide a theoretical reference for further gas prevention and control through PHF.