Issue 47

Z.-Y. Han et alii, Frattura ed Integrità Strutturale, 47 (2019) 74-81; DOI: 10.3221/IGF-ESIS.47.07 76 The factors affecting fracturing efficiency of shale after liquid nitrogen cold impact treatment, such as the complexity of natural cracks, the displacement of fracturing fluid and the difference of ground stress, were mainly discussed. A comparison table of control variables was used to develop an experimental program, as shown in Tab. 1, where σ v is the vertical stress, MPa, σ H is the maximum horizontal principal stress, MPa, and σ h is the minimum horizontal principal stress, MPa. To achieve the goal of fully extending of crack, the pre-freezing displacement is set to an intermediate value of 40 ml/min, and the order from low to high after freezing is 20 ml/min, 40 ml/min and 60 ml/min. The liquid nitrogen treatment times are set as 1h, 2h and 3h, respectively. Figure 2: Change law of tensile strength before and after shale freezing. Figure 3: True triaxial experimental system for liquid nitrogen fracturing. Figure 4: Sample basic model diagram. 0 1 2 3 4 5 6 7 8 0 2 4 6 8 10 Tensile strength/MPa Core data points Before freezing After freezing Face A Face B Face C Face D Face E Face F