Issue 47

Z.-Y. Han et alii, Frattura ed Integrità Strutturale, 47 (2019) 74-81; DOI: 10.3221/IGF-ESIS.47.07 80 Therefore, to get a better fracturing effect, it is suggested that the larger displacement fracturing be adopted firstly to produce the main fracture, and then the lower displacement fracturing be used to develop the fracture network. Figure 9: Curve of fracture pressure with displacement. a) Before fracturing (Face A) b) After fracturing (Face A) c) Before fracturing (Face C) d) After fracturing (Face C) Figure10: Morphology of the SL-7 sample before and after fracturing. C ONCLUSION (1) The original bedding and natural cracks on the surface of the natural shale sample were further developed after the cold treatment by liquid nitrogen. The sound wave test results showed that a large number of cracks or large pores had generated inside the sample. (2) The fracture pressure of the shale decreased with the increase of the impact time when the immersion time was short, and remained stable after immersion of 2 hours. When the freezing time extended, the crack initiation time increased accordingly. (3) After low temperature impact, the fracture pressure of shale decreased with the increase of stress difference, but the effect of stress difference on initiation time was not obvious. (4) From the perspective of crack-forming ability, under the premise of good developed bedding, the larger displacement was more likely to form the main crack inside the shale, while the effect of forming the secondary crack was not good. 9,3 10,5 21 0 5 10 15 20 25 0 20 40 60 80 Fracture pressure/MPa Displacement/ml/min