Issue 47

Z.-Y. Han et alii, Frattura ed Integrità Strutturale, 47 (2019) 74-81; DOI: 10.3221/IGF-ESIS.47.07 78 From the results of fracturing, main hydraulic cracks form near simulated wellbore, and extend along the direction perpendicular to the minimum horizontal principal stress, finally form a vertical fracture surface. Micro-cracks form on each face, and most of them extend in the direction of natural cracks and then cross each other to form a crack network. From the dissection of the sample after fracturing, the traces of the white tracer are clearly, indicating that liquid nitrogen enhances its permeability of shale. Figure 5: Schematic diagram of acoustic wave test points. Figure 6: Acoustic wave results before and after freezing of Face C and Face D on SL-1 sample. Effect of liquid nitrogen cold treatment time The hydraulic fracturing simulations were carried out to study the effect of cold treatment time on fracturing effect. The fracturing construction curve of sample SL-1, SL-2 and SL-3, by changing the length of liquid nitrogen action time while maintaining the stress difference and displacement, are shown in Fig. 7. It can be seen that the fracture pressure is the highest after immersion for 1 hour in liquid nitrogen, about 21 MPa. After immersing for 2 hours and 3 hours, the fracture pressure remains basically unchanged as 10.8 MPa and 10.5 MPa, respectively. This phenomenon shows that the fracture pressure decreases with the increase of impact time within a certain period of time. But this change is limited by a time effect. After immersion of 2 hours, the effect of the low temperature on the physical and mechanical properties of the sample did not change much as the treatment time prolonged. The cracking time reflects that the pre-impact of liquid nitrogen mainly enhances the brittleness of shale. However, the late impact reduces the brittleness of shale and increases the plasticity, and it is not easy for cracking, and the cracking time is greatly increased by about 5 times. The crack gradually forms a cross-sewed network from the general macro-crack, and the fracturing effect is better at about 2h. Influence of stress difference The comparative experimental results of sample SL-3, SL-4 and SL-5 show that the fracture pressure of the sample decreases with the increase of the stress difference, under the condition of only changing the magnitude of the stress difference, as shown in Fig. 8. However, there is no obvious regularity about the initiation time affected by the stress difference. At the same time, the fracture-making ability increases gradually with the increase of the stress difference because the cross-sewed network gradually forms from the general macro-crack. p 1 p 1 ' p 2 p 2 ' p 3 p 4 p 5 p 3 ' p 4 ' p 5 ' Face C Face D 4250 4300 4350 4400 4450 4500 4550 4600 4650 1 2 3 4 5 Wave speed m/s Measuring point Before freezing After freezing