Issue 47

Z.-Y. Han et alii, Frattura ed Integrità Strutturale, 47 (2019) 74-81; DOI: 10.3221/IGF-ESIS.47.07 77 Experiment number σ v /MPa σ H /MPa σ h /MPa Stress difference /MPa Freezing time /h displacement /ml/min S-6 10 8 3 5 0 40 SL-1 10 8 3 5 1 40 SL-2 10 8 3 5 2 40 SL-3 10 8 3 5 3 40 SL-4 10 5 3 2 3 40 SL-5 12 11 3 8 3 40 SL-6 10 8 3 5 3 20 SL-7 10 8 3 5 3 60 Table 1 : Shale fracturing test after liquid nitrogen freeze-thaw. The main steps of the fracturing simulation experiment under liquid nitrogen freezing are as follows: (1) Seven standard samples as shown in Fig. 4 were prepared according to the standard and labeled separately. A silica gel solution with a mass fraction of 4% was used as a fracturing fluid, and an appropriate amount of tracer was added. (2) The sample was placed in a heat preservation container, and the liquid nitrogen was continuously injected into the sample simulation wellbore through the low-temperature pipeline by using the liquid nitrogen self-pressurization system, and the duration varied from 1h to 3h according to the experimental requirements. (3) The sample had been placed at rest for a period of time after taken out from container, and then was placed in the main pressure-bearing cavity. Two steel blocks were placed on both Face A and Face B to adjust the height, and the upper cover of the experimenter was sealed and fasten with bolts. (4) An air compressor and a pneumatic control valve were used to apply the in-situ stress. The pressure was maintained at 0.8 MPa by the air compressor. The gas boosting device and the hydraulic pump were used to apply stress to the sample in three directions to a predetermined value. The load was applied smoothly to prevent pressure fluctuations during the whole process. (5) After the three-direction stresses applied to the presupposed values and kept stably, a pipeline was connected to the reserved hole above the sample. The silica gel solution with a mass fraction of 4% was injected in by a displacement pump controlled by the servo motor, to simulate the pressure variation during the whole fracturing process. The real-time pressure information of the injected fluid was synchronously recorded by the computer. (6) The pump was stopped when the experiment was finished. The wellbore pressure and the stress acting on the sample were unloaded in turn. The pressure in the hydraulic bladder was slowly released to zero to prevent damage to the equipment from instantaneous depressurization. (7) After the sample was taken out from the autoclave, and the crack morphology after fracturing was observed in detail and the fracturing mechanism was discussed. E XPERIMENTAL RESULTS Acoustic wave results total of 7 samples were involved in the experiment. According to the experimental procedure, each sample was initially frozen by liquid nitrogen. From the experimental phenomena, the original bedding and natural cracks on the surface, especially on Face A, of the shale developed well after cold treatment by liquid nitrogen. The fractures showed a radial distribution along the simulated wellbore, and the natural fracture network was perfect. To detect the formation of cracks in detail, five points were randomly selected on Face C and Face D, as shown in Fig. 5. The acoustic wave variations of the sample before and after the liquid nitrogen treatment on both sides of Face C and Face D were determined. Taking the sample SL-1 as an example, as shown in Fig. 6, the acoustic wave values of each monitoring point have decreased to some extent after liquid nitrogen cooling treatment, indicating that a large number of cracks or macropores have generated inside the sample after cold treatment. A