Laboratory Study of Hydraulic Fracturing in Cyclic Injection

In order to study the influence of cyclic injection on the initiation and extension of hydraulic fracturing, Two large-scale tri-axial laboratory experiments, with the sample size of 30in*30in*36in, are conducted with spiral perforation completion on isotropic cement samples by two different ways of cycle and general injection. After the experiment, the pressure data, the actual crack morphology and acoustic emission (AE) data are analyzed. The results show that: (1) Compared with general injection, hydraulic fracturing with cyclic injection can effectively reduce the initiation pressure, which is like the behavior of rock under uniaxial and tri-axial cyclic loading, cyclic injection can bring about fatigue. (2) For spiral perforation completion wellbore, the hydraulic fracture initiation only starts at the weakest perforation. Once a fracture is open, other perforation holes are hard to initiate. The stage spacing and cluster spacing should be chosen reasonably in hydraulic fracturing in order to optimize the efficiency of reservoir stimulation. (3) The occurrence of the Kaiser effect before fracture initiation is the result of pore fluid diffuse to rock and cause the local rise in pore pressure, the failure modes can still be represented by Mohr circle. The research is of guiding significance to choose the proper stage spacing and cluster spacing in hydraulic fracturing. Meanwhile, the exploration to the law of fracture propagation in cyclic injection is of great importance for the development of new fracturing technology.