Experimental Study on the Effects of Perforation Phasing on Fracturing Pressure and Fracture Propagation of Thin Interbeds

Hydraulic fracturing experiment was performed using a large-scale real tri-axial simulation experiment system to extensively investigate the effects of perforation phasing and geo-stress on the fracture-initiation pressure and fracture propagation of thin interbeds. By scanning the fracture section, the hydraulic fracture propagation and distribution status were described, the effects of perforation phasing and geo-stress on the fracture-initiation pressure and fracture propagation as well as the basic mechanisms were analyzed. These researches can be used to provide support to fracturing design and operations. Experimental results showed that, ① rock samples in the test fractured at the end of perforation section, and the fracture diverted to propagate along the direction perpendicular to the direction of the minimum horizontal principal stress. A minimum fracture-initiation pressure existed under the same geo-stress and at 60° perforation phase, and the time spent in initial fracturing and in fracturing process was the shortest. Also under the same conditions, the fractures propagated most extensively, the number of fractures was the highest and the forms of the fractures were complex. ② when the difference between the vertical principal stress and the maximum horizontal principal stress was high, and the differential horizontal principal stresses was low, the fracture-initiation pressure was then low. The process of fracture propagation was steady, and was only weakly affected by rock breakdown. When the difference between the vertical principal stress and the maximum horizontal principal stress was low, the higher the differential horizontal principal stresses, the higher the fracture-initiation pressure, times for the fractures to propagate became less and time spent for fracturing was short. ③ When the differential horizontal principal stresses was high, the propagation of the fractures was evidently along the vertical direction. The fracturing section was flat and perpendicular to the minimum horizontal principal stress. When the differential horizontal principal stresses was low, the direction of the propagation of the fractures was difficult to control; it was easy for the fractures to deflect or to propagate along transverse direction. 4) When the hydraulic fracture met with structural plane, sub-fractures as well as bifurcation, deflection and cross-layer were generated. These were the necessary conditions for a complex fracture network to form. Formation bedding affects cross-layer of fractures, and micro-fissures and micro-pores all affects fracture-initiation pressure and fracture propagation.