射孔相位及地应力对薄互层起裂压力及裂缝扩展影响的实验研究

为了深入研究射孔相位、地应力对薄互层起裂压力及裂缝扩展的影响，采用大尺寸真三轴模拟实验系统进行水力压裂实验，通过扫描裂缝断面描述了水力裂缝扩展和分布状态，分析了射孔相位及地应力对薄互层起裂压力及裂缝扩展的影响，并进行初步的机理分析，为压裂设计和施工提供支持。实验结果表明：①岩心在射孔末端沿射孔方向开裂，随后裂缝转向，沿垂直于最小水平主应力方向扩展；相同地应力下，射孔相位60°时岩心起裂压力最小，初次开裂及压裂过程用时最短，且裂缝扩展或产生新缝数量多、形式复杂。②当垂向主应力与最大水平主应力差较大且水平主应力差较小时，岩心起裂压力较小，裂缝扩展过程平稳，受岩石断裂韧性影响微弱；当垂向主应力与最大水平主应力差较小时，水平主应力差越大，则起裂压力越大，裂缝扩展次数越少，压裂过程用时越短。③当水平主应力差较大时，裂缝沿垂向扩展明显，断裂面平直且垂直于最小水平主应力方向；当水平主应力差较小时，裂缝扩展方向难控制，易产生偏转或横向裂缝。④水力裂缝与结构面相遇时产生支裂缝，以及出现分叉、转向、穿层等现象是形成复杂裂缝网络的必要条件。层理影响裂缝穿层，微裂隙与微空隙对起裂压力和裂缝扩展均有影响。

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.