Crack propagation and hydraulic fracturing in different lithologies

不同岩性岩石中的水力压裂与裂缝扩展规律研究（英文）

We simulated hydraulic fracturing in different lithologic rocks in the horizontal drilling by using the true physical model experiment and large rock specimens, carried out the real-time dynamic monitoring with adding tracer and then did post-fracturing cutting and so on. Based on this monitoring results, we compared and assessed the factors affecting expansion in shale, shell limestone, and tight sandstone and the fracture expansion in these rocks. In shale, the reformed reservoir volume is the highest, fracture network is formed in the process of fracturing. In tight sandstone, the fracture surface boundaries are curved, and the fracture surface area accounts for 25–50% of the entire specimen. In shell limestone, the complexity of the fracture morphology is between shale and tight sandstone, but no fracture network is developed. Brittleness controls the fracture surface area. In highly brittle rocks, the fracture surface area is high. Fracture toughness mainly affects the initiation and propagation of cracks. A fracture network is formed only if bedding planes are present and are more weaker than their corresponding matrix. The horizontal in situ deviatoric stress affects the crack propagation direction, and different lithologies have different horizontal in situ deviatoric stress thresholds. Low f luid injection rate facilitates the formation of complex cracks, whereas high fluid injection rate favors the development of fractures. Fluid injection weakly controls the complexity of hydraulic fracturing in low-brittleness rocks, whereas lowviscosity fracturing fluids favor the formation of complex cracks owing to easy enter microcracks and micro-pore. Displacement has a greater impact on high brittle rocks than low brittle rocks.

本文采用真三轴物理模型试验机针对不同岩石岩石,开展了大尺度真三轴水平井水力压裂物理模拟实验,通过裂缝的动态监测、在裂液中添加示踪剂以及压裂后剖切等方式,对比分析了页岩、介壳灰岩和致密砂岩的裂缝扩展规律,探讨了影响水力裂缝扩展和影响裂缝网络形成的主要因素,得到如下结论:(1)页岩储层的体积改造效果是最好的,改造体积也是最大,压裂过程中形成了多裂缝、裂缝网络;致密砂岩压裂后单一裂缝居多,裂缝面边界为弧形,面积仅占试样平面的25%-50%;介壳灰岩压裂后,裂缝复杂程度介于页岩和致密砂岩之间,但未形成裂缝网络;(2)脆性控制形成裂缝面的面积,高脆性岩石水力裂缝扩展面积大,易于形成复杂裂缝;断裂韧性主要控制裂缝的起裂和扩展的难易程度,储层断裂韧性越小,水力裂缝越容易向前扩展,反之扩展阻力越大;(3)层里面大量存在且相对于基质体足够弱时才有可能形成复杂的裂缝网络;(4)水平地应力差对裂缝扩展方向具有一定的控制作用,不同岩性岩石具有不同的水平地应力差阈值;(5)低黏度使压裂液更容易、低排量使压裂液有足够的时间进入微裂缝、微孔隙,从而增加裂缝的复杂程度;排量对高脆性岩石的影响力度比低脆性岩石大。