Crack propagation and hydraulic fracturing in different lithologies

Hou Zhen-Kun; Cheng Han-Lie; Sun Shu-Wei; Chen Jun; Qi Dian-Qing; Liu Zhi-Bo

摘要

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 fluid 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 low-viscosity fracturing fluids favor the formation of complex cracks owing to easy enter micro-cracks and micro-pore. Displacement has a greater impact on high brittle rocks than low brittle rocks.

机译：利用真实的物理模型实验和大型岩石标本，模拟了水平钻井过程中不同岩性岩石的水力压裂，并添加示踪剂进行了实时动态监测，然后进行了压裂后的切割等。基于此监测结果，我们比较并评估了影响页岩，壳灰岩和致密砂岩膨胀以及这些岩石的裂缝膨胀的因素。页岩中，改造后的储集层体积最大，压裂过程中形成裂缝网络。在致密的砂岩中，断裂面边界是弯曲的，断裂面占整个试样的25-50％。在壳灰岩中，裂缝形态的复杂性介于页岩和致密砂岩之间，但没有形成裂缝网络。脆性控制断裂表面积。在高脆性岩石中，断裂表面积很大。断裂韧性主要影响裂纹的产生和扩展。仅当存在层理平面并且比它们相应的矩阵更弱时，才会形成断裂网络。水平原位偏应力影响裂纹扩展方向，并且不同岩性具有不同的水平原位偏应力阈值。低注水速率有利于复杂裂缝的形成，而高注液速率有利于裂缝的发展。在低脆性岩石中，流体注入几乎不能控制水力压裂的复杂性，而低粘度压裂液由于易于进入微裂纹和微孔，因而有利于形成复杂的裂缝。与低脆性岩石相比，位移对高脆性岩石的影响更大。

Crack propagation and hydraulic fracturing in different lithologies

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