Abstract Macrolithotypes control the pore-fracture distribution heterogeneity in coal impacting stimulation via hydrofracturing and the coalbed methane (CBM) production. Given that it is affected by the discontinuities, hydraulic fracture geometry is complex in the vertical plane and is different from a simple fracture in a homogeneous reservoir. However, the initiation and propagation mechanism in the vertical plane is unclear. To clarify this, the cohesive zone finite element approach, with macrolithotype contributions included, was used to simulate and analyze the hydraulic fracture propagation. The experimental tests showed that, the bright and semi-bright coal usually have higher microfracture (cleat) density accompanied by the lower mechanical properties than that of the semi-dull and dull coals. The behavioral differences are likely to impact the geometry evolution of hydraulic fractures and which appears to vary when fracturing the different coal macrolithtypes. Thus, the cohesive zone finite element approach was used with two models to capture macrolithotype impacts. The result show that, when fracturing the dull coal (model 2), the overall propagation region rapidly displayed a simple plane in shape because of the less development of natural fractures. With the influence of the larger elastic modulus, the high-stress zone would be easy formed and suddenly release to generate pressure pulse when the hydraulic fracture penetrated the interface. As the hydraulic fracture initiates from the bright coal (model 1), the presence of the existing diverse cleat network contribute greatly to the increase of cracks number to form complex fractures. However, the opening of natural fractures will lead to the diversion of fracturing fluid, and the larger elastic modulus of the interlayer also plays a limiting role in the height of the hydraulic fracture. In addition, the monitoring of hydraulic fracture was carried out and shown that the height of the major fracture in model 1 was restricted and limited by the bright coal; and the height in model 2 is usually larger than the dull coal thickness, indicating that the hydraulic fracture has cut through the fracturing section (dull coal) and embedded into the upper and lower layers.

中文翻译：

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煤大岩型对煤层水力裂缝起裂和扩展的影响

摘要 大岩型通过水力压裂和煤层气（CBM）生产控制煤冲击增产的孔隙-裂缝分布非均质性。鉴于受不连续性的影响，水力裂缝几何形状在垂直平面上是复杂的，与均质储层中的简单裂缝不同。然而，垂直平面内的引发和传播机制尚不清楚。为了阐明这一点，使用包含宏观岩型贡献的粘性带有限元方法来模拟和分析水力压裂扩展。试验试验表明，与半暗、暗煤相比，光亮煤和半光亮煤通常具有较高的微裂缝（割理）密度，但力学性能较低。行为差异可能会影响水力压裂的几何演变，并且在压裂不同的煤巨岩类型时似乎会​​发生变化。因此，内聚区有限元方法与两个模型一起使用来捕获宏观岩型影响。结果表明，在压裂暗煤时（模型2），由于天然裂缝发育较少，整个扩展区域迅速呈现出简单的平面形状。在较大弹性模量的影响下，当水力裂缝穿透界面时，高应力区容易形成并突然释放以产生压力脉冲。随着水力压裂从亮煤开始（模型 1），现有多样化割理网络的存在极大地促进了裂缝数量的增加以形成复杂的裂缝。但天然裂缝的张开会导致​​压裂液分流，夹层较大的弹性模量也对水力裂缝的高度起限制作用。此外，对水力压裂进行监测表明，模型1中的主裂缝高度受光亮煤的制约和限制；且模型2中的高度通常大于暗煤厚度，说明水力裂缝已切穿压裂段（暗煤）并嵌入上、下层。夹层较大的弹性模量也对水力裂缝的高度起限制作用。此外，对水力压裂进行监测表明，模型1中的主裂缝高度受光亮煤的制约和限制；且模型2中的高度通常大于暗煤厚度，说明水力裂缝已切穿压裂段（暗煤）并嵌入上、下层。夹层较大的弹性模量也对水力裂缝的高度起限制作用。此外，对水力压裂进行监测表明，模型1中的主裂缝高度受光亮煤的制约和限制；且模型2中的高度通常大于暗煤厚度，说明水力裂缝已切穿压裂段（暗煤）并嵌入上、下层。