Abstract Weak structures like cleats and fractures are abundant in coal rock. It can form a complex fracture system during hydraulic fracturing process. T-type fracture is generally defined as those intersected complex fractures of horizontal and vertical fractures during hydraulic fracturing of the coal bed. T-type fractures can occur during hydraulic fracturing, yet few present theory or experiments are established or carried out to figure out the underlying mechanism of T-type fractures. Compared with horizontal fractures, T-type fractures have a higher fracture conductivity, which is more conducive to the improvement of production. Therefore, it is necessary to establish theoretical models to analyze the initiation and propagation of T-type fractures. We have carried out laboratory simulation of hydraulic experiments on the one hand. On the other hand, we have observed the spatial development of T-type fractures and described the changing pattern of its propagation. The experiment results showed that hydraulic fracture initiated and extended along the vertical direction, and it formed T-type fracture when weak structures opened in the horizontal direction. According to Rupture Mechanics, we have established a calculation model to calculate the rupture and propagation of T-type fractures based on stress intensity factor and analyzed the factors that may affect fracture propagation. The findings of this paper showed that the lengths of vertical and horizontal fractures and the inclination of horizontal fractures can significantly influence the occurrence of T-type fractures. More specifically, longer horizontal fractures, shorter vertical fractures, and smaller horizontal inclination are favorable for the occurrence of T-type fractures. In the current literature, a complete theoretical model for the initiation and propagation of T-type fractures in coal bed hydraulic fracturing is still not available. This paper has established a new theoretical model for T-type fractures in hydraulic fracturing, the results of which serve as a complement and improvement for the extant mechanism study on coal hydraulic fracturing.

中文翻译：

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煤压裂T型裂缝破裂及扩展机理研究

摘要 煤岩中大量存在割理、裂缝等弱结构。在水力压裂过程中可形成复杂的压裂系统。T型裂缝一般定义为煤层水力压裂过程中水平裂缝和垂直裂缝相交的复杂裂缝。T型裂缝在水力压裂过程中可能发生，但目前很少有理论或实验建立或开展以找出T型裂缝的潜在机制。与水平裂缝相比，T型裂缝具有更高的裂缝导流能力，更有利于提高产量。因此，有必要建立理论模型来分析T型裂缝的起裂和扩展。我们一方面进行了水力实验的实验室模拟。另一方面，我们观察到了 T 型裂缝的空间发育并描述了其扩展的变化模式。试验结果表明，水力压裂沿垂直方向起裂和延伸，弱结构在水平方向张开时形成T型裂缝。根据Rupture Mechanics，我们建立了基于应力强度因子计算T型裂缝破裂和扩展的计算模型，并分析了可能影响裂缝扩展的因素。本文研究结果表明，纵、横向裂缝的长度和水平裂缝的倾角对T型裂缝的产生有显着影响。更具体地说，较长的水平裂缝，较短的垂直裂缝，较小的水平倾角有利于T型裂缝的产生。在目前的文献中，还没有一个完整的煤层水力压裂T型裂缝起裂和扩展的理论模型。本文建立了一种新的水力压裂T型裂缝理论模型，其结果对现有煤水力压裂机理研究起到了补充和改进的作用。