Multistage fractures in different reservoirs exhibit competitive extension and mutual feeding mechanisms under different fracturing sequence conditions. To better understand these mechanisms for a more efficient extraction of mine gases, a combination of true triaxial physical tests and numerical simulation was performed in this study. The expansion process of hydraulic fractures in different layers and the comprehensive effect of fracturing were analyzed. The directional deflection effect of the induced stress field on the hydraulic fractures can be summarized as follows. In terms of their behavioral pattern, the fractures in the rock seam extended “in the direction of maximum geo-stress and then deflected toward the interface.” The fracture behavior in the coal seams could be divided into two patterns: “deflection toward the interface and then extension along the direction of maximum geo-stress” and “deviation from the interface and then extension along the direction of maximum geo-stress.” The mutual feedback between the fractures manifested in the form of fracture “phase direction” in the case of stratified fracturing and “phase back” in the case of simultaneous fracturing, i.e., the fracture behaviors in the rock seams and in the first type of coal seams were promoted whereas the fracture behavior was inhibited in the second type of coal seams. In addition, the second fracturing process could be characterized by an increase in the fracture initiation pressure, a decrease in the rate of pressure drop, an increase in the fracture extension duration, and a decrease in the fracture width. When using a fracturing sequence of rock followed by coal, the formation of the seam network structure was found to be more favorable. When using a fracturing sequence of coal followed by rock, it was necessary to continue the injection of the hydraulic fluid into the first fracture during the second fracturing process, so as to obtain a higher fracturing yield. This research provides a certain theoretical support for the efficient co-exploitation of three gases, namely coalbed methane, tight gas, and shale gas, from coal composite reservoirs and in the prevention of gas disasters.

中文翻译：

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不同压裂层序列条件下复合煤-岩储层水力裂缝的相互反馈及压裂效应


不同储层的多级裂缝在不同压裂顺序条件下表现出竞争性延伸和相互补给机制。为了更好地了解这些机制以更有效地提取矿井气体，本研究结合了真正的三轴物理测试和数值模拟。分析了不同层层水力裂缝的膨胀过程和压裂的综合效应。感应应力场对水力裂缝的方向偏转效应可归纳如下。就它们的行为模式而言，岩层中的裂缝“沿最大地应力方向延伸，然后向界面偏转”。煤层中的裂缝行为可分为两种模式：“向界面偏转，然后沿最大地应力方向延伸”和“偏离界面，然后沿最大地应力方向延伸”。裂缝之间的相互反馈在分层压裂的情况下表现为裂缝“相向”，在同步压裂的情况下表现为“相退”，即岩层和第一类煤层的断裂行为得到促进，而第二类煤层的断裂行为受到抑制。此外，第二次压裂过程的特征是裂缝起裂压力增加、压降速率降低、裂缝延伸持续时间增加和裂缝宽度减小。当使用岩石后煤的压裂序列时，发现煤层网络结构的形成更有利。 当采用煤后岩石的压裂序列时，需要在第二次压裂过程中继续将液压油注入第一个裂缝中，以获得更高的压裂收率。本研究为煤层气、致密气和页岩气3种气体的高效协同开采和气灾防治提供了一定的理论支持。