Abstract The extraction of coalbed methane can supply clean energy to society and improve the safety of subsurface coal mining. The low and ultra-low permeability of coal seams has resulted in the significantly low efficiency of methane production in surface and subsurface extraction systems. Fluid stimulation has been extensively applied for the enhancement of formation permeability. However, the response of coal seams to fluid stimulation may be different from other formations due to their unique structure and mechanical properties. Moreover, the presence of shear-fracturing stimulation and its effect on permeability enhancement is more significant in coal seams during fluid stimulation. In this study, a fluid injection-induced hydrofracturing treatment and shear-fracturing stimulation were separately investigated through laboratory experiments. The permeability of both intact and fractured specimens was used to directly evaluate the efficiency of these two types of fluid stimulations in different types of reservoirs. The experimental results indicated that the closure of hydraulic (i.e., tensile) fractures significantly inhibited the permeability enhancement of the hydrofracturing treatment in coal seams, and resulted in the permeability of fractured coal specimens were considerably lower than that of hard rocks (e.g., shale and sandstone). The results also demonstrated that fluid injection-induced shear stimulation in the coal specimens resulted in dilation behavior primarily associated with permeability enhancement. This was attributed to the fact that the exfoliated particles and masses prop shear fractures and increase their stiffness. It was found that, because of the weak mechanical and discontinuous properties of coal seams, an increase in the deviator stress improved the feasibility of shear-fracturing stimulation. Finally, fluid stimulation can be achieved using low viscosity fluids, such as L/Sc-CO2, as the fracturing fluid tended to form a shear dilation zone with high permeability in the coal seams.

中文翻译：

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剪切压裂增产提高煤层渗透性的可行性和有效性试验研究

摘要 煤层气的开采可以为社会提供清洁能源，提高地下煤矿开采的安全性。煤层的低渗透性和超低渗透性导致地表和地下提取系统中甲烷生产的效率显着降低。流体增产已被广泛应用于提高地层渗透率。然而，煤层对流体刺激的响应可能与其他地层不同，因为它们具有独特的结构和机械性能。此外，在流体增产过程中，剪切压裂增产及其对渗透率增强的影响在煤层中更为显着。在这项研究中，通过实验室实验分别研究了流体注入诱导的水力压裂处理和剪切压裂增产。完整和断裂样品的渗透率用于直接评估这两种流体刺激在不同类型储层中的效率。实验结果表明，水力（即拉伸）裂缝的闭合显着抑制了煤层水力压裂处理的渗透率增强，导致煤层裂缝煤样的渗透率明显低于硬岩（如页岩和煤层）的渗透率。砂岩）。结果还表明，煤样中流体注入诱导的剪切刺激导致了主要与渗透率增强相关的膨胀行为。这归因于剥落的颗粒和团块支撑剪切断裂并增加其刚度的事实。结果发现，由于煤层力学性能较弱且不连续，偏应力的增加提高了剪切压裂增产的可行性。最后，可以使用低粘度流体（例如 L/Sc-CO2）实现流体增产，因为压裂液往往会在煤层中形成具有高渗透性的剪切膨胀区。