Abstract Microwave heating may be used to stimulate fracture formation and the release of hydrocarbons in gas shales. Although extensively studied experimentally and numerically, the microscopic observations are not fully explained in current work where the heating, at sample-scale, and fracturing, at the mineral-scale, are represented independently. Furthermore, the geometry, structure and mechanical interaction of different minerals are not fully considered in current approaches. We present a novel simulation approach to investigate the coupled electromagnetic-heating-stress-damage process. Microwave heating is simulated at sample-scale and the resulting stress-damage response is examined at micro-scale where minerals with contrasting thermo-mechanical characteristics are stacked as lamellae, instead of nested internally as in previous representations. A three-stage temperature evolution profile is observed in the shale samples – although some stages may be absent in other rocks. The mathematical model accounts for the three modes of stress generated between minerals: horizontal stress (σh) (tensile stress parallel to the grain-grain interface) and the normal stress(σn) (tensile stress normal to the grain-grain interface) applied on the minerals, and the shear stress (τ) applied on the interface between different minerals. The minerals comprising the shale matrix are categorized into three types – ‘high’, ‘intermediate’ and ‘low’ – conversion efficiency based on their susceptibility to thermal stressing from microwave irradiation. Shear damage and intergranular fracture usually occurs for minerals with high dielectric permittivity. Transgranular fracture may feature both in high permittivity minerals, due to the larger induced horizontal stress (σh), and in low permittivity minerals - due to high volume fraction and larger size. The simulation approach is a powerful way to link the macro-scale characterization and heating to micro-mechanisms of rock failure. Also this work provides mineral classification and criteria to define a priori evaluation of the effectiveness of microwave treatment of shales and other mineral aggregates.

中文翻译：

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页岩微波加热压裂耦合多尺度模拟

摘要 微波加热可用于促进页岩中裂缝的形成和烃类的释放。尽管在实验和数值上进行了广泛的研究，但在目前的工作中，微观观察并没有得到充分解释，其中样品规模的加热和矿物规模的压裂是独立表示的。此外，目前的方法没有充分考虑不同矿物的几何形状、结构和机械相互作用。我们提出了一种新的模拟方法来研究耦合的电磁-加热-应力-损伤过程。在样品尺度上模拟微波加热，并在微观尺度上检查由此产生的应力损伤响应，其中具有对比热机械特性的矿物堆叠为薄片，而不是像以前的表示那样在内部嵌套。在页岩样品中观察到了一个三阶段温度演化剖面——尽管在其他岩石中可能不存在某些阶段。数学模型解释了矿物之间产生的三种应力模式：水平应力 (σh)（平行于晶粒 - 晶粒界面的拉应力）和法向应力（σn）（垂直于晶粒 - 晶粒界面的拉伸应力）。矿物，以及施加在不同矿物之间界面上的剪切应力 (τ)。构成页岩基质的矿物根据其对微波辐射热应力的敏感性分为三类——“高”、“中”和“低”——转换效率。剪切损伤和晶间断裂通常发生在介电常数高的矿物上。由于较大的诱导水平应力 (σh)，穿晶断裂可能以高介电常数矿物和低介电常数矿物（由于高体积分数和较大尺寸）为特征。模拟方法是将宏观表征和加热与岩石破坏的微观机制联系起来的有效方法。此外，这项工作提供了矿物分类和标准，以定义对页岩和其他矿物聚集体的微波处理有效性的先验评估。