In-situ conversion process (ICP) is a promising recovery technology for economically effective exploitation of unmatured oil shale. High temperature treatment during ICP facilitates the chemical conversion of kerogen content in oil shale, while markedly improving the accessibility of pyrolyzed fluid through thermally-induced microcracks. Although extensively studied experimentally, the microstructure development in shale is not fully understood due to the lack of consideration for multiphysics effects during ICP at the mineral scale. This paper presents a numerical study on thermal cracking and permeability evolution in oil shale matrix during in-situ conversion process. To this end, a multiphysics phase-field model for investigating thermo-mechanical response, chemical reaction, and seepage behavior is developed, while arbitrary crack growth in heterogeneous shale matrix is naturally predicted using the phase-field method for fracture. Implementing the proposed numerical model, thermal cracking of heterogeneous granodiorite is first simulated, from which the numerical outcome regarding crack morphology is reasonably consistent with experimental data. Permeability evolution of oil shale matrix is found to be attributed to early-stage thermal crack propagation and late-stage kerogen decomposition by correlating multiple variables. Anisotropic permeability is also observed and investigated by examining crack morphology, pore-space interconnectivity and fluid permeation. Further analysis reveals that the shale matrix microstructure, in-situ stress and heating temperatures play vital roles in influencing thermal cracking and permeation behaviors of the shale matrix. The results provide a unique perspective to understanding thermal cracking and permeability augmentation of heterogeneous rocks.

中文翻译：

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原位转化过程中油页岩基质热裂解和渗透率演化的多物理相场建模

原位转化工艺（ICP）是一种有前景的回收技术，可经济有效地开采未成熟油页岩。 ICP期间的高温处理促进了油页岩中干酪根含量的化学转化，同时通过热诱导的微裂纹显着提高了热解流体的可及性。尽管进行了广泛的实验研究，但由于在矿物尺度的 ICP 过程中缺乏对多物理场效应的考虑，页岩的微观结构发育尚未完全了解。本文对原位转化过程中油页岩基质的热裂解和渗透率演化进行了数值研究。为此，开发了用于研究热机械响应、化学反应和渗流行为的多物理相场模型，同时使用断裂相场方法自然地预测非均质页岩基质中的任意裂纹扩展。实施所提出的数值模型，首先模拟了异质花岗闪长岩的热裂纹，其中裂纹形态的数值结果与实验数据相当一致。通过关联多个变量，发现油页岩基质渗透率演化归因于早期热裂纹扩展和晚期干酪根分解。还通过检查裂纹形态、孔隙空间互连性和流体渗透性来观察和研究各向异性渗透率。进一步分析表明，页岩基质的微观结构、地应力和加热温度对页岩基质的热裂和渗透行为起着重要的影响。这些结果为理解非均质岩石的热裂化和渗透率增加提供了独特的视角。