微米CT扫描尺度下构造煤微裂隙结构特征及其对渗透性的控制

Characteristics of micro-CT scale pore-fracture of tectonic ally deformed coal and their controlling effect on permeability

  • 摘要: 煤中裂隙是煤层气运移的主要通道,是影响和控制煤储层渗透性的直接因素。我国构造煤普遍发育,不同类型构造煤中裂隙特征差异显著,对煤储层渗透性的影响也不一样。因此,对构造煤中裂隙与渗透性的研究十分必要。选取河南平煤十三矿和山西晋城胡底矿中、高煤阶的2种变形程度构造煤,首先利用环氧树脂制作了柱状构造变形煤样品,解决了构造变形煤制样难的问题;其次通过扫描电镜、微米CT扫描结合数字图像处理技术展现了2种构造煤中开度大于15μm裂隙的发育特征、分布规律、形态与结构的差异性;最后基于泊肃叶定律、三轴渗透实验结合CT扫描裂隙结构参数建立了渗透率预测模型,并探讨了裂隙结构对渗透性的控制机理。研究结果表明,在开度>15μm尺度下弱构造变形煤(偏向原生结构煤)多发育形态简单的大裂隙,裂隙连通性与连接强度均较差,而强构造变形煤中微裂隙形态复杂,裂隙的连通性与连接强度均较好,强构造变形煤的裂隙率大于弱构造变形煤。2种构造煤渗透率随有效应力增大均呈负指数下降趋势。煤储层渗透率受多因素综合控制,包括裂隙开度、数量、体积、连通性与连接强度,其中裂隙开度的影响最为显著。在此基础上,综合考虑到这些控制渗透率的因素以及裂隙分形维数与粗糙度建立了管状裂隙渗透率预测模型,并验证了该模型在此尺度下预测渗透率的准确性和适用性。

     

    Abstract: Fractures in coal are the main channels for the migration of coalbed methane and are the direct factor that affects and controls the permeability of coal reservoirs. Tectonically deformed coals are generally developed in China, and the characteristics of fractures in different types of tectonically deformed coals are significantly different, and they have different effects on the permeability of coal reservoirs. Therefore, it is necessary to study the fracture and permeability in the tectonically deformed coal. In this study, the middle and high-rank tectonically deformed coal samples were selected from the Pingdingshan 13th Mine and the Jincheng Hudi Mine. Firstly, the columnar coal samples were made by epoxy resin to solve the problem of difficulty in the sample preparation of tectonically deformed coal.Secondly, by means of scanning electron microscopy, micro-CT scanning and digital image processing technology, the differences of development, distribution, morphology and structure of aperture greater than 15 μm in the two coals were analyzed. Finally, a permeability prediction model was established based on the Poiseuille's law, triaxial permeability experiment and fracture structure parameters of CT scanning. The mechanism of fracture structure controlling permeability was discussed. The results show that large fractures with simple morphology are mostly developed in the weak tectonically deformed coal(tend to primary structural coal) under the scale of aperture greater than 15 μm, and the fracture connectivity and connection strength are poor, while the micro-fractures in the strong tectonically deformed coal are complex in morphology, and the fracture connectivity and connection strength are good. The fracture porosity of coal with the strong tectonic deformation is larger than that of coal with weak tectonic deformation. Permeability decreases in a negative exponential trend with the increase of effective stress. The permeability of coal reservoirs is controlled by multiple factors, including fracture aperture, number, volume, connectivity and connection strength, of which aperture has the most significant influence. In addition, considering the factors of controlling permeability and the fractal dimension and roughness of fractures, a tubular fracture permeability model was established, and the accuracy and applicability of the model in predicting permeability at this scale were verified.

     

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