基于CT扫描的煤中孔裂隙精细表征

为了深入探究煤层气赋存和运移的机理,需要对煤中孔裂隙进行定性分析和精细定量表征。为此,以沁水盆地南部五阳矿3号煤层的原煤为研究对象,应用X射线对原煤煤心进行360°全角度步进式扫描,借助CT分析构建出其三维数字化可视模型,在此基础上,采用DTM灰度阈值分割法求出孔裂隙的灰度阈值分割点,识别煤中孔裂隙,并与扫描电镜观测到的结果进行对比;采用Avizo软件建立孔裂隙系统的球棒模型,实现了对孔裂隙的定量表征,并与压汞实验结果进行对比。研究结果表明:(1)DTM灰度阈值分割法可以准确识别煤中孔裂隙,并且与扫描电镜图像中同一裂隙的形态和发育尺度基本一致;(2)利用CT扫描可以准确识别直径19.20 mm、高度10.59 mm样品中孔径大于12.40μm的孔裂隙,展现孔裂隙在三维空间的分布情况;(3)压汞实验结果与CT扫描结果仅在以非连通孔隙为主的20~40μm孔径段存在差异,而在40μm以上孔径段二者的孔裂隙体积变化趋势则一致,其中CT扫描的分析结果更为可靠。

Fine characterization of pores and fractures in coal based on a CT scan

In order to reveal the occurrence and migration mechanism of coalbed methane (CBM), it is necessary to qualitatively analyze and quantitatively and precisely characterize the pores and fractures in coal beds. In this paper, the raw coal of 3# coal seam in the Wuyang coal mine, southern Qinshui Basin, was taken as a study object. Its core was scanned step by step in a 360° full angle by using X-ray and a 3D digitized visual model was constructed with the aid of CT analysis. Then, the gray threshold segmentation point of the pores and fractures were obtained by using the digital terrain model (DTM) of gray threshold segmentation, and the pores and fractures in coal beds were identified and compared with the scanning electron microscope (SEM) results. Finally, the ball-and-stick model for pore and fracture systems was established by using the software Avizo to characterize pores and fractures quantitatively, and the quantitative characterization was compared with the results of mercury injection tests. And the following research results were obtained. First, the DTM gray threshold segmentation method can accurately identify the pores and fractures in coal beds, and as for the same fracture, it is basically consistent with SEM image in terms of morphology and developmental scale. Second, by means of a CT scan, the pores and fractures with the diameter larger than 12.40 µm in the samples with a diameter of 19.20 mm and a height of 10.59 mm can be identified accurately and their 3D spatial distribution can be presented. Third, mercury injection test results and CT scan results are different only in the sections with non-connected pores of 20–40 µm in diameter as the dominant, while they present a consistent change trend of pore volume in the sections with pore diameters larger than 40 µm, and the analysis results of CT scan are more reliable.