利用纳米透射X射线显微成像技术研究页岩有机孔三维结构特征

Investigation of Organic Matter Pore Structures of Shale in Three Dimensions of Shale Using Nano-X-ray Microscopy

页岩纳米级孔隙的三维结构特征直接决定页岩气微观渗流机理,是完善页岩储层流动模型亟需解决的核心问题。本文以龙马溪组页岩有机孔样品(直径约7μm)为研究对象,分别利用同步辐射纳米CT和实验室纳米CT重建有机孔三维空间结构,针对两个装置获得的孔隙结构参数进行对比研究,结果表明:(1)龙马溪组页岩有机孔样品呈蜂窝状,孔隙度约60%,连通性较好;孔径分布呈双峰模式,集中于60~150nm和500~1400 nm;孔径大于500 nm的孔隙对样品的总孔隙度贡献较大。(2)同步辐射纳米CT与实验室纳米CT结果相较,孔隙度和孔隙总数两参数基本一致,喉道总数和喉道半径偏差较大;孔径分布和配位数分布规律虽然类似,但具体数值存在明显差异,值得进一步深入比较分析和研究。(3)纳米CT方法在页岩纳米孔隙三维结构表征方面存在阈值划分难度大与扫描视场过小的问题,可从切片重构算法、三维数据处理、表征单元体三方面进行改进。

Three dimensional( 3D) structures of nanometer scale pores is a determining factor for the shale gas micro-seepage mechanism,and it is a critical issue needing to be solved urgently to develop a better model for the description of flow behavior in shale. In this study,3D structure of the OM pores of the Longmaxi formation shale sample( Φ = 7 μm) from the Sichuan basin was rebuilt using synchrontron radiation CT and experimental CT respectively. The purpose of the study was to compare the pore structural parameters obtained from the radiation CT and experimental CT. The results indicate:( 1) The OM pores,with a porosity of about 60%,exhibit a honeycomb pattern and good connectivity. The pore size distribution exhibits a bimodal pattern, mainly concentrating in 60-150 nm and 500-1400 nm,and pores with diameters larger than 500 nm contribute the most to the total porosity.( 2) The porosity and the total pore number obtained from synchrontron radiation CT and experimental CT remain consistent within the accepted margin of error,but the total throat number and throat diameters obtained from the second method above suggest a relatively large difference. Although the pore size distribution and coordination number distribution laws are similar,there are obvious deviations considering the exact figures.( 3) For 3D characterization of shale pore structures using nanometer CT,there are problems of threshold partition and small scan view. The problems can be reduced by three plans starting with reconstruction algorithm,3D data analysis and REV.