蜀南地区五峰-龙马溪组页岩微观孔隙结构及分形特征

南方上奥陶统五峰组-下志留统龙马溪组海相页岩是中国页岩气主力开发层位,页岩微观孔隙结构特征的研究对于页岩含气性和开发储量的评价有重要意义。采用场发射扫描电镜和低温氮气吸附实验方法对蜀南地区长宁区块五峰-龙马溪组页岩微观孔隙结构进行了定性评价和定量表征。实验结果表明,蜀南地区五峰-龙马溪组页岩以有机质孔隙为主,局部可见粒间孔和粒内孔发育。氮气吸附回滞环属于H4型,对应纳米级孔隙类型为狭缝型;五峰-龙马溪组页岩平均比表面积17.35 m~2/g,平均孔体积16.70 mm~3/g,平均孔径9.82 nm;页岩纳米级孔隙表面具有分形特征,分形维数平均值为2.681;有机碳含量的增加使得纳米级孔隙数量增多,页岩分形维数增大,孔隙表面粗糙程度增大,页岩比表面积增大,页岩吸附能力增强。

Microscopic pore structure and fractal characteristics of Wufeng-Longmaxi shale, South Sichuan

Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation shale in south China is the main shale gas development zone, and the study of microscopic pore structure is significant to evaluate gas bearing property and development reserves of shale reservoir. By use of focused ion-beam scanning electron microscopy and low temperature nitrogen adsorption method, the experiment study investigated the microscopic pore structure of nine shale samples from Wufeng-Longmaxi Formation, Changning District, south Sichuan Basin. The results show that Wufeng- Longmaxi shales are dominated by organic-matter pores along with interparticle pores and intraparticle pores. Hysteresis loops of nitrogen adsorption isotherms belong to type H4, which corresponds to narrow slit pore. The average specific surface area of nine Wufeng-Longmaxi shale samples is 17.35m2/g, the average pore volume is 16.70mm3/g, and the average pore size is 9.82nm. Nano-scale pore surfaces of shale samples have fractal characteristics, and the average value of fractal dimension is 2.681. With the increase of total organic carbon content, the number of nano-scale pores increases. Meanwhile the value of fractal dimension increases, which corresponds to the higher degree of surface irregularity. These two factors lead to the increase of specific surface area, and finally enhance the methane adsorption capacity of the shale.