四川盆地富有机质页岩孔隙分形特征

Fractal characteristics of organic rich shale pore in Sichuan Basin, China

文中以下寒武统(牛蹄塘组)、上奥陶统(五峰组)、下志留统(龙马溪组),以及上三叠统(须家河组)页岩为研究对象,在低压氮气吸附测试实验结果的基础上,探讨了四川盆地富有机质页岩的分形特征。研究表明:四川盆地富有机质页岩具有双重分形特征,存在明显的孔径分界点,其中小孔隙分形维数的平均值要小于大孔隙,说明大孔隙结构的复杂程度大于小孔隙;分形维数D_1可描述页岩孔隙表面的分形特征,分形维数D_2可描述页岩孔隙结构的分形特征;D_1越大,页岩孔隙表面越不规则,页岩孔隙表面将有更多的吸附位,使页岩吸附气体能力增大;D_2越大,页岩孔隙结构越复杂,使页岩的解吸、扩散和渗流变得困难。因此,页岩孔隙中高表面分形维数D_1值和低孔隙结构分形维数D_2值对页岩气藏的开发有重要的意义。

In this article, taking the Lower Cambrian Niutitang Formation, Upper Ordovician Wufeng Formation, Lower Silurian Longmaxi Formation and Upper Triassic Xujiahe Formation as research objects, the fractal characteristics of marine shale in Sichuan Basin were calculated based on the results of the low-pressure nitrogen adsorption experiment of the marine shale. The results show that the fractal dimensions of shale in Sichuan Basin obviously have the aperture boundary point, that is to say, the investigated shale has the dual fractal characteristics. The big pore structure, with larger fractal dimension, shows higher complex than the small pore structure. The fractal dimension D1 may represent the pore surface fractal dimension, while the fractal dimension O2 may represent the pore structure fractal dimension. The larger the value of the fractal dimension D1 is, the more irregular and rougher the pore surface is. The pore surface can provide more adsorption sites for gas, which can lead to higher adsorption capacity of shale. The larger the value of the fractal dimension O2 is, the more complicated the pore structure is. The complexity of pore structure can make the capacity of desorption, diffusion and percolation of shale gas become more difficult. Therefore, we consider the higher pore surface fractal dimension D1 as well as lower pore structure surface fractal dimension D2 in shale have an important role in the development of shale gas reservoirs.