页岩微纳米孔隙结构分形特征研究

STUDY ON FRACTAL CHARACTERISTICS OF MICRO-NANO PORE STRUCTURE OF SHALE

页岩气作为非常规气,是一种重要能源。页岩的孔隙结构特征是衡量与评价页岩储层存储能力与可压裂性的重要参数。选取威远海相页岩(1#)、龙马溪海相页岩(2#)、瑶曲凝灰岩(4#)以及瑶曲陆相页岩(5#,6#),进行压汞实验、氮气吸附性实验以及核磁共振实验。利用分形理论,表征孔隙结构的非均质性,揭示分形维数和孔隙结构之间的关系。结果表明:孔隙在0.1~100μm范围采用压汞实验,陆相页岩样品得到的分形维数要比海相页岩样品大;孔隙在2~200 nm范围采用氮气吸附实验,海相页岩的分形维数比陆相页岩大;相比之下,孔隙在10 nm^10μm范围采用核磁共振实验得到的焦石坝海相页岩与陆相页岩的分形维数大小比较接近。尤其是本文统计的氮气吸附实验样本中,焦石坝海相页岩的分形维数最大,即焦石坝海相页岩的微孔结构最为发育,非均质性最强。因此分形维数可作为一种用于评价页岩孔隙非均质性与储存压裂效果的重要参数。

Shale gas,as one kind of unconventional gas,is an important energy supplement.The pore structure characteristic is an important index used to measure and evaluate the storage capacity and fracturing of shale reservoir.The Weiyuan marine shale(1#),Jiaoshiba marine shale(2#),Yaoqu tuff(4#)and Yaoqu continental shale(5#and 6#)are selected for tests with mercury porosimetry(MP),N 2 adsorption(NA)and nuclear magnetic resonance(NMR)devices.Based on fractal theory,the relationships between fractal dimensions and pore structure are investigated with MP,NA and NMR to characterize the pore anisotropy.The results illustrate that the fractal dimensions of continental samples 5#and 6#are bigger than that of marine sample 2#in the pore range between 0.1~100μm based on MP method.The fractal dimensions of marine sample 2#is bigger than those of continental samples 5#and 6#in the pore range of 2~200 nm based on NA method.In contrast,the max fractal dimensions(D NMR)of marine sample 2#are about equal to those of continental samples 5#and 6#in the pore range between 10 nm to 10μm by NMR method.Especially,the fractal dimensions of Jiaoshiba marine sample 2#are biggest among all the samples using NA method.The micropores of Jiaoshiba marine shale are the most development,so the heterogeneity of pore characteristic is the strongest.Therefore,the fractal dimension can be used as an important parameter to evaluate the fracturing effect of shale reservoir and the heterogeneity of pore structure.