Multi-scale pore structure characterization of lacustrine shale and its coupling relationship with material composition: An integrated study of multiple experiments

Abstract

Characterization of microscopic pore system of organic-rich shales is of great importance for understanding the occurrence mechanism and flow behavior of shale oil. Currently, there are numerous studies on pore structure characteristics of shales, but research about the coupling relationship between shale material composition and its microscopic pore system is relatively weak. In this paper, we conducted mercury injection capillary pressure (MICP), Low-temperature N2 adsorption (LTNA) and broad ion beam scanning electron microscope (BIB-SEM) experiments combined with low-field nuclear magnetic resonance (NMR) to quantitatively characterize the full-scale pore structure of typical lacustrine shales in the northern Songliao Basin, China, and analyzed the coupling relationship between multi-scale pore structure and material composition. Results show that these shales are mainly develops inorganic interparticle pores related to clay minerals, and a small amount of organic matter pores were also present. A pore size classification scheme for I-micropore (0–20 nm in diameter), II-micropore (20–50 nm), mesopore (50–150 nm), macropore (>150 nm) within shales was proposed by combining fractal theory with MICP tests. The full-scale characterization of shale revealed that it mainly developed micropores and mesopores, and the pore size was distributed primarily in the region less than 150 nm. The pore specific surface area of these shales was relatively high with a mean value of 20.76 m2/g and mainly controlled by clay minerals. The coupling relationship analysis of different pore types and shale material composition reveals that clay minerals and carbonate minerals govern the development of II-micropores and macropores respectively, whereas organic matter controls the development of I- micropores. It's worth mentioning that organic matter inhibits the formation of mesopores, and the microfractures formed in the shale are dominated by shrinkage OM pores. Based on our observations, the applicability of different experimental methods for characterizing the pore structure of shales is also summarized. The difference between these experiments revealed that the method proposed in this paper is an effective way to characterize the full-scale pore structure of shale.