A new application of atomic force microscopy in the characterization of pore structure and pore contribution in shale gas reservoirs

Abstract

The pore structure determined by different materials is extremely significant for shale gas accumulation. Understanding how different materials contribute to the pore remains a knowledge gap. To elucidate the surface property and pore structure qualitative and quantitative characteristics of gas-bearing shales, the Atomic Force Microscopy (AFM) and low-temperature nitrogen gas adsorption (LT-N2GA) experiments were performed on shale samples selected from the Longmaxi Formation (Fm.) in the Sichuan Basin in China in this study. A new method based on AFM technique was used to calculate shale porosity, and the double-threshold discrete integration method was proposed to estimate the pore contributions of main materials. The results show that the organic-rich shale is significantly different from the organic-poor shale in material composition. The average value of mean roughness (Ra) of the organic-rich and organic-poor shale samples are 38.74 nm and 104.58 nm, respectively, indicating that the organic-poor shale has a rougher surface. The average value of mean square roughness (Rq) of the organic-rich and organic-poor shale samples are 49.91 nm and 138.80 nm, respectively, indicating that the organic-poor shale has a severer variation degree. Moreover, organic-poor shale samples also have a more concentrated height distribution. The average surface porosity of the organic-rich shale and organic-poor shale are 17.08% and 3.31% respectively. The average form factor (ff) of the organic-rich shale and organic-poor shale are respectively 0.102 and 0.386. It indicates that organic-rich shale has more pores and more complicated pore boundaries than organic-poor shale. AFM-calculated porosity is consistent with that of LT-N2GA-converted, which indicates that the porosity calculated by AFM is real porosity and reliable. The essence of the phase change on the shale surface is the difference in material composition. Shale pores are mainly contributed by organic matter, followed by chlorite, potash feldspar, and quartz. The results show that our method provides a powerful and effective tool to characterize the nanopore structure of organic rocks, obtain shale reservoir physical properties, and analyze pore contribution of shale components etc. issues, which has a wide application prospect.