Investigation of Multiscaled Pore Structure of Gas Shales using Nitrogen Adsorption and FE-SEM Imaging Experiments

Abstract

Nanopore in shales is the place for hydrocarbon accumulation and migration. However, there is a lack of understanding of the nanopore structure with regard to their ultratight and multiscaled nature. Here, the porous morphology of gas shales from the Sichuan Basin of China was investigated using field emission-scanning electronic microscopy (FE-SEM) with high resolution. Low-pressure nitrogen adsorption experiments at 77 K were conducted to obtain the adsorption-desorption isotherms, BET-specific surface area, pore size distribution, pore volume, and average pore diameter values. Research results show that pores of the studied shales are at the nanometer scale, and the average pore diameter is between 3 and 5 nm. The pore structure of these shales is complicated, which is not only predominately mesopores (pore diameter at 2–50 nm) but also some micropores ( pore diameter < 2 nm ) and macropores ( pore diameter > 50 nm ). The specific surface area of shales ranges from 13 to 30 m2/g. The micropore volume and mesopore volume occupy the total pore volume highly up to 77%–92%, which indicates that micropores and mesopores are the main storage place for shale gas. Through the analysis of adsorption isotherms and hysteretic loops, there are mainly two kinds of pores in shales, including ink-bottle-like pores and slit pores. Micropores of these shales are mainly related to organic matter, while macropores are mainly related to clay minerals. The estimation about porosity using the combined physical model shows that organic matter and clay minerals contribute about 50% and 33% to the porosity of these shales, respectively.