Evolution of aromatic structure and nanopores in shale kerogen by using in-situ HRTEM and in-situ FT-IR experiment

Abstract

The evolution of kerogen molecular structure and nanopores plays critical roles in oil/gas generation and storage of shale gas. However, due to the heterogeneity and complexity of the kerogen molecule and nanopores, the precise process and evolution mechanism of kerogen molecule and nanopores remain obscure. In this study, in-situ Transmission Electron Microscope (TEM) was introduced to directly observe the evolution process of kerogen aromatic fringes and nanopores. Besides, nuclear magnetic resonance (NMR) spectroscopy and in-situ Fourier Transform Infrared (FT-IR) spectroscopy were also used to help analyze the evolution mechanism of kerogen aromatic structures. The results show that with the heating temperature increasing from 150 °C to 900 °C, the kerogen aromatic structure undergoes a three-stage evolution. In stage I (from 150 °C to 300 °C), the small sized aromatic clusters (naphthalene and 2 × 2 rings) drop off from the kerogen molecules and transform into the oil component. Their proportions in total kerogen aromatic rings decrease significantly from 49 % to 27 %. In stage II (from 300 °C to 700 °C), the aliphatic structures are converted to small sized aromatic clusters through aromatization, and the proportion of small sized aromatic clusters increases from 27 % to 38 %. In stage III, aromatic clusters undergo interconnection, resulting in the formation of significantly larger aromatic rings. During this stage, the proportion of large sized aromatic clusters increases obviously from 8 % to 42 %. Based on the in-situ HRTEM images, 2–15 nm pores were also identified. Throughout the heating process, the nanopore area within the kerogen particle (1956 nm2) exhibits an initial increase from 170 nm2 to 377 nm2 (150 °C–600 °C). Subsequently, the nanopore area experiences a decrease from 377 nm2 to 140 nm2 (600 °C–900 °C). By utilizing the benefits of in-situ HRTEM, the evolution of kerogen molecules and nanopores were elucidated in detail, aiding in the comprehension of the oil/gas generation and shale gas storage.