Carbon Dioxide (CO₂) Adsorption Behaviour and Its Relationship with Nano-Structure in an Organic-Rich Shale: A Case Study of the Longmaxi Shale in Southeast Chongqing

Carbon capture and storage (CCS) technologies in shale reservoirs have attracted increasing interest in recent years. To study the CO₂ adsorption behaviour in the Longmaxi shale, isothermal adsorption experiments, scanning electron microscopy (SEM) and other techniques were employed in this study. The results show that when the pressure is less than 7.37 MPa, the Langmuir model fits well with the experimental CO₂ adsorption data in the shale. In contrast, when the pressure is greater than 7.37 MPa, the D+R-K model fits better with the experimental data. From low pressure to high pressure (>20 MPa), the CO₂ adsorption isothermal curves can be divided into four stages: rapid adsorption stage, slow adsorption stage, linear decreasing stage and slightly decreasing stage. With increases in temperature, the CO₂ adsorption capacity decreases as expected. In addition, the effects of the Longmaxi shale nano-structure on the CO₂ adsorption behaviour were also investigated. It was found that organic matter has a greater influence than clay and quartz on the CO₂ adsorption behaviour. Based on the SEM observations, large numbers of nanoscale organic pores were found in the shale samples, and these nanoscale organic pores may control the CO₂ adsorption behaviour in organic-rich Longmaxi shale. With increasing total organic carbon (TOC) content, the CO₂ adsorption capacity increases linearly. In this study, the characteristics of the CO₂ adsorption in shale and its relationship with the shale nano-structure were studied, which may be helpful for understanding CCS technologies and their application in enhanced shale gas recovery.