Application of atom force microscope and nanoindentation to characterize nanoscale mechanical properties of shale before and after supercritical CO2 immersion

Abstract

Understanding the microscopic effects of supercritical CO2 (SC–CO2) on shale plays an essential role in carbon capture utilization and storage (CCUS). The variation of shale microstructures and micromechanical properties induced by SC-CO2 immersion is obscure, which is crucial in CO2-enhanced shale gas recovery. In this study, the organic matter (OM) and minerals of shale are identified in the microscale by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), atom force microscope (AFM) and nanoindentation. The microstructures and micromechanical properties are measured before and after SC-CO2 exposure. The micromechanical properties variation of shale minerals and OM of shale are analyzed. The results show that the mechanical properties of OM have changed significantly after SC-CO2 exposure. Due to the interaction of CO2, the elastic modulus of OM increased by 124%. With the dissolution of CO2, the porosity of calcite increases from 7.1% to 8.1%, while decreasing by 6% in elastic modulus. By comparing the surface roughness calculated by AFM, the clay and OM swell. Although the mechanical properties are more discrete after CO2 immersion, the correlation between Young's modulus and hardness is better. These findings help understand the SC-CO2 exposure effects on shale and provide insights into the applications of SC-CO2 in unconventional resource development from a microscale perspective.