Nanoscale profiling of the relationship between in-situ organic matter roughness, adhesion, and wettability under ScCO2 based on contact mechanics

Abstract

Understanding the interface and wetting phenomenon between CO2 and shale in the reservoir environment plays a key role in realizing efficient shale gas development and long-term stable carbon sequestration in the formation. However, previous studies on the interaction between CO2 and reservoir rocks focused on macro-scale and batch experiments of inorganic minerals, and the mechanism at the nanoscale, especially the micro-wetting behavior of the organic interface, remains unclear and needs to be investigated urgently. In this paper, based on the atomic force microscopy (AFM) tapping mode and the AM-FM viscoelastic mapping mode, the in-situ organic matter (OM) morphology and the change of the relationship between the adhesion force and the interfacial wettability under carbon dioxide immersion are investigated, respectively, and the carbon dioxide wettability behavior at the organic interfaces is deeply revealed in the nanoscale. Among them, the contact angle is obtained by the adhesion work combined with the Johnson-Kendall-Roberts (JKR) adhesive contact theory. The results show that the surface roughness of the OM keeps increasing with the ScCO2 immersion time, while the surface adhesion keeps decreasing at the same time. Although the contact angle decreases continuously with immersion time, the OM is always hydrophobic, i.e., CO2 wetting. It can be seen that the surface roughness has a negative correlation with the contact angle, which keeps decreasing with decreasing adhesion. This indicates that ScCO2 immersion expands the gas adsorption sites and at the same time reduces the interfacial energy of the OM slightly, which is extremely beneficial for CO2 to enhance shale gas recovery and to implement long-term stable carbon geologic sequestration. Therefore, this study reveals the mechanism of the carbon sequestration process at the organic interface in depth within the nanoscale for the first time, which provides a new perspective to deeply understand the long-term adsorption and capture variation process of OM in deep reservoirs.