Wettability of Different Mineral Surfaces in Supercritical CO2 Extraction: Interpretations from the Molecular Composition of Crude Oil

Supercritical CO2 flooding is an efficient method of enhanced oil recovery in which CO2 injection can selectively extract light components and cause the deposition of residue heavy components in reservoir pores. Residues can change the wettability of the rock surface to ultimately reduce crude oil production. In this study, supercritical CO2 extraction experiments were conducted under identical operation conditions using various minerals, including quartz, montmorillonite, Illite, dolomite, albite, calcite, and chlorite. The crude oil, extracts, and residues were comprehensively characterized using high-temperature gas chromatography and high-resolution mass spectrometry. The results indicate that the extraction effect is influenced by the specific surface areas and adsorption strengths of various minerals. The larger the specific surface area, the more readily the heteroatoms in the resins can be adsorbed onto the rock surface. Quartz exhibits weak adsorption strength toward heteroatoms, whereas minerals with high specific surface areas demonstrate significantly higher adsorption strengths compared to those with lower specific surface areas. The wettability changes of various mineral surfaces were also assessed through contact angle measurements. Upon exposure to crude oil, a rock surface undergoes a transition from being water-wet to becoming oil-wet. Subsequently, it reverts back to being more water-wet due to supercritical CO2 extraction which further proves the feasibility of alleviating channeling by injecting water. Furthermore, the alteration in wettability caused by the change in the carbon number of different heteroatoms under supercritical CO2 extraction is also investigated. This study provides a basis for the mechanism study of CO2-crude oil-rock interaction and a comprehensive understanding of the wettability alteration of different reservoirs in carbon dioxide flooding.