Different effects of resins and asphaltenes concentration of crude oil on sandstone wettability

Abstract

The influence of asphaltenes in crude oil on sandstone wettability is widely acknowledged. Although asphaltenes are recognized for their stronger polarity compared to resins, resins also contain the similar acidic and nitrogen-containing compounds to those found in asphaltenes. Nevertheless, the impact of resin adsorption on altering sandstone wettability, the different effects of vary concentrations of resins and asphaltenes, and their microscopic mechanisms influencing sandstone surfaces remain ambiguous. Thus, in this study, resins and asphaltenes were isolated from crude oil individually to investigate the different effects of their concentrations on sandstone wettability and the specific reasons. The contact angle measurements, atomic force microscopy (AFM), energy dispersive spectroscopy (EDS), and Cryo-environmental scanning electron microscopy (ESEM) were used to examine and explain the wettability changes of Berea sandstone slices before and after exposure to varying concentrations kerosene solutions of resins and asphaltenes. The results reveal that sandstone wettability could not be changed from strongly water-wet to oil-wet, even at resin concentrations as high as 50.0 mg/mL kerosene. In contrast, lower concentrations of asphaltenes, equivalent to a 5.0 mg/mL kerosene threshold, can facilitate the wettability transition process. After the adsorption of resins and asphaltenes, the roughness of the clay surface changes much more significantly than that of the quartz surface. Combined with the observation of ESEM and the results of EDS, it is believed that the resins and asphaltenes preferentially adsorb to clay surfaces, especially chlorite, rather than quartz surfaces. Clay could provide favorable sites for the accumulation of oxygen-containing compounds, leading to enduring clay-organic complexes that serve as anchor molecules for the adsorption of other compounds, ultimately causing the sandstone surface to an oil-wet state. These results may provide theoretical support for better core aging process in the laboratory, surfactant development and enhanced oil recovery procedure optimization in the oilfield.