Water-CO2 wettability on sandstone surface with asphaltene adsorption: Molecular dynamics simulation

Abstract

The reactions between CO2 slugs and crude oil induce a substantial amount of asphaltene precipitation and adsorption on the rock surface during the CO2 alternative water flooding process. When the subsequent water slug passes through the core pores after asphaltene adsorption, it will displace the previous CO2 slug. The wettability of water and CO2 on the asphaltene-adsorbed rock surface will directly determine the magnitude and direction of the capillary force of the two media, which in turn affects their flow resistance and flow pattern analysis. In this paper, the systems of CO2/water/sandstone and CO2/water/asphaltene adsorption sandstone were established by molecular dynamics simulation technology. The effects of asphaltene adsorption, thickness of the asphaltene adsorption layer, CO2 density, mass fraction and type of salt in water on the wettability of water on sandstone surfaces were studied. The results reveal that asphaltene adsorption significantly reduces the wettability of water on the original powerful water-wetting sandstone surface, making it easier to see how CO2 density affects the wettability of water on the asphaltene-adsorbed sandstone surface. The increased CO2 density will continue to lower the wettability of water on the asphaltene-adsorbed sandstone surface, even causing it to become wet with CO2. The adsorption thickness of asphaltene does not affect the wettability of water and CO2 on the asphaltene-adsorbed sandstone surface, and just a layer of 5A-thick asphaltene adsorption can significantly reduce the wettability of water on sandstone surfaces. Furthermore, a rise in salinity in water has a detrimental impact on the wettability of water on the asphaltene-adsorbed sandstone surface, with divalent salts having a stronger negative effect than monovalent salts.