Adsorption mechanism study of viscoelastic surfactants with various molecular structures on oil and gas reservoir rocks

The surface charge and clay composition are different between sandstone and limestone, and the molecular structures are various between each VES, which results in different adsorption and wettability alternation results. Sandstone exhibits great adsorption capacity for cationic VES which lead to wettability reversal, however limestone exhibit weak adsorption for each kind of VES and no wettability reversal occurs. [Display omitted] •A series of VESs with various molecular structures were studied to investigate the relationship between adsorption capacity and molecular structure.•The effect of salinity and temperature on the adsorption capacity were studied through static and dynamic adsorption tests.•Mineral content of rocks, molecular structure characteristic of VESs and adsorption free energy, the adsorption mechanism of each VESs with different molecular structure on rocks were studied to optimize VES molecular structures. As clean thickeners used in fracturing fluids or self-diverting acids, viscoelastic surfactants (VESs) have garnered increased attention for their unique properties. While VESs leave little residue, their adsorption on rocks can lead to adverse effects on oil and gas seepage. Herein, sandstone and limestone were utilized as adsorbents to examine a series of VESs with diverse molecular structures. The relationship between adsorption capacity and molecular structure was investigated, along with the impact of salinity and temperature on adsorption capacity through static and dynamic adsorption tests. Experiments were conducted at various temperatures (25 °C–100 °C) and different salinity levels (0–4 wt%) using sandstone and limestone as adsorbents. The wettability alteration caused by each VES solution was assessed to confirm the adsorption capacity of these VESs on sandstone and limestone. Scanning electron microscopy was employed to visually study the microscopic morphologies of rock surfaces after adsorption in VES solutions to investigate the adsorption characteristics. Owing to the differing charges between sandstone and limestone, the adsorption capacity of each VES varied. Cationic VESs tend to be absorbed by sandstone, while zwitterionic VESs are easily adsorbed by limestone. The increase in salinity considerably enhances the adsorption capacity of rocks, with a greater amplification on sandstone compared to limestone. Additionally, the adsorption capacity of cationic VESs on sandstone is highly sensitive to salinity owing to the